MULTIPLE L1 PROGENITORS IN PROSIMIAN PRIMATES - PHYLOGENETIC EVIDENCEFROM ORF1 SEQUENCES

One of the uncertainties regarding the evolution of L1 elements is whe ther there are numerous progenitor genes. We present phylogenetic evid ence from ORF1 sequences of slow loris (Nycticebus coucang) and galago (Galago crassicaudatus) that there were at least two distinct progeni tors, active at the same time, in the ancestor of this family of prosi mian primates. A maximum parsimony analysis that included representati ve L1s from human, rabbit, and rodents, along with the prosimian seque nces, revealed that one of the galago L1s (Gc11) grouped very strongly with the slow loris sequences. The remaining galago elements formed t heir own unique and strongly supported clade. An analysis of replaceme nt and silent site changes for each link of the most parsimonious tree indicated that during the descent of the Gc 11 sequence approximately two times more synonymous than nonsynonymous substitutions had occurr ed, implying that the Gc 11 founder was functional for some time after the split of galago and slow loris. Strong purifying selection was al so evident on the galago branch of the tree. These data indicate that there were two distinct and contemporaneous L1 progenitors in the lori soid ancestor, evolving under purifying selection, that were retained as functional L1s in the galago lineage (and presumably also in the sl ow loris). The prosimian ORF1 sequences could be further subdivided in to subfamilies. ORF1 sequences from both the galago and slow loris hav e a premature termination codon near the 3' end, not shared by the oth er mammalian sequences, that shortens the open reading frame by 288 bp . An analysis of synonymous and nonsynonymous substitutions for the 5' and 3' portions, that included intra- and inter-subfamily comparisons , as well as comparisons among the other mammalian sequences, suggeste d that this premature stop codon is a prosimian acquisition that has r endered the 3' portion of ORF1 in these primates noncoding.