Single-locus microsatellite variation correlated perfectly with chromo
some number in Sitobion miscanthi aphids. The microsatellites were hig
hly heterozygous, with up to 10 alleles per locus in this species. Des
pite this considerable allelic variation, only seven different S. misc
anthi genotypes were discovered in 555 individuals collected from a wi
de range of locations, hosts and sampling periods. Relatedness between
genotypes suggests only two successful colonizations of Australia. Th
ere was no evidence for genetic recombination in 555 S. miscanthi so t
he occurrence of recent sexual reproduction must be near zero. Thus di
versification is by mutation and chromosomal rearrangement alone. Sinc
e the aphids showed no sexual recombination, microsatellites can mutat
e without meiosis. Five of seven microsatellite differences were a sin
gle repeat unit, and one larger jump is likely. The minimum numbers of
changes between karyotypes corresponded roughly one-to-one with micro
satellite allele changes, which suggests very rapid chromosomal evolut
ion. A chromosomal fission occurred in a cultured line, and a previous
ly unknown chromosomal race was detected. All 121 diverse S. near frag
ariae were heterozygous but revealed only one genotype. This species t
oo must have a low rate of sexual reproduction and few colonizations o
f Australia.