GENETIC-STRUCTURE OF AN APHID STUDIED USING MICROSATELLITES - CYCLIC PARTHENOGENESIS, DIFFERENTIATED LINEAGES AND HOST SPECIALIZATION

In a previous study, samples of the grain aphid Sitobion avenae (F.) w ere collected from wheat and adjacent cocksfoot hosts in a population thought to be primarily parthenogenetic, and DNA from individual aphid s was analysed with a multilocus technique. Here we have applied singl e-locus microsatellites and a mitochondrial DNA marker to a subset of the same DNA extracts, and have made several additional inferences abo ut important genetic and population processes in S. avenae. Microsatel lite analysis indicated very high levels of genic and genotypic variat ion. S. avenue fell into three genotypic groups inferred to be almost noninterbreeding, while analysis of linkage and Hardy-Weinberg equilib ria suggested high levels of sexual recombination within each genotypi c group. Host specialization was evident: one lineage was found only o n wheat, and one (bearing many alleles inferred to be introgressed fro m the blackberry-grass aphid S. fragariae (Walker)) was found only on cocksfoot. The third group of interrelated genotypes was found commonl y on both hosts. Although most genotypes were found only once, some we re much more numerous in the sample than expected from the frequency o f the alleles they contained. This, and rapid temporal changes in geno typic composition of samples, indicates strong selective differences b etween genotypes and lineages. In the major genotypic group, the commo nest genotypes were significantly more homozygous than were rare ones: thus these data may help to explain the frequent observation of homoz ygous excess in aphid allozymes. The genotype group showing S. avenae- like as well as S. fragariae-like alleles also carried S. fragariae-li ke mitochondrial DNA in at least 25/31 cases, indicating gender-asymme trical hybridization.