Evolution of sea urchin retroviral-like (SURL) elements: Evidence from 40 echinoid species

We conducted a phylogenetic survey of sea urchin retroviral-like (SURL) ret rotransposable elements in 33 species of the class Echinoidea (sea urchins, sand dollars, and heart urchins). A 263-bp fragment from the coding region of the reverse transcriptase (RT) gene was amplified, cloned, and sequence d. Phylogenetic relationships of the elements isolated from independent clo nes, along with those from seven additional echinoid species obtained earli er by Springer et al.,, were compared with host phylogeny: Vertical transmi ssion and the presence of paralogous sequences that diverged prior to host speciation:can explain most of the phylogenetic relationships among SURL el ements. Rates of evolution were estimated from cases in which SURL and host phylogenies were concordant. In agreement with conclusions reached previou sly be Springer et al., average rates of synonymous substitution were compa rable with those of single-copy sea urchin DNA. High ratios of synonymous t o nonsynonymous substitution suggest that the RT of the elements is under s trong purifying selection. However, a high proportion (similar to 15%) of e lements with deleterious frameshifts and stop codons and an increase of the ratio of synonymous to nonsynonymous substitutions with divergence time sh ow that in the short term this selection is relaxed. Despite, the predomina nce of vertical transmission, sequence similarity of 83%-94% for SURL eleme nts from hosts that have been separated for 240 Myr suggests four cases of apparent horizontal transfer between the ancestors of the extant echinoid s pecies. In three additional cases, elements with identical RT sequences wer e found in sea urchin species separated for a minimum of 3 Myr. Thus, horiz ontal transfer plays a role in the evolution of this retrotransposon family .