In theory, the loss of sexual reproduction is expected to result in the accumulation of
deleterious mutations. In aphids, two main types of life cycle, cyclic and obligate
parthenogenesis, represent respectively “sexual” and “asexual”
reproductive modes. We used the complete pea aphid genome and previously published
expressed sequence tags (ESTs) from two other aphid species. In addition, we obtained
100,000 new ESTs from five more species. The final set comprised four sexual and four
asexual aphid species and served to test the influence of the reproductive mode on the
evolutionary rates of genes. We reconstructed coding sequences from ESTs and annotated
these genes, discovering a novel peptide gene family that appears to be among the most
highly expressed transcripts from several aphid species. From 203 genes found to be 1:1
orthologs among the eight species considered, we established a species tree that partly
conflicted with taxonomy (for Myzus ascalonicus). We then used this
topology to evaluate the dynamics of evolutionary rates and mutation accumulation in the
four sexual and four asexual taxa. No significant increase of the nonsynonymous to
synonymous ratio or of nonsynonymous mutation numbers was found in any of the four
branches for asexual taxa. We however found a significant increase of the synonymous rate
in the branch leading to the asexual species Rhopalosiphum maidis, which
could be due to a change in the mutation rate or to an increased number of generations
implied by its change of life cycle
