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 premise that ecological specialization on different host plants can indirectly result in significant reproductive isolation. A candidate example of host plant shift is provided by the partially reproductively-isolated aphids Aphis fabae fabae and A. f. mordwilkoi , which utilize different hosts for sexual reproduction. However, unexpected isolation based on mating behaviour was found for these two aphids. In olfactometer trials, males responded to the pheromones of females of both subspecies when presented a subspecies in isolation but preferentially to females of the same subspecies in choice tests. On contact, the incidence or duration of between-subspecies copulations was significantly lower than same-subspecies copulations, and spermatozoa were transferred during 88% of the same-subspecies copulations, but only 19% of between-subspecies copulations. These data are indicative of strong pre-zygotic isolation between the subspecies. Complementary restriction analysis of mitochondrial DNA and a plasmid (pAFE/eu) in an obligate symbiotic bacterium (Buchnera) revealed a paraphyletic relationship among four subspecies of A. fabae (A. f. fabae, A. f mordwilkoi, A. f cirsiiacanthoidis and A. f. solanella). We discuss how the lack of clear genetic differentiation between A. f. fabae and A. f. mordwilkoi , despite substantial pre-zygotic isolation, may be a consequence of reinforcement.