P. Sunnucks et al., GENETIC-STRUCTURE OF AN APHID STUDIED USING MICROSATELLITES - CYCLIC PARTHENOGENESIS, DIFFERENTIATED LINEAGES AND HOST SPECIALIZATION, Molecular ecology, 6(11), 1997, pp. 1059-1073
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.