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 between genetic and phenotypical similarity. There are four known chromosomal races of Australian Sitobion miscanthi living on grasses. Three races evolved from a recent common ancestor by mutation and chromosomal rearrangement alone. Australian S. miscanthi reproduce entirely by parthenogenesis, as does the close congeneric S. near fragariae. Mean relative growth rate (MRGR) was investigated in laboratory-raised representative aphid clones of four 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 significant differences in MRGR; most variance was associated with differences among forms, some among clones within forms and very little with aphids within clones. Developmental time and adult weight both contributed to the differences in MRGR. There was a significant interaction for clone(nested within forms) x host for all three dependent variables. No one clone performed significantly better over all hosts than other clones of its form (clonal MRGRs on the three hosts were negatively correlated). Nearly all clones performed 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 monophyletic parthenogenetic aphids can rapidly evolve substantial differences in host relations and suggest a possible association of chromosomal rearrangements with MRGR.