GENETIC DIVERSITY AND MODE OF REPRODUCTION IN FRENCH POPULATIONS OF THE APHID RHOPALOSIPHUM-PADI L

Holocyclic individuals of the aphid Rhopalosiphum padi reproduce by cy clical parthenogenesis, while anholocyclic individuals are obligate pa rthenogens. Although admired on the secondary hosts in summer, holocyc lic and anholocyclic populations occur separately on the primary and s econdary hosts during winter and spring. In this study, we compared th e genetic diversity and population structure of holocyclic and anholoc yclic populations collected in spring in the northern half of France. We also analysed the genetic composition of summer populations of R. p adi on its secondary hosts. In spring, holocyclic populations were in Hardy-Weinberg equilibrium at individual loci and had a relatively hig h genotypic diversity. Conversely, anholocyclic populations deviated f rom Hardy-Weinberg equilibrium and often consisted of a single clone. Moreover, these populations showed very low mean heterozygosities comp ared with holocyclic populations. Analysis of summer populations sugge sted that in regions with cold winters, summer populations were largel y recruited from holocyclic clones, whereas in areas with mild winters , summer populations were mainly derived from anholocyclic clones. The se results permit an assessment of the geographical distribution of th e two modes of reproduction on a large scale. The reasons for the dimi nished heterozygosity of anholocyclic populations are also discussed i n relation to the mechanisms which may induce transitions to asexualit y in aphids.