Evidence for predominant clones in a cyclically parthenogenetic organism provided by combined demographic and genetic analyses

Aphids are particularly interesting models in the study of genetic and demo graphic components of plant adaptation because of their breeding system whi ch combines parthenogenesis and sexual reproduction (i.e. cyclical partheno genesis), and the frequent emergence of host-adapted races reported in this group. In this paper, patterns of host adaptation were assessed on local p opulations of the aphid Sitobion avenae by following their demographic and genetic structure in a maize field for two consecutive years. The existence of putative generalist (polyphagous) or specialized (host-adapted) genotyp es was also investigated by comparing the genotypic distribution of this ap hid on maize and other cultivated host plants, using five microsatellite lo ci. Although population dynamics revealed strong variation in aphid abundan ce during the colonization period on maize, two genotypes identified at sev en additional microsatellite loci were predominant and exhibited stable fre quencies over cropping season and between years. Based on present and earli er studies, these two prevalent genotypes were shown to survive on differen t host plants other than maize, to colonize large geographical zones and to persist parthenogenetically for several years. All these data strongly sug gest that these two genotypes are asexual generalist clones that could have been favoured by agricultural practices encountered in western Europe. Bes ides these two clones, a continual replacement of rare genotypes was observ ed on maize in both years. Hypotheses involving selection via aphid-plant i nteractions and natural enemies were proposed for explaining the disappeara nce of these genotypes on maize.