Proximate and ultimate factors influencing oviposition site selection by endoparasites on conifer seed cones: two sympatric dipteran species on larch

The taxonomically controversial Galerucella nymphaeae species complex (Coleoptera, Chrysomelidae) contains at least two distinct species in northern Europe, G. nymphaeae (L.) consuming Nuphar luteum, and G. sagittariae (Gyll.), living mainly on Comarum palustre in southern Finland and on Rubus chamaemorus in northern Finland. By analyzing chromosomal polymorphisms in sympatric populations of G. nymphaeae and G. sagittariae, it was shown that the two species do not hybridize in nature. Chorion polypeptide analysis revealed fixed genetic differences between the two species. However, differences between species must be small, since hybridization is easily induced in the laboratory and F1 hybrids are fertile. Larvae of G. sagittariae cannot survive on Nuphar, and those of G. nymphaeae survive very poorly on Comarum and Rubus. Adults of G. sagittariae from Comarum or Rubus do not choose between these two food plants in laboratory conditions regarding dwelling, egg laying or feeding, but their avoidance of Nuphar is complete. Galerucella nymphaeae clearly prefers Nuphar, but may occasionally also sit, lay eggs or feed on terrestrial plants. It is suggested that G. nymphaeae and G. sagittariae represent sibling species and that the two forms of G. sagittariae, one feeding on Comarum and the other one on Rubus, are habitat races adapted to semiaquatic and terrestrial habitats, respectively. Adaptation to different habitats has created an effective premating barrier to gene flow, supporting the possibility that a sympatric mode of divergence has been involved in habitat race formation and speciation in the species complex.