The ecology and genetics of sympatric speciation in Rhagoletis pomonella.

摘要

Speciation in sexually reproducing animals is generally thought to require the geographic isolation of populations. But as early as 1864, Benjamin Walsh proposed that certain host plant specific phytophagous insects could speciate in the absence of geographic isolation (i.e., in sympatry) in the process of shifting and adapting to new host plants. In particular, Walsh cited the recent (150 year) shift of Rhagoletis pomonella from its ancestral host hawthorn (Crataegus mollis) to domesticated apple (Malus pumila) as an example of sympatric speciation in action. Subsequent studies have confirmed the taxonomic status of apple- and hawthorn-flies as reproductively isolated and genetically differentiated “host races”. However, critical questions remain concerning the nature of ecological factors separating the races.; The goal of my dissertation was to determine how ecological specialization has resulted in the divergence of apple and hawthorn host races of Rhagoletis pomonella. Previous studies have implied that the earlier fruiting phenology of apples differentially selects on life-history traits (especially diapause) between the fly races. To verify this, I conducted four experiments in which I varied rearing conditions and tested for a genetic response at allozyme loci displaying allele frequency differences between apple and haw flies.; My results suggested that: (1) Apple and hawthorn fruit necroses rapidly in the field, and that this selects for larvae of both host races to develop quickly, (2) Cooler temperatures experienced by haw flies in nature due to the later phenology of haw tree select for allozyme alleles more common to the hawthorn race. In Contrast, warmer temperatures, which mirror conditions that apple fly larvae experience, select for “apple race” alleles, (3) Variation in photoperiod influences diapause regulation in Rhagoletis, but has minimal genetic consequences, and (4) Light intensity does not appear to affect diapause.; In conclusion, my dissertation verified that environmental conditions associated with host plant phenology are critical to the genetic divergence of R. pomonella host races. Perhaps most importantly, differential temperature conditions experienced by fly larvae feeding within apples and hawthorns produces a fundamental genetic fitness tradeoff between the races. The resulting divergent selection is an important aspect of sympatric host race formation in R. pomonella that may have general consequences for insect diversity.