Speciation in Phytophagous Insects
Determining the role host-plant shifts play in sympatric speciation for phytophagous insects
This research examines two important questions in speciation theory: a) the relationship between ecological adaptation and reproductive isolation, and b) whether geographic isolation is a prerequisite for animal speciation. Work in this area is concentrated on the Rhagoletis pomonella (Diptera: Tephritidae) sibling species complex, a model for sympatric host race formation and speciation for phytophagous insects. We have found that traits related to the overwintering pupal diapause and host plant recognition are the principal barriers restricting gene flow among R. pomonella taxa.
Host Odor Discrimination
Research on host choice behavior is being conducted in collaboration with Dr. Wendell Roelofs and Dr. Charlie Linn (Cornell Agricultural Experiment Station at Geneva) and Dr. Stewart Berlocher (Univ. Illinois). We have focused our attention on fruit odor recognition as a key element in assortative mating among sympatric R. pomonella host races. Volatile compounds characteristic of apple, haw and dogwood fruit have been identified and apple, haw and dogwood flies have been shown to discriminate for their natal odor in flight tunnel assays.
A second ongoing area of Rhagoletis research involves field studies testing the synthetic odor blends of apple, hawthorn and dogwood fruits for their attractiveness to flies in nature. The combination of lab and field research will help us resolve how ecology influences the formation of new races and incipient species.
Another ongoing aspect of this project involves the neurophysiology behind the various olfactory preferences of the Rhagoletis host races. This is being investigated in collaboration with Dr. Shannon Olsson (National Centre for Biological Sciences, Bangalore, India).
Shannon Olsson, NCBS, Bangalore
Website: Shannon’s lab page : Naturalist Inspired Chemical Ecology
Another component of current Rhagoletis research is directed at understanding the genetic and physiological basis for diapause. Variation in diapause timing between R. pomonella host races, corresponding to differences in host plant phenology, leads to allochromic separation of mature flies. These studies emphasize integrating a genomics based approach, with phylogenetic, ecological and behavioral/developmental analyses to resolve the genetic and environmental components of diapause and its role in population divergence. We are currently collaborating with the Hahn lab at the University of Florida to investigate gene expression patterns during the breakage of diapause using pyrosequencing and microarray techniques.
Western Pomonella State of Washington
R. pomonella has been described from the Pacific Northwest, infesting a native hawthorn species, and introduced hawthorn, and introduced commercial apples. We are currently collaborating with Wee Yee from the USDA-ARS in Yakima, Washington and the Linn/Roelofs Lab at Cornell to investigate potential host race formation in this system. One particularly interesting component of this project is the reversal of apple and hawthorn relative phenology compared to the classic haw-apple shift in the Midwest.
Wee Yee, USDA, Yakima, Washington
Dietmar Schwarz, Western Washington University, College of Science and Engineering
Southern U.S. Populations of R. Pomonella
R. pomonella in the northeastern United States is known to infest only a single hawthorn species, Crataegus mollis. The shift into apples, therefore, occurred under relatively simple ecological conditions, with clear trade-offs between the two suitable host plants. Host plant usage for R. pomonella is much more complex in unglaciated North America, with at least five Crataegus species regularly infested. These hawthorn species differ from one another in the two primary traits on which host plant adaptation is based in the north: fruiting phenology and fruit volatile profiles. Our current investigation is aimed at understanding host plant adaptation and potential reproductive isolation among southern R. pomonella populations. We are also interested in how the phenotypic and genetic variance present in the south may have influenced the recent sympatric shift into apples.
Cascading Sympatric Speciation
The result of cascading or sequential speciation is a chain reaction of biodiversity where the modification of one species triggers the sequential modification of a second, dependent species. If sympatric speciation has played a significant role in phytophagous insect diversity, we expect that higher trophic levels, dependent on phytophagous specialists would be similarly affected.