Evolution of an ecological trait in parthenogenetic Sitobion aphids

The aim of this study was to test whether host plant responses of Sitobion aphids have evolved under parthenogenesis and to examine the relationship b etween genetic and phenotypical similarity. There are four known chromosoma l races of Australian Sitobion miscanthi living on grasses. Three races evo lved from a recent common ancestor by mutation and chromosomal rearrangemen t alone. Australian S. miscanthi reproduce entirely by parthenogenesis, as does the close congeneric S. near fragariae. Mean relative growth rate (MRG R) was investigated in laboratory-raised representative aphid clones of fou r races of S, miscanthi, and also S. near fragariae (i.e. five aphid 'forms ') on three host plants, with 15 replicate aphids per clone. There were sig nificant differences in MRGR; most variance was associated with differences among forms, some among clones within forms and very little with aphids wi thin clones. Developmental time and adult weight both contributed to the di fferences in MRGR. There was a significant interaction for clone(nested wit hin forms)x host for all three dependent variables. No one clone performed significantly better over all hosts than other clones of its form (clonal M RGRs on the three hosts were negatively correlated). Nearly all clones perf ormed best on barley (which was the only 'familiar' host, in that previous generations had been raised on it), next best on cocksfoot and worst on rye . MRGR was found to be under genetic control. The data show that monophylet ic parthenogenetic aphids can rapidly evolve substantial differences in hos t relations and suggest a possible association of chromosomal rearrangement s with MRGR.