Postzygotic isolating factor in sympatric speciation in Rhagoletis flies: Reduced response of hybrids to parental host-fruit odors

doi:10.1073/pnas.0408255101

Postzygotic isolating factor in sympatric speciation in Rhagoletis flies: Reduced response of hybrids to parental host-fruit odors

^*Department of Entomology, Barton Laboratory, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456; ^†Department of Biological Sciences, P.O. Box 369, Galvin Life Science Center, University of Notre Dame, Notre Dame, IN 46556-0369; and ^‡Department of Entomology, University of Illinois, 320 Morrill Hall, 505 South Goodwin Avenue, Urbana, IL 61801

Contributed by Wendell L. Roelofs, November 5, 2004

Abstract

Rhagoletis pomonella is a model for sympatric speciation (divergence without geographic isolation) by means of host-plant shifts. Many Rhagoletis species are known to use fruit odor as a key olfactory cue to distinguish among their respective host plants. Because Rhagoletis rendezvous on or near the unabscised fruit of their hosts to mate, behavioral preferences for fruit odor translate directly into premating reproductive isolation among flies. Here, we report that reciprocal F₁ hybrids between the apple and hawthorn host races of R. pomonella, as well as between the host races and an undescribed sibling species infesting Cornus florida (flowering dogwood) do not respond to host fruit volatiles in wind-tunnel assays at doses that elicit maximal directed flight in parental flies. The reduced ability of hybrids to orient to fruit volatiles could result from a conflict between neural pathways for preference and avoidance behaviors, and it suggests that hybrids might suffer a fitness disadvantage for finding fruit in nature. Therefore, host-specific mating may play a dual role as an important postzygotic as well as a premating reproductive barrier to isolate sympatric Rhagoletis flies.

Footnotes

↵ § To whom correspondence should be addressed. E-mail: wlr1{at}cornell.edu.
Author contributions: C.E.L.J., J.L.F., S.H.B., S.N., and W.L.R. designed research; C.E.L.J., H.R.D., J.L.F., S.H.B., and S.N. performed research; C.E.L.J., H.R.D., J.L.F., S.H.B., S.N., and W.L.R. analyzed data; and C.E.L.J., H.R.D., J.L.F., S.H.B., and W.L.R. wrote the paper.