Clade E, or the clade, is one of the major Brassicaceae (Crucifereae) clades, comprising some 48 genera and 351 species classified into seven tribes and is distributed predominantly across arid and montane regions of Asia. Several taxa have socioeconomic significance, being important ornamental but also weedy and invasive species. From the comparative genomic perspective, the clade is noteworthy as it harbors species with the largest crucifer genomes but low numbers of chromosomes ( = 5-7). By applying comparative cytogenetic analysis and whole-chloroplast phylogenetics, we constructed, to our knowledge, the first partial and complete cytogenetic maps for selected representatives of clade E tribes and investigated their relationships in a family-wide context. The clade is a well-supported monophyletic lineage comprising seven tribes: Anchonieae, Buniadeae, Chorisporeae, Dontostemoneae, Euclidieae, Hesperideae, and Shehbazieae. The clade diverged from other Brassicaceae crown-group clades during the Oligocene, followed by subsequent Miocene tribal diversifications in central/southwestern Asia. The inferred ancestral karyotype of clade E (CEK; = 7) originated from an older = 8 genome, which also was the purported progenitor of tribe Arabideae (KAA genome). In most taxa of clade E, the seven linkage groups of CEK either remained conserved (Chorisporeae) or were reshuffled by chromosomal translocations (Euclidieae). In 50% of Anchonieae and Hesperideae species, the CEK genome has undergone descending dysploidy toward = 6 (-5). These genomic data elucidate early genome evolution in Brassicaceae and pave the way for future whole-genome sequencing and assembly efforts in this as yet genomically neglected group of crucifer plants.
Background and Aims Most crucifer species (Brassicaceae) have small nuclear genomes (mean 1C-value 617 Mb). The species with the largest genomes occur within the monophyletic Hesperis clade (Mandáková et al., Plant Physiology174: 2062–2071; also known as Clade E or Lineage III). Whereas most chromosome numbers in the clade are 6 or 7, monoploid genome sizes vary 16-fold (256–4264 Mb). To get an insight into genome size evolution in the Hesperis clade (~350 species in ~48 genera), we aimed to identify, quantify and localize in situ the repeats from which these genomes are built. We analysed nuclear repeatomes in seven species, covering the phylogenetic and genome size breadth of the clade, by low-pass whole-genome sequencing. Methods Genome size was estimated by flow cytometry. Genomic DNA was sequenced on an Illumina sequencer and DNA repeats were identified and quantified using RepeatExplorer; the most abundant repeats were localized on chromosomes by fluorescence in situ hybridization. To evaluate the feasibility of bacterial artificial chromosome (BAC)-based comparative chromosome painting in Hesperis-clade species, BACs of arabidopsis were used as painting probes. Key Results Most biennial and perennial species of the Hesperis clade possess unusually large nuclear genomes due to the proliferation of long terminal repeat retrotransposons. The prevalent genome expansion was rarely, but repeatedly, counteracted by purging of transposable elements in ephemeral and annual species. Conclusions The most common ancestor of the Hesperis clade has experienced genome upsizing due to transposable element amplification. Further genome size increases, dominating diversification of all Hesperis-clade tribes, contrast with the overall stability of chromosome numbers. In some subclades and species genome downsizing occurred, presumably as an adaptive transition to an annual life cycle. The amplification versus purging of transposable elements and tandem repeats impacted the chromosomal architecture of the Hesperis-clade species.
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