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The nutrient-rich tubers of the greater yam, Dioscorea alata L., provide food and income security for millions of people around the world. Despite its global importance, however, greater yam remains an orphan crop. Here, we address this resource gap by presenting a highly contiguous chromosome-scale genome assembly of D. alata combined with a dense genetic map derived from African breeding populations. The genome sequence reveals an ancient allotetraploidization in the Dioscorea lineage, followed by extensive genome-wide reorganization. Using the genomic tools, we find quantitative trait loci for resistance to anthracnose, a damaging fungal pathogen of yam, and several tuber quality traits. Genomic analysis of breeding lines reveals both extensive inbreeding as well as regions of extensive heterozygosity that may represent interspecific introgression during domestication. These tools and insights will enable yam breeders to unlock the potential of this staple crop and take full advantage of its adaptability to varied environments.

Background: The genus Pulmonaria (Boraginaceae) represents a taxonomically complex group of species in which morphological similarity contrasts with striking karyological variation. The presence of different numbers of chromosomes in the diploid state suggests multiple hybridization/polyploidization events followed by chromosome rearrangements (dysploidy). Unfortunately, the phylogenetic relationships and evolution of the genome, have not yet been elucidated. Our study focused on the P. officinalis group, the most widespread species complex, which includes two morphologically similar species that differ in chromosome number, i.e. P. obscura (2n = 14) and P. officinalis (2n = 16). Ornamental cultivars, morphologically similar to P. officinalis (garden escapes), whose origin is unclear, were also studied. Here, we present a pilot study on genome size and repeatome dynamics of these closely related species in order to gain new information on their genome and chromosome structure. Results: Flow cytometry confirmed a significant difference in genome size between P. obscura and P. officinalis, corresponding to the number of chromosomes. Genome-wide repeatome analysis performed on genome skimming data showed that retrotransposons were the most abundant repeat type, with a higher proportion of Ty3/Gypsy elements, mainly represented by the Tekay lineage. Comparative analysis revealed no species-specific retrotransposons or striking differences in their copy number between the species. A new set of chromosome-specific cytogenetic markers, represented by satellite DNAs, showed that the chromosome structure in P. officinalis was more variable compared to that of P. obscura. Comparative karyotyping supported the hybrid origin of putative hybrids with 2n = 15 collected from a mixed population of both species and outlined the origin of ornamental garden escapes, presumably derived from the P. officinalis complex. Conclusions: Large-scale genome size analysis and repeatome characterization of the two morphologically similar species of the P. officinalis group improved our knowledge of the genome dynamics and differences in the karyotype structure. A new set of chromosome-specific cytogenetic landmarks was identified and used to reveal the origin of putative hybrids and ornamental cultivars morphologically similar to P. officinalis. [ABSTRACT FROM AUTHOR]

The majority of cultivated bananas originated from inter- and intra(sub)specific crosses between two wild diploid species, Musa acuminata and Musa balbisiana. Hybridization and polyploidization events during the evolution of bananas led to the formation of clonally propagated cultivars characterized by a high level of genome heterozygosity and reduced fertility. The combination of low fertility in edible clones and differences in the chromosome structure among M. acuminata subspecies greatly hampers the breeding of improved banana cultivars. Using comparative oligo-painting, we investigated large chromosomal rearrangements in a set of wild M. acuminata subspecies and cultivars that originated from natural and human-made crosses. Additionally, we analyzed the chromosome structure of F1 progeny that resulted from crosses between Mchare bananas and the wild M. acuminata 'Calcutta 4' genotype. Analysis of chromosome structure within M. acuminata revealed the presence of a large number of chromosomal rearrangements showing a correlation with banana speciation. Chromosome painting of F1 hybrids was complemented by Illumina resequencing to identify the contribution of parental subgenomes to the diploid hybrid clones. The balanced presence of both parental genomes was revealed in all F1 hybrids, with the exception of one clone, which contained only Mchare-specific SNPs and thus most probably originated from an unreduced diploid gamete of Mchare. [ABSTRACT FROM AUTHOR]

Apomictic genera have a complex evolutionary history, including reticulate hybridization, polyploidization and variation in reproduction modes. Restrictions of functional meiosis in polyploid apomictic taxa considerably hamper gene conversion, leaving footprints of past hybridization events in their genomes, which are masked by the homogenization of tandemly organized arrays of nuclear ribosomal DNA in sexual species. Dandelions (Taraxacum) have a highly complex reticulate evolutionary history. Detailed knowledge remains hazy; earlier investigations of rDNA sequences have uncovered the complex pattern derived from evolution but without direct evidence for any particular processes. We investigated the position and number of 45S and 5S rDNA loci in 38 Taraxacum taxa (covering different reproduction modes, geographical regions and putative phylogenetic groups) using fluorescent in situ hybridization (FISH) and measured genome size and GC content. The ITS1-5.8S-ITS2 regions of four sexual and five apomictic taxa were sequenced to investigate inter- and intra-individual variation. Most species considerably differ in the chromosome positions of loci and karyotype patterns, but mostly share the same number of studied loci (45S:5S) in a 1:2 ratio per haploid genome (x = 8), with six exceptions (up to a 4:2 ratio). Genome size (2C) varies sixfold and with GC content partly distinguishes major evolutionary groups. Sexual taxa show limited variation in sequenced regions (with two to eight variants), but apomictic taxa vary significantly (with 20–36 variants). Extensive reticulate evolution in Taraxacum and subsequent phenomena such as genome repatterning and non-effective concerted evolution are probably the cause of the dynamic nature of Taraxacum karyotypes and the large variation in genome size and rDNA sequences. [ABSTRACT FROM AUTHOR]

Abstract Background The classification of the Musaceae (banana) family species and their phylogenetic inter-relationships remain controversial, in part due to limited nucleotide information to complement the morphological and physiological characters. In this work the evolutionary relationships within the Musaceae family were studied using 13 species and DNA sequences obtained from a set of 19 unlinked nuclear genes. Results The 19 gene sequences represented a sample of ~16 kb of genome sequence (~73% intronic). The sequence data were also used to obtain estimates for the divergence times of the Musaceae genera and Musa sections. Nucleotide variation within the sample confirmed the close relationship of Australimusa and Callimusa sections and showed that Eumusa and Rhodochlamys sections are not reciprocally monophyletic, which supports the previous claims for the merger between the two latter sections. Divergence time analysis supported the previous dating of the Musaceae crown age to the Cretaceous/Tertiary boundary (~ 69 Mya), and the evolution of Musa to ~50 Mya. The first estimates for the divergence times of the four Musa sections were also obtained. Conclusions The gene sequence-based phylogeny presented here provides a substantial insight into the course of speciation within the Musaceae. An understanding of the main phylogenetic relationships between banana species will help to fine-tune the taxonomy of Musaceae.

Abstract Background The centromeric and pericentromeric regions of plant chromosomes are colonized by Ty3/gypsy retrotransposons, which, on the basis of their reverse transcriptase sequences, form the chromovirus CRM clade. Despite their potential importance for centromere evolution and function, they have remained poorly characterized. In this work, we aimed to carry out a comprehensive survey of CRM clade elements with an emphasis on their diversity, structure, chromosomal distribution and transcriptional activity. Results We have surveyed a set of 190 CRM elements belonging to 81 different retrotransposon families, derived from 33 host species and falling into 12 plant families. The sequences at the C-terminus of their integrases were unexpectedly heterogeneous, despite the understanding that they are responsible for targeting to the centromere. This variation allowed the division of the CRM clade into the three groups A, B and C, and the members of each differed considerably with respect to their chromosomal distribution. The differences in chromosomal distribution coincided with variation in the integrase C-terminus sequences possessing a putative targeting domain (PTD). A majority of the group A elements possess the CR motif and are concentrated in the centromeric region, while members of group C have the type II chromodomain and are dispersed throughout the genome. Although representatives of the group B lack a PTD of any type, they appeared to be localized preferentially in the centromeres of tested species. All tested elements were found to be transcriptionally active. Conclusions Comprehensive analysis of the CRM clade elements showed that genuinely centromeric retrotransposons represent only a fraction of the CRM clade (group A). These centromeric retrotransposons represent an active component of centromeres of a wide range of angiosperm species, implying that they play an important role in plant centromere evolution. In addition, their transcriptional activity is consistent with the notion that the transcription of centromeric retrotransposons has a role in normal centromere function.