Evolution of oviposition strategies and speciation in the globeflower flies Chiastocheta spp. (Anthomyiidae)

Trollius europaeus (Ranunculaceae) is involved in an intimate interaction w ith several species of Chiastocheta flies (Anthamyiidae) that are both seed predators and pollinators. In this paper, we analyse the oviposition strat egy of the six Chiastocheta species found to coexist on T. europaeus in 19 populations from the French Alps. We shaw that the species are not equivale nt in their oviposition behaviour:, C. rotundiventris usually deposits no m ore:than one egg per flower in first-day flowers whereas C. dentifera aggre gates ifs eggs on fruits and thus does not contribute to pollination at all ; the four remaining species deposit eggs sequentially during the flowering period from the 2nd to the :7th day. Hence, the outcomes of the interactio n in terms of net Seed production for the plant greatly depend on the Chias tocheta species visiting it,ranging from a mutualistic to a purely-parasiti c interaction. We assessed mitochondrial divergence between Chiastocheta sp p: by sequencing a 1320-bp mitochondrial DNA fragment. The low divergence o bserved between species (0-4-15%) suggests that genus diversification took place recently. Unlike in other plant-insect systems where diversification is usually thought to be driven by cospeciation or host shifts we propose t hat Chiastocheta speciation took place within the host plant. Basal separat ion of a particularly mutualistic species provided favourable conditions fa r plant specialization on this seed-parasite as: a pollinator early in the evolution of the association. The parasitic species ovipositing on fruits d erived from a species ovipositing on flowers. Diversification of the interm ediate strategies probably occurred in relation with the Pleistocene climat ic events, reproductive isolation between species being reinforced by niche partitioning for oviposition and/or sexual selection.