Multiple and independent rearrangements revealed by comparative cytogenetic mapping in the dysploid Leptostachyus group (Phaseolus L., Leguminosae)

Comparative analysis of repetitive DNA in dysploid and non-dysploid Phaseolus beans

Structural karyotype changes result from ectopic recombination events frequently associated with repetitive DNA. Although most Phaseolus species present relatively stable karyotypes with 2n = 22 chromosomes, the karyotypes of species of the Leptostachyus group show high rates of structural rearrangements, including a nested chromosome fusion that led to the dysploid chromosome number of the group (2n = 20). We examined the roles of repetitive landscapes in the rearrangements of species of the Leptostachyus group using genome-skimming data to characterize the repeatome in a range of Phaseolus species and compared them to species of that group (P. leptostachyus and P. macvaughii). LTR retrotransposons, especially the Ty3/gypsy lineage Chromovirus, were the most abundant elements in the genomes. Differences in the abundance of Tekay, Retand, and SIRE elements between P. macvaughii and P. leptostachyus were reflected in their total amounts of Ty3/gypsy and Ty1/copia. The satellite DNA fraction was the most divergent among the species, varying both in abundance and distribution, even between P. leptostachyus and P. macvaughii. The rapid turnover of repeats in the Leptostachyus group may be associated with the several rearrangements observed.

High rates of structural rearrangements have shaped the chromosome evolution in dysploid Phaseolus beans

KEY MESSAGE

Karyotypes evolve through numerical and structural chromosome rearrangements. We show that Phaseolus leptostachyus, a wild bean, underwent a rapid genome reshuffling associated with the reduction from 11 to 10 chromosome pairs, but without whole genome duplication, the highest chromosome evolution rate known for plants. Plant karyotypes evolve through structural rearrangements often associated with polyploidy or dysploidy. The genus Phaseolus comprises ~ 90 species, five of them domesticated due to their nutritional relevance. Most of the species have 2n = 22 karyotypes and are highly syntenic, except for three dysploid karyotypes of species from the Leptostachyus group (2n = 20) that have accumulated several rearrangements. Here, we investigated the degrees of structural rearrangements among Leptostachyus and other Phaseolus groups by estimating their chromosomal evolution rates (CER). For this, we combined our oligo-FISH barcode system for beans and chromosome-specific painting probes for chromosomes 2 and 3, with rDNA and a centromeric probe to establish chromosome orthologies and identify structural rearrangements across nine Phaseolus species. We also integrated the detected rearrangements with a phylogenomic approach to estimate the CERs for each Phaseolus lineage. Our data allowed us to identify translocations, inversions, duplications and deletions, mostly in species belonging to the Leptostachyus group. Phaseolus leptostachyus showed the highest CER (12.31 rearrangements/My), a tenfold increase in contrast to the 2n = 22 species analysed. This is the highest rate known yet for plants, making it a model species for investigating the mechanisms behind rapid genome reshuffling in early species diversification.

Comparative cytogenomics reveals genome reshuffling and centromere repositioning in the legume tribe Phaseoleae

The tribe Phaseoleae includes several legume crops with assembled genomes. Comparative genomic studies have evidenced the preservation of large genomic blocks among legumes, although chromosome dynamics during Phaseoleae evolution has not been investigated. We conducted a comparative genomic analysis to define an informative genomic block (GB) system and to reconstruct the ancestral Phaseoleae karyotype (APK). We identified GBs based on the orthologous genes between Phaseolus vulgaris and Vigna unguiculata and searched for GBs in different genomes of the Phaseolinae (P. lunatus) and Glycininae (Amphicarpaea edgeworthii) subtribes and Spatholobus suberectus (sister to Phaseolinae and Glycininae), using Medicago truncatula as the outgroup. We also used oligo-FISH probes of two P. vulgaris chromosomes to paint the orthologous chromosomes of two non-sequenced Phaseolinae species. We inferred the APK as having n = 11 and 19 GBs (A to S), hypothesizing five chromosome fusions that reduced the ancestral legume karyotype to n = 11. We identified the rearrangements among the APK and the subtribes and species, with extensive centromere repositioning in Phaseolus. We also reconstructed the chromosome number reduction in S. suberectus. The development of the GB system and the proposed APK provide useful approaches for future comparative genomic analyses of legume species.

Mixed-Ploidy and Dysploidy in Hypericum perforatum: A Karyomorphological and Genome Size Study

Karyomorphology and genome size of 15 St John's wort (Hypericum perforatum L.) populations are reported for the first time. Root tips and fresh young leaves were used for karyological studies and flow cytometric (FCM) measurements, respectively. The chromosome length varied from 0.81 µm to 1.16 µm, and chromosome types were determined as "m". Eight different somatic chromosome numbers were found (2n = 16, 22, 24, 26, 28, 30, 32, 38). Based on the observed basic (x) chromosome numbers of x = 8, 11, 13, 14, 15, 19, this may correspond to diploid (2x), triploid (3x), tetraploid (4x), respectively. Interestingly, we found mixoploidy (3x - 4x) in the root tips of one of the populations. Hybridization, polyploidy and dysploid variation may be the main factors associated with the chromosome number evolution of this species. FCM showed that 2C DNA contents vary from 0.87 to 2.02 pg, showing more than a 2-fold variation. The mean amount of 2C DNA/chromosome and the mean of monoploid genome size were not proportional to ploidy.

Molecular and Cytogenetic Analysis of rDNA Evolution in Crepis Sensu Lato

Although Crepis was the first model plant group in which chromosomal changes were considered to play an important role in speciation, their chromosome structure and evolution have been barely investigated using molecular cytogenetic methods. The aim of the study was to provide a better understanding of the patterns and directions of Crepis chromosome evolution, using comparative analyses of rDNA loci number and localisation. The chromosome base number and chromosomal organisation of 5S and 35S rDNA loci were analysed in the phylogenetic background for 39 species of Crepis, which represent the evolutionary lineages of Crepis sensu stricto and Lagoseris, including Lapsana communis. The phylogenetic relationships among all the species were inferred from nrITS and newly obtained 5S rDNA NTS sequences. Despite high variations in rDNA loci chromosomal organisation, most species had a chromosome with both rDNA loci within the same (usually short) chromosomal arm. The comparative analyses revealed several independent rDNA loci number gains and loci repositioning that accompanied diversification and speciation in Crepis. Some of the changes in rDNA loci patterns were reconstructed for the same evolutionary lineages as descending dysploidy.

Oligo-FISH barcode in beans: a new chromosome identification system

An Oligo-FISH barcode system was developed for two model legumes, allowing the identification of all cowpea and common bean chromosomes in a single FISH experiment, and revealing new chromosome rearrangements. The FISH barcode system emerges as an effective tool to understand the chromosome evolution of economically important legumes and their related species. Current status on plant cytogenetic and cytogenomic research has allowed the selection and design of oligo-specific probes to individually identify each chromosome of the karyotype in a target species. Here, we developed the first chromosome identification system for legumes based on oligo-FISH barcode probes. We selected conserved genomic regions between Vigna unguiculata (Vu, cowpea) and Phaseolus vulgaris (Pv, common bean) (diverged ~ 9.7-15 Mya), using cowpea as a reference, to produce a unique barcode pattern for each species. We combined our oligo-FISH barcode pattern with a set of previously developed FISH probes based on BACs and ribosomal DNA sequences. In addition, we integrated our FISH maps with genome sequence data. Based on this integrated analysis, we confirmed two translocation events (involving chromosomes 1, 5, and 8; and chromosomes 2 and 3) between both species. The application of the oligo-based probes allowed us to demonstrate the participation of chromosome 5 in the translocation complex for the first time. Additionally, we detailed a pericentric inversion on chromosome 4 and identified a new paracentric inversion on chromosome 10. We also detected centromere repositioning associated with chromosomes 2, 3, 5, 7, and 9, confirming previous results for chromosomes 2 and 3. This first barcode system for legumes can be applied for karyotyping other Phaseolinae species, especially non-model, orphan crop species lacking genomic assemblies and cytogenetic maps, expanding our understanding of the chromosome evolution and genome organization of this economically important legume group.