Genome Size Evolution Mediated by Gypsy Retrotransposons in Brassicaceae

The dynamic activity of transposable elements (TEs) contributes to the vast diversity of genome size and architecture among plants. Here, we examined the genomic distribution and transposition activity of long terminal repeat retrotransposons (LTR-RTs) in Arabidopsis thaliana (Ath) and three of its relatives, Arabidopsis lyrata (Aly), Eutrema salsugineum (Esa), and Schrenkiella parvula (Spa), in Brassicaceae. Our analyses revealed the distinct evolutionary dynamics of Gypsy retrotransposons, which reflects the different patterns of genome size changes of the four species over the past million years. The rate of Gypsy transposition in Aly is approximately five times more rapid than that of Ath and Esa, suggesting an expanding Aly genome. Gypsy insertions in Esa are strictly confined to pericentromeric heterochromatin and associated with dramatic centromere expansion. In contrast, Gypsy insertions in Spa have been largely suppressed over the last million years, likely as a result of a combination of an inherent molecular mechanism of preferential DNA removal and purifying selection at Gypsy elements. Additionally, species-specific clades of Gypsy elements shaped the distinct genome architectures of Aly and Esa.