Variation in clonal diversity in glasshouse infestations of the aphid, Aphis gossypii Glover in southern France

Aphis gossypii is an aphid species that is found throughout the world and i s extremely polyphagous. It is considered a major pest of cotton and cucurb it species. In Europe, A. gossypii is assumed to reproduce exclusively by a pomictic parthenogenesis. The present study investigates the genetic divers ity of A. gossypii in a microgeographic, fragmented habitat consisting of e ight glasshouses of cucurbit crops. This analysis, which was based on the r esults from seven microsatellite loci, has confirmed that A. gossypii popul ations in southern France are primarily asexual, as only 12 nonrecombinant genotypic classes (clones) were identified from 694 aphids. Moreover, a hig h proportion of the aphids (87%) had one of three common genotypes. No sign ificant correlation was found between genotypic class and host plant specie s. Within a glasshouse population of A. gossypii, a significant reduction i n clonal diversity was observed as the spring/summer season progressed. The final predominance of a clone could result from interclonal competition. A t the microgeographic level (i.e. glasshouses within a 500-m radius), signi ficant genetic subdivision was detected and could be attributed to founder effects and the limitation of gene flow imposed by the enclosed nature of t he glasshouse structure. Finally, the three common clones of A. gossypii de tected in 1996 reappeared in spring 1997 following the winter extinction, t ogether with rare clones that had not previously been seen. The probability that A. gossypii overwinters within refuges at a microgeographic scale fro m which populations are renewed each spring is discussed.