The dynamic loss and gain of introns during the evolution of the Brassicaceae

Sequence comparison allows the detailed analysis of evolution at the nucleotide and amino acid levels, but much less information is known about the structural evolution of genes, i.e. how the number, length and distribution of introns change over time. We constructed a parsimonious model for the evolutionary rate of intron loss (IL) and intron gain (IG) within the Brassicaceae and found that IL/IG has been highly dynamic, with substantial differences between and even within lineages. The divergence of the Brassicaceae lineages I and II marked a dramatic change in the IL rate, with the common ancestor of lineage I losing introns three times more rapidly than the common ancestor of lineage II. Our data also indicate a subsequent declining trend in the rate of IL, although in Arabidopsis thaliana introns continue to be lost at approximately the ancestral rate. Variations in the rate of IL/IG within lineage II have been even more remarkable. Brassica rapa appears to have lost introns approximately 15 times more rapidly than the common ancestor of B. rapa and Schenkiella parvula, and approximately 25 times more rapidly than its sister species Eutrema salsugineum. Microhomology was detected at the splice sites of several dynamic introns suggesting that the non-homologous end-joining and double-strand break repair is a common pathway underlying IL/IG in these species. We also detected molecular signatures typical of mRNA-mediated IL, but only in B. rapa.