Multiple routes to asexuality in an aphid species

Cyclical parthenogens, including aphids, are important models for studying the evolution of sex. However, little is known about transitions to asexual ity in aphids, although the mode of origin of asexual lineages has importan t consequences for their level of genetic diversity, ecological adaptabilit y and the outcome of competition with their sexual relatives. Thus, we surv eyed nuclear, mitochondrial and biological data obtained on cyclical and ob ligate parthenogens of the bird cherry-oat aphid, Rhopalosiphum padi (L.), to investigate the frequency of transitions from sexuality to permanent ase xuality. Many instances of asexual lineages retaining the ability to produc e males are known in aphids, so particular attention was paid to the existe nce of occasional matings between females from sexual lineages and males pr oduced by asexual lineages, which have the potential to produce new asexual lineages. Phylogenetic inference based an microsatellite and mitochondrial data indicates at least three independent origins of asexuality in R. padi , yielding the strongest evidence to date for multiple origins of asexualit y in an aphid. Moreover, several lines of evidence demonstrate that transit ions to asexuality result from two mechanisms: a complete spontaneous loss of sex and repeated gene flow from essentially asexual lineages into sexual ones.