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This research was funded by Discovery grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) to D.T.S. (grant number 217175) and S.B. (grant number 435656). E.E.C. was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement No713750, carried out with the financial support of the Regional Council of Provence- Alpes-Cote d'Azur and with the financial support of the A*MIDEX (n degrees ANR- 11-IDEX-0001-02), funded by the Investissements d'Avenir project funded by the French Government, managed by the French National Research Agency [ANR]. B.M.R. was supported by an NSERC CGS-D award, Killam Predoctoral Scholarship, and a NS Graduate Scholarship. M.A.G.-R. was supported by a Harrison McCain Visiting Professorship Award at Acadia University from the Harrison McCain Foundation., Doubly uniparental inheritance (DUI) of mitochondrial DNA (mtDNA) in bivalve mollusks is one of the most notable departures from the paradigm of strict maternal inheritance of mtDNA among metazoans. Recently, work on the Mediterranean mussel Mytilus galloprovincialis suggested that a nucleotide motif in the control region of this species, known as the sperm transmission element (STE), helps protect male-transmitted mitochondria from destruction during spermatogenesis. Subsequent studies found similar, yet divergent, STE motifs in other marine mussels. Here, we extend the in silico search for mtDNA signatures resembling known STEs. This search is carried out for the large unassigned regions of 157 complete mitochondrial genomes from within the Mytiloida, Veneroida, Unionoida, and Ostreoida bivalve orders. Based on a sliding window approach, we present evidence that there are additional putative STE signatures in the large unassigned regions of several marine clams and freshwater mussels with DUI. We discuss the implications of this finding for interpreting the origin of doubly uniparental inheritance in ancestral bivalve mollusks, as well as potential future in vitro and in silico studies that could further refine our understanding of the early evolution of this unusual system of mtDNA inheritance., Natural Sciences and Engineering Research Council of Canada (NSERC) 217175 435656, European Commission 713750, Region Provence-Alpes-Cote d'Azur, French National Research Agency (ANR) ANR- 11-IDEX-0001-02 NSERC CGS-D award, Killam Predoctoral Scholarship, NS Graduate Scholarship, Harrison McCain Visiting Professorship Award at Acadia University from the Harrison McCain Foundation

Comparative genomics of fungal mitochondrial genomes (mitogenomes) have revealed a remarkable pattern of rearrangement between and within major phyla owing to horizontal gene transfer (HGT) and recombination. The role of recombination was exemplified at a finer evolutionary time scale in basidiomycetes group of fungi as they display a diversity of mitochondrial DNA (mtDNA) inheritance patterns. Here, we assembled mitogenomes of six species from the Hymenochaetales order of basidiomycetes and examined 59 mitogenomes from two genetic lineages ofPyrrhoderma noxium. Gene order is largely colinear while intergene regions are major determinants of mitogenome size variation. Substantial sequence divergence was found in shared introns consistent with high HGT frequency observed in yeasts, but we also identified a rare case where an intron was retained in five species since speciation. In contrast to the hyperdiversity observed in nuclear genomes ofP. noxium, mitogenomes’ intraspecific polymorphisms at protein coding sequences are extremely low. Phylogeny based on introns revealed turnover as well as exchange of introns between two lineages. Strikingly, some strains harbor a mosaic origin of introns from both lineages. Analysis of intergenic sequence indicated substantial differences between and within lineages, and an expansion may be ongoing as a result of exchange between distal intergenes. These findings suggest that the evolution in mtDNAs is usually lineage specific but chimeric mitotypes are frequently observed, thus capturing the possible evolutionary processes shaping mitogenomes in a basidiomycete. The large mitogenome sizes reported in various basidiomycetes appear to be a result of interspecific reshuffling of intergenes.

Some essential hypertension (EH) patients show maternal inheritance, which is the mode of mitochondrial DNA inheritance. This study examines the mechanisms by which mitochondrial mutations cause EH characterized by maternal inheritance. The study enrolled 115 volunteers, who were divided into maternally inherited EH (group A, n = 17), non-maternally inherited EH (group B, n = 65), and normal control (group C, n = 33) groups. A mitochondrial tRNA (15910 C>T) gene mutation was significantly correlated with EH and may play an important role in the pathogenesis of maternally inherited EH. Examining two families carrying the mitochondrial tRNA 15910 C>T mutation, which disrupted base pairing and may affect the stability and function of mitochondrial tRNAThr, we find that the overall incidence of EH was 59.3% in the maternal family members and 90% in males, significantly higher than in the general population in China (23.2%), and that the EH began at a younger age in those carrying mitochondrial tRNA 15910 C>T. To reveal the mechanism through which mitochondrial tRNA 15910 C>T causes maternally inherited EH, we cultured human peripheral blood mononuclear cells from family A2 in vitro. We find that cells carrying mitochondrial tRNA 15910 C>T were more viable and proliferative, and the increased ATP production resulted in raised intracellular reactive oxygen species (ROS). Moreover, the mitochondrial dysfunction resulted in reduced APN levels, causing hypoadiponectinemia, which promoted cell proliferation, and produced more ROS. This vicious cycle promoted the occurrence of EH with maternally inherited mitochondrial tRNA 15910 C>T. The mitochondrial tRNA 15910 C>T mutation may induce hypertension by changing the APN, AdipoR1, PGC-1α, and ERRα signaling pathways to elevate blood pressure. We discover a new mitochondrial mutation (tRNA 15910 C>T) related to EH, reveal part of the mechanism by which mitochondrial mutations lead to the occurrence and development of maternally inherited EH, and discuss the role of APN in it.

Mitochondrial DNA (mtDNA) in yeast is biparentally inherited, but colonies rapidly lose one type of parental mtDNA, thus becoming homoplasmic. Therefore, hybrids between the yeast species possess two homologous nuclear genomes, but only one type of mitochondrial DNA. We hypothesise that the choice of mtDNA retention is influenced by its contribution to hybrid fitness in different environments, and the allelic expression of the two nuclear sub-genomes is affected by the presence of different mtDNAs in hybrids. Saccharomyces cerevisiae/S. uvarum hybrids preferentially retained S. uvarum mtDNA when formed on rich media at colder temperatures, while S. cerevisiae mtDNA was primarily retained on non-fermentable carbon source, at any temperature. Transcriptome data for hybrids harbouring different mtDNA showed a strong environmentally dependent allele preference, which was more important in respiratory conditions. Co-expression analysis for specific biological functions revealed a clear pattern of concerted allelic transcription within the same allele type, which supports the notion that the hybrid cell works preferentially with one set of parental alleles (or the other) for different cellular functions. Given that the type of mtDNA retained in hybrids affects both nuclear expression and fitness, it might play a role in driving hybrid genome evolution in terms of gene retention and loss.

The mitochondrial outer membrane (OM) contains single and multiple membrane-spanning proteins which need to contain signals that ensure correct targeting and insertion into the OM. The biogenesis of such proteins has so far essentially only been studied in yeast and related organisms. Here we show that POMP10, an OM protein of the early diverging protozoan Trypanosoma brucei, is signal-anchored. Transgenic cells expressing variants of POMP10 fused to GFP demonstrate that the N-terminal membrane-spanning domain flanked by a few positively charged or neutral residues is both necessary and sufficient for mitochondrial targeting. Carbonate extraction experiments indicate that, while the presence of neutral instead of positively charged residues did not interfere with POMP10 localization, it weakened its interaction with the OM. Expression of GFP-tagged POMP10 in inducible RNAi cell lines shows that its mitochondrial localization depends on pATOM36 but does neither require Sam50 nor ATOM40, the trypanosomal analogue of the Tom40 import pore. pATOM36 is a kinetoplastid-specific OM protein that has previously been implicated in the assembly of OM proteins and in mitochondrial DNA inheritance. In summary, our results show that while the features of the targeting signal in signal-anchored proteins are widely conserved, the protein machinery that mediates their biogenesis is not.

Marine mussels of the family Mytilidae, as well as a number of other bivalves, have a unique system of mitochondrial DNA inheritance called doubly uniparental inheritance (DUI). DUI is characterized by the presence of an ‘F’ mitochondrial genome that is transmitted through mothers to daughters and sons, and an ‘M’ mitochondrial genome that is transmitted only from fathers to sons. In this paper, we demonstrate that DUI exists in the horse mussel, Modiolus modiolus (Linnaeus, 1758) and compare the pattern of molecular evolution of the M and F types in this species. Total DNA was isolated from M. modiolus male and female gonad tissues, as well as from spawned sperm cells. From these DNA samples, partial mitochondrial DNA fragments were amplified from both cytochrome c oxidase subunit I (cox1), and 16S ribosomal RNA (rrnL) genes. Based on cox1 and rrnL sequences, heteroplasmy was observed in M. modiolus and characterized by the resolution of two mitotypes: an F mitotype present in tissues of both males and females, and an M mitotype present in spawned sperm. Using standardized p-distance and Tamura-Nei values, M. modiolus is found to display the highest M/F conspecific sequence divergence for any member of the family Mytilidae (i.e. 38% M/F sequence divergence, which is 9% higher than any other intraspecific M/F comparison for the family Mytilidae when standardized using p-distances across all taxa observed). Sequence analysis also indicated that the M. modiolus M mitotype evolves significantly faster than its conspecific F type. The findings discussed herein broaden the range of mytilid species known to exhibit DUI and they also establish a new threshold for the genetic divergence of male mytilid mitochondrial genomes.

Sex-biased demographic events have played a crucial role in shaping human history. Many of these processes affect genetic variation and can therefore leave detectable signatures in the genome because autosomal, X-linked, Y-linked, and mitochondrial DNA inheritance differ between sexes. Here, we discuss how sex-biased processes shape patterns of genetic diversity across the genome, review recent genomic evidence for sex-biased demography in modern human populations, and suggest directions for future research.

Studies of mitochondrial (mt)DNA replication, which forms the basis of mitochondrial inheritance, have demonstrated that a rolling-circle replication mode exists in yeasts and human cells. In yeast, rolling-circle mtDNA replication mediated by homologous recombination is the predominant pathway for replication of wild-type mtDNA. In human cells, reactive oxygen species (ROS) induce rolling-circle replication to produce concatemers, linear tandem multimers linked by head-to-tail unit-sized mtDNA that promote restoration of homoplasmy from heteroplasmy. The event occurs ahead of mtDNA replication mechanisms observed in mammalian cells, especially under higher ROS load, as newly synthesized mtDNA is concatemeric in hydrogen peroxide-treated human cells. Rolling-circle replication holds promise for treatment of mtDNA heteroplasmy-attributed diseases, which are regarded as incurable. This review highlights the potential therapeutic value of rolling-circle mtDNA replication.

Since the discovery of mtDNA in 1963 (1), it has been a common notion that inheritance of this small, but very essential, piece of DNA occurs in a strict maternal manner. One reason for this is that the paternal contribution of sperm mitochondria to the fertilized egg is about 1,000-fold less than the number of mitochondria present in the oocyte. In addition to this, there seem to be selective mechanisms, which are still poorly understood, that target paternal mtDNA for destruction. Sperm mitochondria are marked with ubiquitin, which likely facilitates this deselection of paternal mtDNA molecules (2), and paternal mtDNA molecules are usually undetectable after the four- to eight-cell stage after fertilization (3). It is therefore interesting that Luo et al. (4) in PNAS report on biparental inheritance in three families, which challenges the notion of strict maternal transmission of mtDNA to offspring. Nuclear and mitochondrial DNAs have separate evolutionary origins. The circular mtDNA genome is derived from bacteria and founded the basis for multicellular organisms when single, eukaryote cells engulfed proteobacteria ∼1.5 billion y ago, according to the endosymbiotic theory (5). Although mtDNA is thought to be transmitted by strict maternal inheritance in humans, paternal inheritance of mtDNA has been described for several insects such as fruit flies (6, 7), honey bees … [↵][1]1Email: john.vissing{at}regionh.dk. [1]: #xref-corresp-1-1

Mitochondrial DNA (mtDNA) in budding yeast is biparentally inherited, but colonies rapidly lose one type of parental mtDNA, becoming homoplasmic. Therefore, hybrids between different yeast species possess two homologous nuclear genomes, but only one type of mitochondrial DNA. We hypothesise that the choice of mtDNA retention is influenced by its contribution to hybrid fitness in different environments, and that the allelic expression of the two nuclear sub-genomes is affected by the presence of different mtDNAs in hybrids. Here, we crossedSaccharomyces cerevisiaewithS. uvarumunder different environmental conditions and examined the plasticity of the retention of mtDNA in each hybrid. We showed that on fermentable carbon sources at warm temperatures each parental mtDNA was equally likely to be retained, while at colder temperatures, hybrids preferentially retained mtDNA derived fromS. uvarum. On a non-fermentable carbon source, hybrids retainedS. cerevisiaemtDNA, independent of temperature. By acquiring transcriptome data and co-expression profiles for hybrids harbouring different mtDNA in a selection of environments, we found a clear pattern of concerted allelic transcription of one or the other sub-genome for specific biological pathways, supporting the notion that the hybrid cell works preferentially with one set of parental alleles or the other according to specific cellular functions. We argue that the type of mtDNA retained in hybrids affects the expression of the nuclear genome and the organism fitness in different environments, and therefore may have a role in driving the evolution of the hybrid nuclear genome in terms of gene retention and loss.

Mitochondrial DNA (mtDNA) sequence variation and maternal inheritance are valuable tools in assessing ancestry of different human populations and for clinical practice. A new study (Wei et al., 2019) reports that the fate of new mtDNA variants in the female germline is non-random as they report functional selection and matching to nuclear ancestry to shape human mtDNA variation.

Body weight and adiposity are determined by the balance between energy intake, energy expenditure, and nutrient deposition. We have identified differences in appetite-regulating peptides in sheep selectively bred to be either lean or fat, wherein gene expression for orexin and melanin-concentrating hormone are elevated in the lean group. Despite this, the underlying mechanisms leading to differences in body composition in the lean and fat lines remains unknown. We measured postprandial temperature in adipose tissue and muscle to ascertain whether a difference in thermogenesis is associated with the difference in body composition in genetically lean (n = 8) and fat (n = 12) ewes. Body weight was higher (P < 0.01) but percent fat mass was lower (P < 0.001) in the lean group. The percent lean mass was similar in lean and fat groups. Animals received intracerebroventricular cannulae and temperature probes implanted into the retroperitoneal fat and the hind-limb skeletal muscle (vastus lateralis). Animals were meal fed (1100-1600 h) to entrain postprandial thermogenesis. Food intake was similar between lean and fat animals. Postprandial thermogenesis was greater (P < 0.05) in the retroperitoneal adipose tissue of lean animals but not in skeletal muscle. Intracerebroventricular infusion of leptin reduced (P< 0.05) food intake by an equal extent in both groups. Postprandial expression of UCP1 mRNA was greater (P < 0.05) in retroperitoneal fat of lean animals, with similar UCP3 expression in skeletal muscle. Mitochondrial genome sequencing indicated haplotypic clustering in lean and fat animals within both the encoding and nonencoding regions. This demonstrates that differences in body composition may be underpinned by differences in thermogenesis, specifically within adipose tissue. Furthermore, thermogenic differences may be associated with specific mitochondrial DNA haplotypes, suggesting a strong genetic component inherited through the maternal lineage.

The inheritance of mitochondrial DNA (mtDNA) is predominantly uniparental in most sexual eukaryotes. In this study, we examined the mitochondrial inheritance pattern of Cryptococcus gattii, a basidiomycetous yeast responsible for the recent and ongoing outbreak of cryptococcal infections in the US Pacific Northwest and British Columbia (especially Vancouver Island) in Canada. Using molecular markers, we analyzed the inheritance of mtDNA in 14 crosses between strains within and between divergent lineages in C. gattii. Consistent with results from recent studies, our analyses identified significant variations in mtDNA inheritance patterns among strains and crosses, ranging from strictly uniparental to biparental. For two of the crosses that showed uniparental mitochondrial inheritance in standard laboratory conditions, we further investigated the effects of the following environmental variables on mtDNA inheritance: UV exposure, temperature, and treatments with the methylation inhibitor 5-aza-2'-deoxycytidine and with the ubiquitination inhibitor ammonium chloride. Interestingly, one of these crosses showed no response to these environmental variables while the other exhibited diverse patterns ranging from complete uniparental inheritance of the MATa parent mtDNA, to biparental inheritance, and to a significant bias toward inheritance of the MATα parental mtDNA. Our results indicate that mtDNA inheritance in C. gattii differs from that in its closely related species Cryptococcus neoformans.

Mitochondria cannot form de novo but require mechanisms that mediate their inheritance to daughter cells. The parasitic protozoan Trypanosoma brucei has a single mitochondrion with a single-unit genome that is physically connected across the two mitochondrial membranes with the basal body of the flagellum. This connection, termed the tripartite attachment complex (TAC), is essential for the segregation of the replicated mitochondrial genomes prior to cytokinesis. Here we identify a protein complex consisting of three integral mitochondrial outer membrane proteins—TAC60, TAC42 and TAC40—which are essential subunits of the TAC. TAC60 contains separable mitochondrial import and TAC-sorting signals and its biogenesis depends on the main outer membrane protein translocase. TAC40 is a member of the mitochondrial porin family, whereas TAC42 represents a novel class of mitochondrial outer membrane β-barrel proteins. Consequently TAC40 and TAC42 contain C-terminal β-signals. Thus in trypanosomes the highly conserved β-barrel protein assembly machinery plays a major role in the biogenesis of its unique mitochondrial genome segregation system., Author summary Trypanosoma brucei and its relatives are important human and animal pathogens. Unlike most other eukaryotes trypanosomes have a single mitochondrion with a single unit mitochondrial genome, termed the kinetoplast DNA (kDNA). During each cell cycle the kDNA is replicated and subsequently segregated into the two organelles that are formed during binary fission of the mitochondrion. Segregation depends on the tripartite attachment complex (TAC) which physically links the kDNA to the basal body of the flagellum. Thus, the TAC couples the segregation of the replicated kDNA to the segregation of the old and new flagella. We have characterized the outer membrane section of the TAC and shown that it contains a complex of three integral membrane proteins, TAC60, TAC42 and TAC40, each of which is essential for TAC function. Furthermore, we have identified which protein import systems are required for their biogenesis. In the case of TAC60 we demonstrate that membrane insertion and sorting to the TAC are separate processes requiring distinct cis-elements. Finally, we show that TAC42 is a novel mitochondrial beta-barrel protein whose biogenesis depends on the beta-signal in its C-terminus. Thus, TAC60, TAC42 and TAC40 are essential trypanosomatid-specific proteins that may be exploited as drug targets.

Mitochondria cannot form de novo but require mechanisms that mediate their inheritance to daughter cells. The parasitic protozoanTrypanosoma bruceihas a single mitochondrion with a single-unit genome that is physically connected across the mitochondrial membranes to the basal body of the flagellum. This connection, termed tripartite attachment complex (TAC), is essential for the segregation of the replicated mitochondrial genomes prior to cytokinesis. Here we identify a protein complex consisting of three integral mitochondrial outer membrane proteins - TAC60, TAC42 and TAC40 - which are essential subunits of the TAC. TAC60 contains separable mitochondrial import and TAC-sorting signals and its biogenesis depends on the main outer membrane protein translocase. TAC40 is a member of the mitochondrial porin family, whereas TAC42 represents a novel class of mitochondrial outer membrane β-barrel proteins. Consequently TAC40 and TAC42 contain C-terminal β-signals. Thus in trypanosomes the highly conserved β-barrel protein assembly machinery plays a major role in the biogenesis of its unique mitochondrial genome segregation system.

Mitochondria cannot form de novo but require mechanisms allowing their inheritance to daughter cells. In contrast to most other eukaryotes Trypanosoma brucei has a single mitochondrion whose single-unit genome is physically connected to the flagellum. Here we identify a β-barrel mitochondrial outer membrane protein, termed tripartite attachment complex 40 (TAC40), that localizes to this connection. TAC40 is essential for mitochondrial DNA inheritance and belongs to the mitochondrial porin protein family. However, it is not specifically related to any of the three subclasses of mitochondrial porins represented by the metabolite transporter voltage-dependent anion channel (VDAC), the protein translocator of the outer membrane 40 (TOM40), or the fungi-specific MDM10, a component of the endoplasmic reticulum-mitochondria encounter structure (ERMES). MDM10 and TAC40 mediate cellular architecture and participate in transmembrane complexes that are essential for mitochondrial DNA inheritance. In yeast MDM10, in the context of the ERMES, is postulated to connect the mitochondrial genomes to actin filaments, whereas in trypanosomes TAC40 mediates the linkage of the mitochondrial DNA to the basal body of the flagellum. However, TAC40 does not colocalize with trypanosomal orthologs of ERMES components and, unlike MDM10, it regulates neither mitochondrial morphology nor the assembly of the protein translocase. TAC40 therefore defines a novel subclass of mitochondrial porins that is distinct from VDAC, TOM40, and MDM10. However, whereas the architecture of the TAC40-containing complex in trypanosomes and the MDM10-containing ERMES in yeast is very different, both are organized around a β-barrel protein of the mitochondrial porin family that mediates a DNA-cytoskeleton linkage that is essential for mitochondrial DNA inheritance.

The dihydrolipoyl succinyltransferase (E2) of the multisubunit α-ketoglutarate dehydrogenase complex (α-KD) is an essential Krebs cycle enzyme commonly found in the matrices of mitochondria. African trypanosomes developmentally regulate mitochondrial carbohydrate metabolism and lack a functional Krebs cycle in the bloodstream of mammals. We found that despite the absence of a functional α-KD, bloodstream form (BF) trypanosomes express α-KDE2, which localized to the mitochondrial matrix and inner membrane. Furthermore, α-KDE2 fractionated with the mitochondrial genome, the kinetoplast DNA (kDNA), in a complex with the flagellum. A role for α-KDE2 in kDNA maintenance was revealed in α-KDE2 RNA interference (RNAi) knockdowns. Following RNAi induction, bloodstream trypanosomes showed pronounced growth reduction and often failed to equally distribute kDNA to daughter cells, resulting in accumulation of cells devoid of kDNA (dyskinetoplastic) or containing two kinetoplasts. Dyskinetoplastic trypanosomes lacked mitochondrial membrane potential and contained mitochondria of substantially reduced volume. These results indicate that α-KDE2 is bifunctional, both as a metabolic enzyme and as a mitochondrial inheritance factor necessary for the distribution of kDNA networks to daughter cells at cytokinesis.

Trypanosomatids are one of the earliest diverging eukaryotes that have fully functional mitochondria. pATOM36 is a trypanosomatid-specific essential mitochondrial outer membrane protein that has been implicated in protein import. Changes in the mitochondrial proteome induced by ablation of pATOM36 and in vitro assays show that pATOM36 is required for the assembly of the archaic translocase of the outer membrane (ATOM), the functional analog of the TOM complex in other organisms. Reciprocal pull-down experiments and immunofluorescence analyses demonstrate that a fraction of pATOM36 interacts and colocalizes with TAC65, a previously uncharacterized essential component of the tripartite attachment complex (TAC). The TAC links the single-unit mitochondrial genome to the basal body of the flagellum and mediates the segregation of the replicated mitochondrial genomes. RNAi experiments show that pATOM36, in line with its dual localization, is not only essential for ATOM complex assembly but also for segregation of the replicated mitochondrial genomes. However, the two functions are distinct, as a truncated version of pATOM36 lacking the 75 C-terminal amino acids can rescue kinetoplast DNA missegregation but not the lack of ATOM complex assembly. Thus, pATOM36 has a dual function and integrates mitochondrial protein import with mitochondrial DNA inheritance.

Selective elimination of mitochondria by autophagy (mitophagy) is a crucial developmental process to dispose of disintegrated or superflous organelles. However, little is known about underlying regulatory mechanisms. We have investigated mitophagy in response to conditional overexpression of the a2 mating-type locus gene lga2 , which encodes a small mitochondrial protein critically involved in uniparental mitochondrial DNA inheritance during sexual development of Ustilago maydis . In this study, we show that conditional overexpression of lga2 efficiently triggers mitophagy that is dependent on atg8 and atg11 , consistent with selective autophagy. lga2 -triggered mitophagy is preceded by mitochondrial dysfunction, including depletion of mitochondrial RNA transcripts, and is mechanistically distinct from starvation-induced mitophagy despite a common requirement for atg11 . In particular, lga2 -triggered mitophagy strongly depends on the mitochondrial fission factor Dnm1, but it is only slightly affected by N -acetylcysteine, which is an inhibitor of starvation-induced mitophagy. To further delineate the role of mitochondrial fission, we analyzed lga2 effects in Δfis1 mutants. This revealed that mitochondrial fragmentation was only attenuated and mitophagy was largely unaffected. In further support of a Fis1-independent role for Dnm1, mitochondrial association of green fluorescent protein-tagged Dnm1 as well as Dnm1-opposed mitochondrial fusion during sexual development were fis1 independent. In conclusion, our results specify the role of the mitochondrial fission factor Dnm1 in mitophagy and uncover differences between mitophagy pathways in the same cellular system.

Lager beer brewing relies on strains collectively known as Saccharomyces carlsbergensis, which are hybrids between S. cerevisiae and S. eubayanus-like strains. Lager yeasts are particularly adapted to low-temperature fermentations. Selection of new yeast strains for improved traits or fermentation performance is laborious, due to the allotetraploid nature of lager yeasts. Initially, we have generated new F1 hybrids by classical genetics, using spore clones of lager yeast and S. cerevisiae and complementation of auxotrophies of the single strains upon mating. These hybrids were improved on several parameters, including growth at elevated temperature and resistance against high osmolarity or high ethanol concentrations. Due to the uncertainty of chromosomal make-up of lager yeast spore clones, we introduced molecular markers to analyse mating-type composition by PCR. Based on these results, new hybrids between a lager and an ale yeast strain were isolated by micromanipulation. These hybrids were not subject to genetic modification. We generated and verified 13 hybrid strains. All of these hybrid strains showed improved stress resistance as seen in the ale parent, including improved survival at the end of fermentation. Importantly, some of the strains showed improved fermentation rates using 18° Plato at 18-25°C. Uniparental mitochondrial DNA inheritance was observed mostly from the S. cerevisiae parent.

Epigenetic parental genetic effects are important in many biological processes but their roles in the evolution of adaptive traits and their consequences in naturally evolving populations remain to be addressed. By comparing two divergent blind cave-dwelling cavefish populations with a sighted surface-dwelling population (surface fish) of the teleost Astyanax mexicanus, we report here that convergences in vibration attraction behavior (VAB), the lateral line sensory receptors underlying this behavior, and the feeding benefits of this behavior are controlled by parental genetic effects, either maternal or paternal inheritance. From behavioral studies and mathematical evolutionary simulations, we further demonstrate that disparity in nuclear and mitochondrial DNA in one of these cavefish populations that has hybridized with surface fish can be explained by paternal inheritance of VAB. The results suggest that parental genetic effects in adaptive behaviors may be important factors in biasing mitochondrial DNA inheritance in natural populations that are subject to introgression.

Late Quaternary (Eemian) deposits of the Netherlands contain shells that resemble those of living Mytilus galloprovincialis. Similar broad-shelled mytilids also occur in estuaries of the southwestern Netherlands together with slender individuals typical of M. edulis. We sampled living mussels along a depth gradient in the Oosterschelde to a) investigate whether a relation exists between shell shape and depth, b) test if the broadshelled specimens might represent M. galloprovincialis (or a hybrid with M. edulis) and c) assess by inference if the Quaternary specimens might be attributed to M. galloprovincialis as well. In order to do so, we compared genetic (length polymorphism of Me 15/16, COIII sequences and AFLPs) and shellmorphological characteristics (juvenile L/W ratios and socalled Verduin parameters) of the same specimens. The obtained dataset indicates that all studied mussels from the Oosterschelde should be attributed to M. edulis, including those with broad shell outlines. No correlation of shell-morphology and depth-distribution was found. The worn and generally damaged state of the Eemian specimens precluded measurement of the Verduin parameters, while juvenile L/W ratios turned out not to be diagnostic. Therefore the shell characters examined in this study are insufficient to demonstrate the possible presence of M. galloprovincialis shells in Quaternary deposits of the Netherlands.

The unusual mode of mitochondrial DNA inheritance, with two separate: maternal (F) and paternal (M) lineages, gives unique opportunities to study the evolution of the mitochondrial genome. This system was first discovered in the marine mussels Mytilus. The three related species: Mytilus edulis, Mytilus galloprovincialis and Mytilus trossulus form a complex in which the divergence of M and F lineages pre-dates the speciation. The complete mitochondrial genomes of both lineages were known for all species except Pacific M. trossulus. Here we report, for the first time, the complete sequences of both mitochondrial genomes of Pacific M. trossulus, filling the gap. While the reported M and F genomes are highly diverged (26%), they have similar organisation. The only difference is the translocation of one tRNA gene into the long, mosaic control region of the F genome. Consistent presence of an ORF which most likely represents the atp8 gene was confirmed in both genomes. The predicted protein has characteristics expected of the functional atp8 even though the M and F versions are markedly different in length. Comparative analysis involving all three species led to the conclusion that the cause of a faster evolution of atp8 and Mytilus mtDNA in general is most likely the Compensation-Draft Feedback process coupled with relatively relaxed selection in the M lineage. Thus, we postulate that the adaptive changes may have played a role in the emergence of highly diverged, barely recognizable atp8 in Mytilus mussels.

Doubly uniparental inheritance (DUI) of mitochondrial DNA in marine mussels (Mytiloida), freshwater mussels (Unionoida), and marine clams (Veneroida) is the only known exception to the general rule of strict maternal transmission of mtDNA in animals. DUI is characterized by the presence of gender-associated mitochondrial DNA lineages that are inherited through males (male-transmitted or M types) or females (female-transmitted or F types), respectively. This unusual system constitutes an excellent model for studying basic aspects of mitochondrial DNA inheritance and the evolution of mtDNA genomes in general. Here we compare published mitochondrial genomes of unionoid bivalve species with DUI, with an emphasis on characterizing unassigned regions, to identify regions of the F and M mtDNA genomes that could (i) play a role in replication or transcription of the mtDNA molecule and/or (ii) determine whether a genome will be transmitted via the female or the male gamete. Our results reveal the presence of one F-specific and one M-specific open reading frames (ORFs), and we hypothesize that they play a role in the transmission and/or gender-specific adaptive functions of the M and F mtDNA genomes in unionoid bivalves. Three major unassigned regions shared among all F and M unionoid genomes have also been identified, and our results indicate that (i) two of them are potential heavy-strand control regions (OH) for regulating replication and/or transcription and that (ii) multiple and potentially bidirectional light-strand origins of replication (OL) are present in unionoid F and M mitochondrial genomes. We propose that unassigned regions are the most promising candidate sequences in which to find regulatory and/or gender-specific sequences that could determine whether a mitochondrial genome will be maternally or paternally transmitted.

Mitochondrial DNA plays a crucial role in cellular homeostasis; however, the molecular mechanisms underlying mitochondrial DNA inheritance and propagation are only beginning to be understood. To ensure the distribution and propagation of the mitochondrial genome, mitochondrial DNA is packaged into macromolecular assemblies called nucleoids, composed of one or more copies of mitochondrial DNA and associated proteins. We review current research on the mitochondrial nucleoid, including nucleoid-associated proteins, nucleoid dynamics within the cell, potential mechanisms to ensure proper distribution of nucleoids, and the impact of nucleoid organization on mitochondrial dysfunction. The nucleoid is the molecular organizing unit of mitochondrial genetics, and is the site of interactions that ultimately determine the bioenergetic state of the cell as a whole. Current and future research will provide essential insights into the molecular and cellular interactions that cause bioenergetic crisis, and yield clues for therapeutic rescue of mitochondrial dysfunction.

13 pages, 2 figures, 5 tables.-- PMID: 18794572 [PubMed].-- PMCID: PMC2635125.-- Available online Sep15, 2008., Study of the genetic basis of gene expression variation is central to attempts to understand the causes of evolutionary change. Although there are many transcriptomics studies estimating genetic variance and heritability in model organisms such as humans there is a lack of equivalent proteomics studies. In the present study, the heritability underlying egg protein expression was estimated in the marine mussel Mytilus. We believe this to be the first such measurement of genetic variation for gene expression in eggs of any organism. The study of eggs is important in evolutionary theory and life history analysis because maternal effects might have profound effects on the rate of evolution of offspring traits. Evidence is presented that the egg proteome varies significantly between individual females and that heritability of protein expression in mussel eggs is moderate to high suggesting abundant genetic variation on which natural selection might act. The study of the mussel egg proteome is also important because of the unusual system of mitochondrial DNA inheritance in mussels whereby different mitochondrial genomes are transmitted independently through female and male lineages (doubly uniparental inheritance). It is likely that the mechanism underlying this system involves the interaction of specific egg factors with sperm mitochondria following fertilization, and its elucidation might be advanced by study of the proteome in females having different progeny sex ratios. Putative identifications are presented here for egg proteins using MS/MS in Mytilus lines differing in sex ratio. Ontology terms relating to stress response and protein folding occur more frequently for proteins showing large expression differences between the lines. The distribution of ontology terms in mussel eggs was compared with those for previous mussel proteomics studies (using other tissues) and with mammal eggs. Significant differences were observed between mussel eggs and mussel tissues but not between the two types of eggs., This work was supported by a Marine Genomics Europe Network grant (European Union FP6 Contract GOCE-CT-2004-505403) and postdoctoral fellowship (to A. P. D.).

The mechanism of maternal mitochondrial DNA (mtDNA) inheritance in animals can be said to be the selective elimination of sperm mtDNA via the elimination factor of the egg and a sperm mitochondria-specific factor. In 2005, we clarified that t-tpis (Spag1 isoform 1) is a mitochondria-specific translocator and the sperm factor, and furthermore estimated that the elimination factors of the egg are the divalent cation-dependent endonuclease and s-tpis (Spag1 isoform 2 and isoform 3) as the elimination system-specific chaperone [K. Hayashida, K. Omagari, J. Masuda, H. Hazama, Y. Kadokawa, K. Ohba, S. Kohno, The sperm mitochondria-specific translocator has a key role in maternal mitochondrial inheritance, Cell Biol. Int. 29 (2005) 472-481]. This time, using a recombinant Spag1 isoform 1 protein, a pull-down assay of ovary cytosol was performed and the elimination factors searched for. Surprisingly, an endogenous retroviral integrase fragment (Eri15) was identified using mass spectrometry of the electrophoresis band of the pull-down protein. Eri15 was detected as a complex of approximately 500kDa with Spag1 isoform 2 or isoform 3 in native PAGE of the ovary cytosol. This strongly suggested that Eri15 is selectively transported into the sperm mitochondria matrix by Spag1 isoform 2 and 3 via Spag1 isoform 1 and that sperm mtDNA is destroyed, thus causing the establishment of maternal mtDNA inheritance., Biochemical and biophysical research communications, 366(1), pp.206-211; 2008

Blue mussels of the genus Mytilus have an unusual mode of mitochondrial DNA inheritance termed doubly uniparental inheritance (DUI). Females are homoplasmic for the F mitotype which is inherited maternally, whereas males are heteroplasmic for this and the paternally inherited M mitotype. In areas where species distributions overlap a varying degree of hybridization occurs; yet genetic differences between allopatric populations are maintained. Observations from natural populations and previous laboratory experiments suggest that DUI may be disrupted by hybridization, giving rise to heteroplasmic females and homoplasmic males. We carried out controlled laboratory crosses between Mytilus edulis and M. galloprovincialis to produce pure species and hybrid larvae of known parentage. DNA markers were used to follow the fate of the F and M mitotypes through larval development. Disruption of the mechanism which determines whether the M mitotype is retained or eliminated occurred in an estimated 38% of M. edulis x M. galloprovincialis hybrid larvae, a level double that previously observed in adult mussels from a natural M. edulis x M. galloprovincialis hybrid population. Furthermore, reciprocal hybrid crosses exhibited contrasting types of DUI disruption. The results indicate that disruption of DUI in hybrid mussels may be associated with increased mortality and hence could be a factor in the maintenance of genetic integrity for each species.

Mitochondrial fusion has been observed in a great variety of organisms from yeast to man. It serves to mix and unify the mitochondrial compartment and plays roles in cellular aging, cell development, energy dissipation and mitochondrial DNA inheritance. Large GTPases in the mitochondrial outer membrane, termed Fzo or mitofusins, have been identified as key components of the mitochondrial fusion machinery in yeast, flies and mammalian cells. Recent studies in yeast suggest an involvement of a dynamin-related protein in the intermembrane space. Additional components have been identified by genetic screens. These findings suggest a unique and evolutionarily conserved mechanism for mitochondrial membrane fusion.

UNLABELLED Mitochondria are the cell's powerhouse when organisms are grown in the presence of oxygen. They are also the source of reactive oxygen species that cause damage to the biochemical components of the cell and lead to cellular ageing and death. Under winemaking conditions, Saccharomyces yeasts exclusively have a fermentative metabolism due to the high sugar content of grape must. However, their production as an active dry yeast (ADY) form required aerobic propagation and a dehydration process. In these industrial steps, oxidative stress is particularly harmful for the cell. In this work, we analysed the impact of the mitochondrial genome on oxidative stress response, longevity and dehydration tolerance using the synthetic interspecific hybrids obtained between two S. cerevisiae and S. uvarum strains. The isogenic nature of nuclear DNA of such hybrids allowed us to analyse the impact of mitochondrial DNA for fermentative and oxidative stress conditions. Under grape must conditions, the inheritance of mitochondrial DNA poorly impacted the fermentative performance of interspecific hybrids, unlike the hybrids with S. cerevisiae mitochondrial inheritance, which displayed increased tolerance to oxidative stress and dehydration, and showed an extended chronological longevity when cells were grown with aeration. SIGNIFICANCE AND IMPACT OF THE STUDY In modern oenology, yeast starters are employed to inoculate grape juice, usually in the form of active dry yeast (ADY). The dehydration process implies stressful conditions that lead to oxidative damage. Other yeast species and interspecific hybrids other than Saccharomyces cerevisiae may be used to confer novel properties to the final product. However, these yeasts are usually more sensitive to drying. Understanding the causes of oxidative stress tolerance is therefore necessary for developing the use of these organisms in industry. This study indicates the impact of mitochondrial DNA inheritance for oxidative stress resistance in an interspecific context using isogenic Saccharomyces cerevisiae × Saccharomyces uvarum hybrids.

Bivalves of the families Mytilidae and Unionidae show a unique mode of mitochondrial DNA inheritance called doubly uniparental inheritance. In addition to receiving the maternally transmitted mtDNA lineage, males receive a separate mtDNA genome from their fathers. This system is sometimes compromised, in that female genomes are occasionally recruited into the male cycle of inheritance. These masculinization events are common in the Mytilidae but have not been reported in the Unionidae. In order to estimate the age of the male and the female lineages in the Unionidae and to look for evidence of masculinization, we sequenced the junction between the cytochrome c oxidase II gene and the cytochrome c oxidase I gene. The unionid male and female lineages diverged approximately 450 MYA. There is no evidence for masculinization during this period, suggesting that there are taxon-specific differences in the rate of masculinization. Coincidentally, a 200-codon extension of the COII gene is present in the male genome of the Unionidae and may be responsible for the absence of masculinization.

Analysing the current mitochondrial DNA patterns biologists have concluded that we all descend from the same mitochondrial Eve, who is postulated to have lived around 200.000 years ago. Such a result is in agreement with the coalescence theory. Here we represent the mitochondrial DNAs as bitstrings that are maternally transmitted with mutations, and that may also participate in the selection process for survival together with the nuclear DNAs. We end up with the same common ancestor, whose mitochondrial DNA can be traced back from the current population, despite the mitochondrial mutations considered. For a given mutation rate, the degree of confidence of this tracing-back process increases even further when the selection mechanism is included.

The maternal inheritance of mitochondrial DNA (mtDNA) in eukaryotic organisms occurs because of the selective destruction of paternal mtDNA molecules that may be present in the zygote. The elimination of sperm mtDNA is less efficient in interspecific crosses, and biparental inheritance of mtDNA has been observed in a variety of species. Because interspecific crosses are likely to be extremely rare in nature, parental inheritance of mtDNA has been deemed of little relevance to population genetics. The mtDNA of the parasitic trematode Schistosoma mansoni was examined for its utility in addressing epidemiological questions related to the transmission and spread of schistosomiasis. Prior to embarking on such experiments, we sought to confirm the mode of inheritance of this molecule using the highly polymorphic mtDNA minisatellite as a marker. In 3 separate crosses, mtDNA apparently identical to paternal DNA was observed in some individuals of the F2 and F3 generations. These observations thus suggest the intraspecific paternal inheritance of mtDNA across multiple generations in Schistosoma mansoni.

Uniparental inheritance of mitochondrial DNA is pervasive in nonisogamic higher eukaryotes during sexual reproduction, and postzygotic and/or prezygotic factors are shown to be important in ensuring such an inheritance pattern. Although the fungus Cryptococcus neoformans undergoes sexual production with isogamic partners of opposite mating types a and α, most progeny derived from such mating events inherit the mitochondrial DNA (mtDNA) from the a parent. The homeodomain protein complex Sxi1α/Sxi2a, formed in the zygote after a-α cell fusion, was previously shown to play a role in this uniparental mtDNA inheritance. Here, we defined the timing of the establishment of the mtDNA inheritance pattern during the mating process and demonstrated a critical role in determining the mtDNA inheritance pattern by a prezygotic factor, Mat2. Mat2 is the key transcription factor that governs the pheromone sensing and response pathway, and it is critical for the early mating events that lead to cell fusion and zygote formation. We show that Mat2 governs mtDNA inheritance independently of the postzygotic factors Sxi1α/Sxi2a, and the cooperation between these prezygotic and postzygotic factors helps to achieve stricter uniparental mitochondrial inheritance in this eukaryotic microbe., IMPORTANCE Mitochondrial DNA is inherited uniparentally from the maternal parent in the majority of eukaryotes. Studies done on higher eukaryotes such as mammals have shown that the transmission of parental mitochondrial DNA is controlled at both the prefertilization and postfertilization stages to achieve strict uniparental inheritance. However, the molecular mechanisms underlying such uniparental mitochondrial inheritance have been investigated in detail mostly in anisogamic multicellular eukaryotes. Here, we show that in a simple isogamic microbe, Cryptococcus neoformans, the mitochondrial inheritance is controlled at the prezygotic level as well as the postzygotic level by regulators that are critical for sexual development. Furthermore, the cooperation between these two levels of control ensures stricter uniparental mitochondrial inheritance, echoing what has been observed in higher eukaryotes. Thus, the investigation of uniparental mitochondrial inheritance in this eukaryotic microbe could help advance our understanding of the convergent evolution of this widespread phenomenon in the eukaryotic domain.

Mitochondrial fusion is a highly coordinated process that mixes and unifies the mitochondrial compartment for normal mitochondrial functions and mitochondrial DNA inheritance. Dysregulated mitochondrial fusion causes mitochondrial fragmentation, abnormal mitochondrial physiology and inheritance, and has been causally linked with a number of neuronal diseases. Here, we identified a diterpenoid derivative 15-oxospiramilactone (S3) that potently induced mitochondrial fusion to restore the mitochondrial network and oxidative respiration in cells that are deficient in either Mfn1 or Mfn2. A mitochondria-localized deubiquitinase USP30 is a target of S3. The inhibition of USP30 by S3 leads to an increase of non-degradative ubiquitination of Mfn1/2, which enhances Mfn1 and Mfn2 activity and promotes mitochondrial fusion. Thus, through the use of an inhibitor of USP30, our study uncovers an unconventional function of non-degradative ubiquitination of Mfns in promoting mitochondrial fusion.

An exceptional mode of mtDNA inheritance involving separate maternal and paternal transmission routes has been reported recently in the mussel Mytilus edulis. This mode of inheritance provides an explanation for the high levels of heteroplasmy for two highly diverged genomes observed in males of this species. Here we provide evidence for a similar pattern of heteroplasmy in Atlantic and Mediterranean forms of the related mussel M. galloprovincialis. The results support the hypothesis that this mode of mtDNA inheritance has an ancient origin. In addition, the detection of some heteroplasmic females suggests preferential, rather than exclusive, transmission within male and female lines of descent. We also present evidence that the two highly diverged genomes display a parallel split between the Atlantic and Mediterranean forms, consistent with neutral evolution.

Loss of mitochondrial DNA activates the DNA damage checkpoint kinase Rad53 to inhibit G1- to S-phase progression in budding yeast., The Chk2-mediated deoxyribonucleic acid (DNA) damage checkpoint pathway is important for mitochondrial DNA (mtDNA) maintenance. We show in this paper that mtDNA itself affects cell cycle progression. Saccharomyces cerevisiae rho0 cells, which lack mtDNA, were defective in G1- to S-phase progression. Deletion of subunit Va of cytochrome c oxidase, inhibition of F1F0 adenosine triphosphatase, or replacement of all mtDNA-encoded genes with noncoding DNA did not affect G1- to S-phase progression. Thus, the cell cycle progression defect in rho0 cells is caused by loss of DNA within mitochondria and not loss of respiratory activity or mtDNA-encoded genes. Rad53p, the yeast Chk2 homologue, was required for inhibition of G1- to S-phase progression in rho0 cells. Pif1p, a DNA helicase and Rad53p target, underwent Rad53p-dependent phosphorylation in rho0 cells. Thus, loss of mtDNA activated an established checkpoint kinase that inhibited G1- to S-phase progression. These findings support the existence of a Rad53p-regulated checkpoint that regulates G1- to S-phase progression in response to loss of mtDNA.

In animals, mitochondrial DNA (mtDNA) inheritance is predominantly maternal. In a few cases incidental transmission of paternal mtDNA was observed and estimated to account for only 10(-4)-10(-3) of an individual's mtDNA content. In contrast, biparental inheritance is common in mussels of the genus Mytilus. Here we present direct evidence that sex and mtDNA inheritance are coupled in Mytilus. Females inherit mtDNA only from their mother, but they transmit it to both daughters and sons. Males inherit mtDNA from both parents, but they transmit to sons only the mtDNA they inherited from their father. In pair matings, this mtDNA inheritance pattern is associated with a strong sex-ratio bias. These findings establish a newly discovered type of cytoplasmic DNA transmission. We also present evidence that the phenomenon breaks down in interspecific hybrids.

In contrast to the nuclear genome, the mitochondrial genome does not follow Mendelian laws of inheritance. The nuclear genome of meiotic progeny comes from the recombination of both parental genomes, whereas the meiotic progeny could inherit mitochondria from one, the other, or both parents. In fact, one fascinating phenomenon is that mitochondrial DNA in the majority of eukaryotes is inherited from only one particular parent. Typically, such unidirectional and uniparental inheritance of mitochondrial DNA can be explained by the size of the gametes involved in mating, with the larger gamete contributing towards mitochondrial DNA inheritance. However, in the human fungal pathogen Cryptococcus neoformans, bisexual mating involves the fusion of two isogamous cells of mating type (MAT) a and MATα, yet the mitochondrial DNA is inherited predominantly from the MATa parent. Although the exact mechanism underlying such uniparental mitochondrial inheritance in this fungus is still unclear, various hypotheses have been proposed. Elucidating the mechanism of mitochondrial inheritance in this clinically important and genetically amenable eukaryotic microbe will yield insights into general mechanisms that are likely conserved in higher eukaryotes. In this review, we highlight studies on Cryptococcus mitochondrial inheritance and point out some important questions that need to be addressed in the future.

In animal mitochondrial DNA inheritance, it remains largely unclear where the mitochondrial genetic bottleneck localizes and how it works in rewinding Muller’s ratchet. In a variety of different animals germ plasm mRNAs typically aggregate along with numerous mitochondria to form the mitochondrial cloud (MC) during oogenesis. The MC has been found to serve as messenger transport organizer for germ plasm mRNAs. Germ plasm RNAs in MC will specifically distribute to the primordial germ cells of the future embryo. It has been proposed that the MC might be the site where selected mitochondria accumulate for specific transmission to grandchildren but this idea received relatively little attention and the criterion by which mitochondria are selected remains unknown. Our recent results in zebrafish provided further evidence for selective mitochondria accumulation in the MC by showing that mitochondria with high-inner membrane potential tend to be recruited preferentially into the MC, and these mitochondria are transported along with germ plasm to the cortex of the vegetal pole. By analyzing the composition, behavior and functions of the MC, and in reviewing related literature, we found strong support for the proposition that the MC corresponds to the position and function of the mitochondrial genetic bottleneck.

Individuals heteroplasmic for mitochondrial DNA were observed in seven Mytilus populations at frequencies varying between 7 and 38%. The majority of heteroplasmic individuals possessed two genomes, referred to as the F and M genomes, which appear to exhibit base sequence divergence estimated to be about 10%. All homoplasmic individuals examined possessed the F genome. A large sex difference in the incidence of heteroplasmy was observed with 37% of males but only 5% of females possessing the M genome. Analysis of mtDNA extracted from different tissues provided evidence that the F genome predominates in female gonads and the M genome predominates in male gonads. Biparental transmission of mtDNA is considered as an explanation of these results but is not supported by a statistical analysis of the association of genotypes within heteroplasmic individuals. Because the F genome predominates in the gonads of most females, it is suggested that the M genome must have a replicative advantage to be detected in large quantity within individuals.

Mitochondrial biogenesis and function is under dual genetic control and requires extensive interaction between biparentally inherited nuclear genes and maternally inherited mitochondrial genes. Standard SCNT procedures deprive an oocytes’ mitochondrial DNA (mtDNA) of the corresponding maternal nuclear DNA and require it to interact with an entirely foreign nucleus that is again interacting with foreign somatic mitochondria. As a result, most SCNT embryos, -fetuses, and -offspring carry somatic cell mtDNA in addition to recipient oocyte mtDNA, a condition termed heteroplasmy. It is thus evident that somatic cell mtDNA can escape the selective mechanism that targets and eliminates intraspecific sperm mitochondria in the fertilized oocyte to maintain homoplasmy. However, the factors responsible for the large intra- and interindividual differences in heteroplasmy level remain elusive. Furthermore, heteroplasmy is probably confounded with mtDNA recombination. Considering the essential roles of mitochondria in cellular metabolism, cell signalling, and programmed cell death, future experiments will need to assess the true extent and impact of unorthodox mtDNA transmission on various aspects of SCNT success.

Nuclear remodeling and nuclear reprogramming for making transgenic pigs by nuclear transfer.- Somatic cell nuclei in cloning: strangers traveling in a foreign land.- Cloning cattle: the methods in the madness.- Centrosome inheritance after fertilization and nuclear transfer in mammals.- Developmental, behavioral, and physiological phenotype of cloned mice.- Nucleolar remodelingin nuclear transfer embryos.- Somatic cell nuclear transfer (scnt) in mammals: the cytoplast and its reprogramming activities.- Mitochondrial DNA inheritance after SCNT.- Activation of fertilized and nuclear transfer eggs.

Unlike nuclear genes and genomes, the inheritance of organelle genes and genomes does not follow Mendel's laws. In this mini-review, I summarize recent research progress on the patterns and mechanisms of the inheritance of organelle genes and genomes. While most sexual eukaryotes show uniparental inheritance of organelle genes and genomes in some progeny at least part of the time, increasing evidence indicates that strictly uniparental inheritance is rare and that organelle inheritance patterns are very diverse and complex. In contrast with the predominance of uniparental inheritance in multicellular organisms, organelle genes in eukaryotic microorganisms, such as protists, algae, and fungi, typically show a greater diversity of inheritance patterns, with sex-determining loci playing significant roles. The diverse patterns of inheritance are matched by the rich variety of potential mechanisms. Indeed, many factors, both deterministic and stochastic, can influence observed patterns of organelle inheritance. Interestingly, in multicellular organisms, progeny from interspecific crosses seem to exhibit more frequent paternal leakage and biparental organelle genome inheritance than those from intraspecific crosses. The recent observation of a sex-determining gene in the basidiomycete yeast Cryptococcus neoformans, which controls mitochondrial DNA inheritance, has opened up potentially exciting research opportunities for identifying specific molecular genetic pathways that control organelle inheritance, as well as for testing evolutionary hypotheses regarding the prevalence of uniparental inheritance of organelle genes and genomes.Key words: isogamy, anisogamy, paternal leakage, mating type, quantitative organelle inheritance.

Comparisons among loci with differing modes of inheritance can reveal unexpected aspects of population history. We employ a multilocus approach to ask whether two types of independently assorting mitochondrial DNAs (maternally and paternally inherited: F- and M-mtDNA) and a nuclear locus (ITS) yield concordant estimates of gene flow and population divergence. The blue mussel, Mytilus edulis, is distributed on both North American and European coastlines and these populations are separated by the waters of the Atlantic Ocean. Gene flow across the Atlantic Ocean differs among loci, with F-mtDNA and ITS showing an imprint of some genetic interchange and M-mtDNA showing no evidence for gene flow. Gene flow of F-mtDNA and ITS causes trans-Atlantic population divergence times to be greatly underestimated for these loci, although a single trans-Atlantic population divergence time (1.2 MYA) can be accommodated by considering all three loci in combination in a coalescent framework. The apparent lack of gene flow for M-mtDNA is not readily explained by different dispersal capacities of male and female mussels. A genetic barrier to M-mtDNA exchange between North American and European mussel populations is likely to explain the observed pattern, perhaps associated with the double uniparental system of mitochondrial DNA inheritance.