Transposable Element Evolution in Stramenopiles and Choanoflagellates

Transposable elements are parasitic mobile elements that can transpose within host genomes and may have a large impact on the host evolution. Despite this, very few studies have looked into how transposable elements themselves have evolved within unicellular organisms, which make up the vast majority of organisms in existence. In this study, evidence is presented for selection for transposable element codon usage in both the closest relatives to Metaoza –choanoflagellates, and stramenopiles. Both choanoflagellates studied show evidence for selective accuracy whilst neither of the stramenopiles do. Selection appears to be stronger within the choanoflagellates than the stramenopiles. Phaeodactylum tricorntum transposable elements are likely to be evolving under mutation pressure rather than selection pressure but there is evidence to suggest Thalassiosira pseudonana transposable element’s codon usage is evolving via selection. The most likely explanation for these findings is that the choanoflagellates have a larger effective population size than the two stramenopiles in this study.