MICROSATELLITE AND CHROMOSOME EVOLUTION OF PARTHENOGENETIC SITOBION APHIDS IN AUSTRALIA

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.