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.; 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), Petra Hloušková and colleagues aimed to identify, quantify and localize in situ the repeats from which these genomes are built. They analysed nuclear repeatomes in seven species, covering the phylogenetic and genome size breadth of the clade, by low-pass whole-genome sequencing.
The authors found that 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.
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.