CHARACTERIZATION AND MOLECULAR ANALYSIS OF ADH RETROSEQUENCES IN SPECIES OF THE DROSOPHILA-OBSCURA GROUP

Retrosequences, genes, and pseudogenes originated by retrotranscriptio n are frequent components of vertebrate genomes, but they have only oc casionally been described in invertebrates. In Drosophila, very few re trosequences have been reported, among them those of alcohol dehydroge nase (Adh) and phosphoglyceromutase (Pglym). Although 52 Adh gene sequ ences are available for comparison, Adh retrosequences have been descr ibed only in the sibling species D. teissieri and D. yakuba (melanogas ter subgroup) and in D. subobscura (obscura subgroup). Here, we report the presence of Adh retrosequences in two closely related species of D. subobscura: D. madeirensis and D. guanche. Extensive sequence compa risons with their functional paralogs suggest separate retrotranscript ional events: one in the melanogaster subgroup in the ancestor of D. t eissieri and D. yakuba, and the other in the obscura subgroup before t he radiation of the lineages leading to D. subobscura, D. madeirensis, and D. guanche. In the former, the Adh retrotranscript originated a n ew expressed gene, named jingwei. However, in the obscura Adh retroseq uences, retention of codon bias and higher K-s than K-a values, both d istinctive evolutionary features supporting functionality, have to be considered together with a frameshift, premature stop codons, and othe r nucleotide substitutions, which, added to the lack of the original p romoter elements, suggest that they are pseudogenes. At least two diff erent Adh retrosequences have been characterized in each of the obscur a species, and their phylogenetic analysis indicates that paralogs and their flanking genomic regions share a higher degree of similarity th an orthologous sequences. Two alternative hypotheses could explain thi s current organization and structure: either a multiplication event oc curred independently in each species, or gene conversion events should be invoked after a single duplication in the species ancestor. The si gnificance of retrotranscriptional events in the evolution of inverteb rate genomes is discussed.