One Host Shift Leads to Another: Formation of Host Races in a Gall- and Stem-Boring Beetle

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DEB-9806339 Warren G. Abrahamson & Micky Eubanks New species are known to arise when geographic barriers divide an existing species into two or more isolated populations. When mountains, oceans, or other barriers prevent reproduction between the isolated populations, each can evolve along an independent path. If geographic isolation lasts long enough, the populations may diverge so much that interbreeding is no longer possible -- one species divides into several. With parasites such as herbivorous insects, barriers to genetic exchange may arise not out of geography, but out of behavior. When mating occurs only on the host plant, a subpopulation consistently choosing one host will not mate with that choosing the other. Thus, speciation can occur in the absence of geographic separation when the two subpopulations are sympatric. Many have argued that the conditions for sympatric speciation are too restrictive for it to be important. Our extensive work on flies that induce galls on goldenrod plants and our preliminary work with a stem and gall-boring beetle indicate that host choice can be a barrier to genetic exchange. We will study a host shift and potential host-race formation by a common tumbling flower beetle associated with two species of goldenrod in the eastern United States. Our study system is unique in that a host shift and subsequent host-race formation by one herbivorous insect may have directly contributed to the host shift and host-race formation of a second, unrelated species. Given that nearly half of animal species are parasites, sympatric speciation may be an important source of the earth's biodiversity.