Sequential radiation of the parasitoid wasp Diachasma alloeum.

The various modes by which new animal species are generated are still in many ways contentious amongst evolutionary biologists. Some tout the primacy of geographic isolation in driving divergence, while others insist that sympatric speciation can and does occur with appreciable frequency. The latter view has gained traction in recent decades, due to an ever-enlarging pool of examples demonstrating the feasibility of sympatric divergence. However, there remains a bias towards an assumption of an allopatric contribution to any speciation event. Perhaps this bias is justified, but perhaps it is a consequence of the type of organisms that speciation studies typically focus on: the free-living, habitat and/or diet generalist, vertebrate. But these organisms do not represent the majority of animal life. The millions upon millions of small specialist organisms (including most parasites), for whom highly developed interactions with their environment are crucial to survival and reproduction, may move along evolutionary trajectories influenced more by ecology than physical isolation. The goal of my dissertation was to test a hypothesis of sequential radiation, wherein the speciation of a phytophagous insect drives parallel divergence in one of its parasitoids. I used the wasp Diachasma alloeum (Hymenoptera: Braconidae) as a study organism. Its host, Rhagoletis pomonella (Diptera: Tephritidae) is one of the preeminent examples of incipient sympatric speciation via host shifting; flies shifted from their ancestral hawthorn host into introduced European apples ~160 years ago. Diachasma alloeum wasps were reared from apple and hawthorn races of R. pomonella, as well as from the blueberry maggot, R. mendax, and snowberry maggot, R. zephyria. Wasps were scored for 21 polymorphic microsatellite loci and a 603 bp segment of the mitochondrial COI gene. Wasps were also assayed for host odor discrimination behavior using a y-tube olfactometer. Finally, host-related differences in wasp eclosion times were also investigated as a potential mechanism of allochronic isolation. Three main findings emerged from the dissertation research: 1) Genetic distances based on microsatellite frequencies revealed host-associated differences among apple, hawthorn, and blueberry fly-origin D. alloeum populations, independent of geographic origin. Furthermore, patterns of differentiation were different from locus to locus; some loci showed pronounced differences between host-associated populations, others showed a marked isolation by distance effect, irrespective of host type. 2) Apple, hawthorn and blueberry-origin D. alloeum showed preferences for the odor of their natal fruit and were antagonized by non-natal odors. 3) Apple, blueberry and hawthorn-origin D. alloeum differed in their average time to eclosion, and eclosion timing was correlated with the frequencies of certain microsatellite alleles. Taken together, the results from this study support sequential radiation for the R. pomonella / D. alloeum system. For small specialist animals, diversity may generate further diversity.