Ecological speciation in a multi-trophic context: Goldenrods, gall midges, and parasitoids

Background/Question/Methods There is a growing realization of the importance of ecological interactions in the origin and maintenance of species diversity. Studying the ecology of actively diversifying lineages such as Asteromyia carbonifera, a species complex of gall-making midges (Cecidomyiidae), promises to reveal how ecological interactions shape the likelihood and pattern of diversification. A. carbonifera form four distinctive blister-like galls types (flat, cushion, crescent, and irregular) which have been observed to coexist on a single plant species (Solidago altissima). Previous DNA sequencing and AFLP markers provides evidence of genetic isolation between these gall morphologies. Our research focuses around the primary questions: What ecological forces, if any, are responsible for the divergence of A. carbonifera gall morphs and how is the diversity of gall morphs maintained? We examined the following hypotheses to explain the observed pattern of diversification: 1. Gall morphs are specialized on plant genotypes or ploidy races. 2. Phenological differences in morphs reduce gene flow and/or competition among gall morphs. 3. Parasitoid pressure may drive divergent evolution of gall morphology and facilitate coexistence Ten 1m2 plots were surveyed monthly (June-September) at three sites to establish if distribution of gall morphs is significantly clumped on certain plant genotypes. I monitored six 50m transects weekly (June-September) to follow galls from initiation through adult emergence to determine if temporal differences in emergence could result in genetic and ecological isolation. Monthly collections of 100 individuals of each gall type were reared to eclosion of either midge or parasitoid to estimate parasitism rates among these different gall morphs.

Results/Conclusions

We found no significant evidence of phenological isolation of gall morphs. A significant non-random distribution was observed among the S. altissima host-plant genets; however, multiple gall morphs have been observed on a single leaf. This non-random distribution could be caused by preference of particular host-plant genotypes by A. carbonifera or by low dispersal of females after emergence from her natal gall. At least five species of parasitoids were found to parasitize Asteromyia galls. Preliminary analyses of plot data indicate that gall morphotypes exhibit significant differences in overall parasitism; however, these results are not consistent among sites. In addition, certain parasitoid species are significantly associated with particular gall morphs. In summary, it appears that host-plant and enemy-related ecological factors may contribute to the divergence and ecological coexistence of A. carbonifera; however, further research needs to be done to understand the mechanisms of this divergence.