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Although introns are typically tens to thousands of nucleotides, there are notable exceptions. In flies as well as humans, a small number of genes contain introns that are more than 1000 times larger than typical introns, exceeding hundreds of kilobases (kb) to megabases (Mb). It remains unknown why gigantic introns exist and how cells overcome the challenges associated with their transcription and RNA processing. The Drosophila Y chromosome contains some of the largest genes identified to date: multiple genes exceed 4Mb, with introns accounting for over 99% of the gene span. Here we demonstrate that co-transcriptional splicing of these gigantic Y-linked genes is important to ensure successful transcription: perturbation of splicing led to the attenuation of transcription, leading to a failure to produce mature mRNA. Cytologically, defective splicing of the Y-linked gigantic genes resulted in disorganization of transcripts within the nucleus suggestive of entanglement of transcripts, likely resulting from unspliced long RNAs. We propose that co-transcriptional splicing maintains the length of nascent transcripts of gigantic genes under a critical threshold, preventing their entanglement and ensuring proper gene expression. Our study reveals a novel biological significance of co-transcriptional splicing., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Fingerhut et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Across species, sperm maturation involves the dramatic reconfiguration of chromatin into highly compact nuclei that enhance hydrodynamic ability and ensure paternal genomic integrity. This process is mediated by the replacement of histones by sperm nuclear basic proteins, also referred to as protamines. In humans, a carefully balanced dosage between two known protamine genes is required for optimal fertility. However, it remains unknown how their proper balance is regulated and how defects in balance may lead to compromised fertility. Here, we show that a nucleolar protein, modulo , a homolog of nucleolin , mediates the histone-to-protamine transition during Drosophila spermatogenesis. We find that modulo mutants display nuclear compaction defects during late spermatogenesis due to decreased expression of autosomal protamine genes (including Mst77F ) and derepression of Y-linked multicopy Mst77F homologs ( Mst77Y ), leading to the mutant's known sterility. Overexpression of Mst77Y in a wild-type background is sufficient to cause nuclear compaction defects, similar to modulo mutant, indicating that Mst77Y is a dominant-negative variant interfering with the process of histone-to-protamine transition. Interestingly, ectopic overexpression of Mst77Y caused decompaction of X-bearing spermatids nuclei more frequently than Y-bearing spermatid nuclei, although this did not greatly affect the sex ratio of offspring. We further show that modulo regulates these protamine genes at the step of transcript polyadenylation. We conclude that the regulation of protamines mediated by modulo , ensuring the expression of functional ones while repressing dominant-negative ones, is critical for male fertility.

From insects to mammals, oocytes and sperm develop within germline cysts comprising cells connected by intercellular bridges (ICBs). In numerous insects, formation of the cyst is accompanied by growth of the fusome-a membranous organelle that permeates the cyst. Fusome composition and function are best understood in Drosophila melanogaster: during oogenesis, the fusome dictates cyst topology and size and facilitates oocyte selection, while during spermatogenesis, the fusome synchronizes the cyst's response to DNA damage. Despite its distinct and sex-specific roles during insect gametogenesis, elucidating fusome growth and inheritance in females and its structure and connectivity in males has remained challenging. Here, we take advantage of advances in three-dimensional (3D) confocal microscopy and computational image processing tools to reconstruct the topology, growth, and distribution of the fusome in both sexes. In females, our experimental findings inform a theoretical model for fusome assembly and inheritance and suggest that oocyte selection proceeds through an 'equivalency with a bias' mechanism. In males, we find that cell divisions can deviate from the maximally branched pattern observed in females, leading to greater topological variability. Our work consolidates existing disjointed experimental observations and contributes a readily generalizable computational approach for quantitative studies of gametogenesis within and across species., Competing Interests: The authors declare no competing interests., (Copyright: © 2023 Diegmiller et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Karyotype refers to the configuration of the genome into a set of chromosomes. The karyotype difference between species is expected to impede various biological processes, such as chromosome segregation and meiotic chromosome pairing, potentially contributing to incompatibility. Karyotypes can rapidly change between closely related species and even among populations of the same species. However, the forces driving karyotype evolution are poorly understood. Here we describe a unique karyotype of a Drosophila melanogaster strain isolated from the Seychelles archipelago. This strain has lost the ribosomal DNA (rDNA) locus on the X chromosome. Because the Y chromosome is the only other rDNA-bearing chromosome, all females carry at least one Y chromosome as the source of rDNA. Interestingly, we found that the strain also carries a truncated Y chromosome (YS) that is stably maintained in the population despite its inability to support male fertility. Our modeling and cytological analysis suggest that the Y chromosome has a larger negative impact on female fitness than the YS chromosome. Moreover, we generated an independent strain that lacks X rDNA and has a karyotype of XXY females and XY males. This strain quickly evolved multiple karyotypes: two new truncated Y chromosomes (similar to YS), as well as two independent X chromosome fusions that contain the Y-derived rDNA fragment, eliminating females' dependence on the Y chromosome. Considering that Robertsonian fusions frequently occur at rDNA loci in humans, we propose that rDNA loci instability may be one of driving forces of karyotype evolution., (© The Author(s) 2022. Published by Oxford University Press on behalf of Society for Molecular Biology and Evolution.)

Cytoplasmic cilia, a specialized type of cilia in which the axoneme resides within the cytoplasm rather than within the ciliary compartment, are proposed to allow for the efficient assembly of very long cilia. Despite being found diversely in male gametes (e.g., Plasmodium falciparum microgametocytes and human and Drosophila melanogaster sperm), very little is known about cytoplasmic cilia assembly. Here, we show that a novel RNP granule containing the mRNAs for axonemal dynein motor proteins becomes highly polarized to the distal end of the cilia during cytoplasmic ciliogenesis in Drosophila sperm. This allows for the incorporation of these axonemal dyneins into the axoneme directly from the cytoplasm, possibly by localizing translation. We found that this RNP granule contains the proteins Reptin and Pontin, loss of which perturbs granule formation and prevents incorporation of the axonemal dyneins, leading to sterility. We propose that cytoplasmic cilia assembly requires the precise localization of mRNAs encoding key axonemal constituents, allowing these proteins to incorporate efficiently into the axoneme., (© 2020 Fingerhut and Yamashita.)

Intron gigantism, where genes contain megabase-sized introns, is observed across species, yet little is known about its purpose or regulation. Here we identify a unique gene expression program utilized for the proper expression of genes with intron gigantism. We find that two Drosophila genes with intron gigantism, kl-3 and kl-5, are transcribed in a spatiotemporal manner over the course of spermatocyte differentiation, which spans ~90 hours. The introns of these genes contain megabases of simple satellite DNA repeats that comprise over 99% of the gene loci, and these satellite-DNA containing introns are transcribed. We identify two RNA-binding proteins that specifically localize to kl-3 and kl-5 transcripts and are needed for the successful transcription or processing of these genes. We propose that genes with intron gigantism require a unique gene expression program, which may serve as a platform to regulate gene expression during cellular differentiation., Competing Interests: The authors have declared that no competing interests exist.

A central principle underlying the ubiquity and abundance of pericentromeric satellite DNA repeats in eukaryotes has remained poorly understood. Previously we proposed that the interchromosomal clustering of satellite DNAs into nuclear structures known as chromocenters ensures encapsulation of all chromosomes into a single nucleus (Jagannathan et al., 2018). Chromocenter disruption led to micronuclei formation, resulting in cell death. Here we show that chromocenter formation is mediated by a 'modular' network, where associations between two sequence-specific satellite DNA-binding proteins, D1 and Prod, bound to their cognate satellite DNAs, bring the full complement of chromosomes into the chromocenter. D1 prod double mutants die during embryogenesis, exhibiting enhanced phenotypes associated with chromocenter disruption, revealing the universal importance of satellite DNAs and chromocenters. Taken together, we propose that associations between chromocenter modules, consisting of satellite DNA binding proteins and their cognate satellite DNA, package the Drosophila genome within a single nucleus., Competing Interests: MJ, RC No competing interests declared, YY Reviewing editor, eLife, (© 2019, Jagannathan et al.)

Satellite DNAs are highly repetitive sequences that account for the majority of constitutive heterochromatin in many eukaryotic genomes. It is widely recognized that sequences and locations of satellite DNAs are highly divergent even in closely related species, contributing to the hypothesis that satellite DNA differences may underlie speciation. However, due to its repetitive nature, the mapping of satellite DNAs has been mostly left out of recent genomics analyses, hampering the use of molecular genetics techniques to better understand their role in speciation and evolution. Satellite DNAs are most extensively and comprehensively mapped in Drosophila melanogaster , a species that is also an excellent model system with which to study speciation. Yet the lack of comprehensive knowledge regarding satellite DNA identity and location in its sibling species ( D. simulans , D. mauritiana , and D. sechellia ) has prevented the full utilization of D. melanogaster in studying speciation. To overcome this problem, we initiated the mapping of satellite DNAs on the genomes of the D. melanogaster species complex ( D. melanogaster , D. simulans , D. mauritiana , and D. sechellia ) using multi-color fluorescent in situ hybridization (FISH) probes. Our study confirms a striking divergence of satellite DNAs in the D. melanogaster species complex, even among the closely related species of the D. simulans clade ( D. simulans , D. mauritiana , and D. sechellia ), and suggests the presence of unidentified satellite sequences in these species., (Copyright © 2017 Jagannathan et al.)

Asymmetric cell division (ACD) is utilized in many stem cell systems to produce two daughter cells with different cell fates. Despite the fundamental importance of ACD during development and tissue homeostasis, the nature of ACD is far from being fully understood. Step-by-step observation of events during ACD allows us to understand processes that lead to ACD. Here we describe examples of how we evaluate ACD in vivo using the Drosophila male germline stem cell system.

Due to their strict maternal inheritance in most animals and plants, mitochondrial genomes are predicted to accumulate mutations that are beneficial or neutral in females but harmful in males. Although a few male-harming mtDNA mutations have been identified, consistent with this 'Mother's Curse', their effect on females has been largely unexplored. Here, we identify COII(G177S), a mtDNA hypomorph of cytochrome oxidase II, which specifically impairs male fertility due to defects in sperm development and function without impairing other male or female functions. COII(G177S) represents one of the clearest examples of a 'male-harming' mtDNA mutation in animals and suggest that the hypomorphic mtDNA mutations like COII(G177S) might specifically impair male gametogenesis. Intriguingly, some D. melanogaster nuclear genetic backgrounds can fully rescue COII(G177S) -associated sterility, consistent with previously proposed models that nuclear genomes can regulate the phenotypic manifestation of mtDNA mutations.

Stem cell niches provide resident stem cells with signals that specify their identity. Niche signals act over a short range such that only stem cells but not their differentiating progeny receive the self-renewing signals. However, the cellular mechanisms that limit niche signalling to stem cells remain poorly understood. Here we show that the Drosophila male germline stem cells form previously unrecognized structures, microtubule-based nanotubes, which extend into the hub, a major niche component. Microtubule-based nanotubes are observed specifically within germline stem cell populations, and require intraflagellar transport proteins for their formation. The bone morphogenetic protein (BMP) receptor Tkv localizes to microtubule-based nanotubes. Perturbation of microtubule-based nanotubes compromises activation of Dpp signalling within germline stem cells, leading to germline stem cell loss. Moreover, Dpp ligand and Tkv receptor interaction is necessary and sufficient for microtubule-based nanotube formation. We propose that microtubule-based nanotubes provide a novel mechanism for selective receptor-ligand interaction, contributing to the short-range nature of niche-stem-cell signalling.

Many stem cells divide asymmetrically in order to balance self-renewal with differentiation. The essence of asymmetric cell division (ACD) is the polarization of cells and subsequent division, leading to unequal compartmentalization of cellular/extracellular components that confer distinct cell fates to daughter cells. Because precocious cell division before establishing cell polarity would lead to failure in ACD, these two processes must be tightly coupled; however, the underlying mechanism is poorly understood. In Drosophila male germline stem cells, ACD is prepared by stereotypical centrosome positioning. The centrosome orientation checkpoint (COC) further serves to ensure ACD by preventing mitosis upon centrosome misorientation. In this study, we show that Bazooka (Baz) provides a platform for the correct centrosome orientation and that Baz-centrosome association is the key event that is monitored by the COC. Our work provides a foundation for understanding how the correct cell polarity may be recognized by the cell to ensure productive ACD.

Asymmetric cell division is utilized by a broad range of cell types to generate two daughter cells with distinct cell fates. In stem cell populations asymmetric cell division is believed to be crucial for maintaining tissue homeostasis, failure of which can lead to tissue degeneration or hyperplasia/tumorigenesis. Asymmetric cell divisions also underlie cell fate diversification during development. Accordingly, the mechanisms by which asymmetric cell division is achieved have been extensively studied, although the check points that are in place to protect against potential perturbation of the process are poorly understood. Drosophila melanogaster male germline stem cells (GSCs) possess a checkpoint, termed the centrosome orientation checkpoint (COC), that monitors correct centrosome orientation with respect to the component cells of the niche to ensure asymmetric stem cell division. To our knowledge, the COC is the only checkpoint mechanism identified to date that specializes in monitoring the orientation of cell division in multicellular organisms. Here, by establishing colcemid-induced microtubule depolymerization as a sensitive assay, we examined the characteristics of COC activity and find that it functions uniquely in GSCs but not in their differentiating progeny. We show that the COC operates in the G2 phase of the cell cycle, independently of the spindle assembly checkpoint. This study may provide a framework for identifying and understanding similar mechanisms that might be in place in other asymmetrically dividing cell types., (© 2015. Published by The Company of Biologists Ltd.)

Specialized microenvironments, or niches, provide signaling cues that regulate stem cell behavior. In the Drosophila testis, the JAK-STAT signaling pathway regulates germline stem cell (GSC) attachment to the apical hub and somatic cyst stem cell (CySC) identity. Here, we demonstrate that chickadee, the Drosophila gene that encodes profilin, is required cell autonomously to maintain GSCs, possibly facilitating localization or maintenance of E-cadherin to the GSC-hub cell interface. Germline specific overexpression of Adenomatous Polyposis Coli 2 (APC2) rescued GSC loss in chic hypomorphs, suggesting an additive role of APC2 and F-actin in maintaining the adherens junctions that anchor GSCs to the niche. In addition, loss of chic function in the soma resulted in failure of somatic cyst cells to maintain germ cell enclosure and overproliferation of transit-amplifying spermatogonia.

The immortal strand hypothesis, which emerged four decades ago, proposes that certain cells retain a template copy of chromosomal DNA to protect against replication-induced mutations. As the interest in stem cells rose in recent years, researchers speculated that stem cells, which must maintain proliferative capacity throughout the life of the organism, may be the population that most needs the strong protection afforded by immortal strand segregation. Alternative hypotheses have also been proposed to explain observed non-random sister chromatid segregation. We recently found that Drosophila male germline stem cells segregate sister chromatids non-randomly, but such bias was limited to the sex chromosomes. Interestingly, the biased segregation does not lead to immortal strand segregation. We will discuss the implications of this observation and molecular mechanisms, which might be applicable to non-random sister chromatid segregation in other systems as well., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Adult stem cells undergo asymmetric cell division to self-renew and give rise to differentiated cells that comprise mature tissue. Sister chromatids may be distinguished and segregated nonrandomly in asymmetrically dividing stem cells, although the underlying mechanism and the purpose it may serve remain elusive. Here we develop the CO-FISH (chromosome orientation fluorescence in situ hybridization) technique with single-chromosome resolution and show that sister chromatids of X and Y chromosomes, but not autosomes, are segregated nonrandomly during asymmetric divisions of Drosophila male germline stem cells. This provides the first direct evidence, to our knowledge, that two sister chromatids containing identical genetic information can be distinguished and segregated nonrandomly during asymmetric stem-cell divisions. We further show that the centrosome, SUN-KASH nuclear envelope proteins and Dnmt2 (also known as Mt2) are required for nonrandom sister chromatid segregation. Our data indicate that the information on X and Y chromosomes that enables nonrandom segregation is primed during gametogenesis in the parents. Moreover, we show that sister chromatid segregation is randomized in germline stem cell overproliferation and dedifferentiated germline stem cells. We propose that nonrandom sister chromatid segregation may serve to transmit distinct information carried on two sister chromatids to the daughters of asymmetrically dividing stem cells.

Sister chromatids are the product of DNA replication, which is assumed to be a very precise process. Therefore, sister chromatids should be exact copies of each other. However, reports have indicated that sister chromatids are segregated nonrandomly during cell division, suggesting that sister chromatids are not the same, although their DNA sequences are the same. Researchers have speculated that stem cells may retain template strands to avoid replication-induced mutations. An alternative proposal is that cells may segregate distinct epigenetic information carried on sister chromatids. Recently, we found that Drosophila male germline stem cells segregate sister chromatids of X and Y chromosomes with a strong bias. We discuss this finding in relation to existing models for nonrandom sister chromatid segregation.

Male germline stem cells (GSCs) in Drosophila melanogaster divide asymmetrically by orienting the mitotic spindle with respect to the niche, a microenvironment that specifies stem cell identity. The spindle orientation is prepared during interphase through stereotypical positioning of the centrosomes. We recently demonstrated that GSCs possess a checkpoint ("the centrosome orientation checkpoint") that monitors correct centrosome orientation prior to mitosis to ensure an oriented spindle and thus asymmetric outcome of the division. Here, we show that Par-1, a serine/threonine kinase that regulates polarity in many systems, is involved in this checkpoint. Par-1 shows a cell cycle-dependent localization to the spectrosome, a germline-specific, endoplasmic reticulum-like organelle. Furthermore, the localization of cyclin A, which is normally localized to the spectrosome, is perturbed in par-1 mutant GSCs. Interestingly, overexpression of mutant cyclin A that does not localize to the spectrosome and mutation in hts, a core component of the spectrosome, both lead to defects in the centrosome orientation checkpoint. We propose that the regulation of cyclin A localization via Par-1 function plays a critical role in the centrosome orientation checkpoint., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Tight regulation of stem cell proliferation is fundamental to tissue homeostasis, aging and tumor suppression. Although stem cells are characterized by their high potential to proliferate throughout the life of the organism, the mechanisms that regulate the cell cycle of stem cells remain poorly understood. Here, we show that the Cdc25 homolog String (Stg) is a crucial regulator of germline stem cells (GSCs) and cyst stem cells (CySCs) in Drosophila testis. Through knockdown and overexpression experiments, we show that Stg is required for stem cell maintenance and that a decline in its expression during aging is a critical determinant of age-associated decline in stem cell function. Furthermore, we show that restoration of Stg expression reverses the age-associated decline in stem cell function but leads to late-onset tumors. We propose that Stg/Cdc25 is a crucial regulator of stem cell function during tissue homeostasis and aging.

Many adult stem cells reside in a special microenvironment known as the niche, where they receive essential signals that specify stem cell identity. Cell-cell adhesion mediated by cadherin and integrin plays a crucial role in maintaining stem cells within the niche. In Drosophila melanogaster, male germline stem cells (GSCs) are attached to niche component cells (i.e., the hub) via adherens junctions. The GSC centrosomes and spindle are oriented toward the hub-GSC junction, where E-cadherin-based adherens junctions are highly concentrated. For this reason, adherens junctions are thought to provide a polarity cue for GSCs to enable proper orientation of centrosomes and spindles, a critical step toward asymmetric stem cell division. However, understanding the role of E-cadherin in GSC polarity has been challenging, since GSCs carrying E-cadherin mutations are not maintained in the niche. Here, we tested whether E-cadherin is required for GSC polarity by expressing a dominant-negative form of E-cadherin. We found that E-cadherin is indeed required for polarizing GSCs toward the hub cells, an effect that may be mediated by Apc2. We also demonstrated that E-cadherin is required for the GSC centrosome orientation checkpoint, which prevents mitosis when centrosomes are not correctly oriented. We propose that E-cadherin orchestrates multiple aspects of stem cell behavior, including polarization of stem cells toward the stem cell-niche interface and adhesion of stem cells to the niche supporting cells.

Asymmetric division of adult stem cells generates one self-renewing stem cell and one differentiating cell, thereby maintaining tissue homeostasis. A decline in stem cell function has been proposed to contribute to tissue ageing, although the underlying mechanism is poorly understood. Here we show that changes in the stem cell orientation with respect to the niche during ageing contribute to the decline in spermatogenesis in the male germ line of Drosophila. Throughout the cell cycle, centrosomes in germline stem cells (GSCs) are oriented within their niche and this ensures asymmetric division. We found that GSCs containing misoriented centrosomes accumulate with age and that these GSCs are arrested or delayed in the cell cycle. The cell cycle arrest is transient, and GSCs appear to re-enter the cell cycle on correction of centrosome orientation. On the basis of these findings, we propose that cell cycle arrest associated with centrosome misorientation functions as a mechanism to ensure asymmetric stem cell division, and that the inability of stem cells to maintain correct orientation during ageing contributes to the decline in spermatogenesis. We also show that some of the misoriented GSCs probably originate from dedifferentiation of spermatogonia.

The piRNA pathway protects germline genomes from selfish genetic elements (e.g. transposons) through their transcript cleavage in the cytoplasm and/or their transcriptional silencing in the nucleus. Here, we describe a mechanism by which the nuclear and cytoplasmic arms of the piRNA pathway are linked. We find that during mitosis of Drosophila spermatogonia, nuclear Piwi interacts with nuage, the compartment that mediates the cytoplasmic arm of the piRNA pathway. At the end of mitosis, Piwi leaves nuage to return to the nucleus. Dissociation of Piwi from nuage occurs at the depolymerizing microtubules of the central spindle, mediated by a microtubule-depolymerizing kinesin, Klp10A. Depletion of klp10A delays the return of Piwi to the nucleus and affects piRNA production, suggesting the role of nuclear-cytoplasmic communication in piRNA biogenesis. We propose that cell cycle-dependent communication between the nuclear and cytoplasmic arms of the piRNA pathway may play a previously unappreciated role in piRNA regulation.

The piRNA pathway plays an important role in maintaining genome stability in the germ line by silencing transposable elements (TEs) from fly to mammals. As a highly conserved piRNA pathway component, Piwi is widely expressed in both germ cells and somatic cells in the Drosophila ovary and is required for piRNA production in both cell types. In addition to its known role in somatic cap cells to maintain germline stem cells (GSCs), this study has demonstrated that Piwi has novel functions in somatic cells and germ cells of the Drosophila ovary to promote germ cell differentiation. Piwi knockdown in escort cells causes a reduction in escort cell (EC) number and accumulation of undifferentiated germ cells, some of which show active BMP signaling, indicating that Piwi is required to maintain ECs and promote germ cell differentiation. Simultaneous knockdown of dpp, encoding a BMP, in ECs can partially rescue the germ cell differentiation defect, indicating that Piwi is required in ECs to repress dpp. Consistent with its key role in piRNA production, TE transcripts increase significantly and DNA damage is also elevated in the piwi knockdown somatic cells. Germ cell-specific knockdown of piwi surprisingly causes depletion of germ cells before adulthood, suggesting that Piwi might control primordial germ cell maintenance or GSC establishment. Finally, Piwi inactivation in the germ line of the adult ovary leads to gradual GSC loss and germ cell differentiation defects, indicating the intrinsic role of Piwi in adult GSC maintenance and differentiation. This study has revealed new germline requirement of Piwi in controlling GSC maintenance and lineage differentiation as well as its new somatic function in promoting germ cell differentiation. Therefore, Piwi is required in multiple cell types to control GSC lineage development in the Drosophila ovary. [ABSTRACT FROM AUTHOR]

In the homeostatic state, adult stem cells divide either symmetrically to increase the stem cell number to compensate stem cell loss, or asymmetrically to maintain the population while producing differentiated cells. We have investigated the mode of stem cell division in the testes of Drosophila melanogaster by lineage tracing and confirm the presence of symmetric stem cell division in this system. We found that the rate of symmetric division is limited to 1–2% of total germline stem cell divisions, but it increases with expression of a cell adhesion molecule, E-cadherin, or a regulator of the actin cytoskeleton, Moesin, which may modulate adhesiveness of germ cells to the stem cell niche. Our results indicate that the decision regarding asymmetric vs. symmetric division is a dynamically regulated process that contributes to tissue homeostasis, responding to the needs of the tissue. [ABSTRACT FROM AUTHOR]

The ability of dividing cells to produce daughters with different fates is an important developmental mechanism conserved from bacteria to fungi, plants, and metazoan animals. Asymmetric outcomes of a cell division can be specified by two general mechanisms: asymmetric segregation of intrinsic fate determinants or asymmetric placement of daughter cells into microenvironments that provide extrinsic signals that direct cells to different states. For both, spindle orientation must be coordinated with the localization of intrinsic determinants or source of extrinsic signals to achieve the proper asymmetric outcome. Recent work on spindle orientation in Drosophila melanogaster male germline stem cells and neuroblasts has brought into sharp focus the key role of differential centrosome behavior in developmentally programmed asymmetric division (for reviews see Cabernard, C., and C.Q. Doe. 2007. Curr. Biol. 17:R465-R467; Gonzalez, C. 2007. Nat. Rev. Genet. 8:462-472). These findings provide new insights and suggest intriguing new models for how cells coordinate spindle orientation with their cellular microenvironment to regulate and direct cell fate decisions within tissues. [ABSTRACT FROM AUTHOR]

Crosses between Drosophila melanogaster females and Drosophila simulans males produce hybrid sons that die at the larval stage. This hybrid lethality is suppressed by loss-of-function mutations in the D. melanogaster Hybrid male rescue (Hmr) or in the D. simulans Lethal hybrid rescue (Lhr) genes. Previous studies have shown that Hmr and Lhr interact with heterochromatin proteins and suppress expression of transposable elements within D. melanogaster. It also has been proposed that Hmr and Lhr function at the centromere. We examined mitotic divisions in larval brains from Hmr and Lhr single mutants and Hmr; Lhr double mutants in D. melanogaster. In none of the mutants did we observe defects in metaphase chromosome alignment or hyperploid cells, which are hallmarks of centromere or kinetochore dysfunction. In addition, we found that Hmr-HA and Lhr-HA do not colocalize with centromeres either during interphase or mitotic division. However, all mutants displayed anaphase bridges and chromosome aberrations resulting from the breakage of these bridges, predominantly at the euchromatin–heterochromatin junction. The few dividing cells present in hybrid males showed fuzzy and irregularly condensed chromosomes with unresolved sister chromatids. Despite this defect in condensation, chromosomes in hybrids managed to align on the metaphase plate and undergo anaphase. We conclude that there is no evidence for a centromeric function of Hmr and Lhr within D. melanogaster nor for a centromere defect causing hybrid lethality. Instead, we find that Hmr and Lhr are required in D. melanogaster for detachment of sister chromatids during anaphase. [ABSTRACT FROM AUTHOR]