On the processes shaping reproductive isolation in aphids of the Aphis fabae (Scop.) complex (Aphididae : Homoptera)

Models of sympatric speciation for phytophagous insects are based on the pr emise that ecological specialization on different host plants can indirectl y result in significant reproductive isolation. A candidate example of host plant shift is provided by the partially reproductively-isolated aphids Ap his fabae fabae and A. f. mordwilkoi, which utilize different hosts for sex ual reproduction. However, unexpected isolation based on mating behaviour w as found for these two aphids. In olfactometer trials, males responded to t he pheromones of females of both subspecies when presented a subspecies in isolation but preferentially to females of the same subspecies in choice te sts. On contact, the incidence or duration of between-subspecies copulation s was significantly lower than same-subspecies copulations, and spermatozoa were transferred during 88% of the same-subspecies copulations, but only 1 9% of between-subspecies copulations. These data are indicative of strong p re-zygotic isolation between the subspecies. Complementary restriction anal ysis of mitochondrial DNA and a plasmid (pAFEleu) in an obligate symbiotic bacterium (Buchnera) revealed a paraphyletic relationship among four subspe cies of A. fabae (A. f. fabae, A. f. mordwilkoi, A. f. cirsiiacanthoidis an d A. f. solanella). We discuss how the lack of clear genetic differentiatio n between A. f. fabae and A. f. mordwilkoi, despite substantial pre-zygotic isolation, may be a consequence of reinforcement. (C) 2001 The Linnean Soc iety of London.