Evol Ecol Res 9: 829-841 (2007)     Full PDF if your library subscribes.

When ecological isolation breaks down: sexual isolation is an incomplete barrier to hybridization between Rhagoletis species

Dietmar Schwarz* and Bruce A. McPheron

Department of Entomology, The Pennsylvania State University, 501 ASI, University Park, PA 16802, USA

Author to whom all correspondence should be addressed.
e-mail: dxs332@psu.edu


Question: Environmental disturbance can disrupt habitat choice as an ecological barrier to hybridization between host-specific parasites that mate on their host. Is environment-independent mate choice a sufficient barrier to prevent hybridization when ecological isolation breaks down?

Hypothesis: Males and females will not discriminate between conspecific and heterospecific mating partners in the absence of host cues.

Study system: Rhagoletis mendax and R. zephyria (Diptera: Tephritidae). Hybridization between these two taxa resulted in the Lonicera fly, an example of hybrid speciation in animals. The Lonicera fly is found only on non-native honeysuckle. Rhagoletis mendax and R. zephyria discriminate against each other’s host but not honeysuckle. This suggests the local breakdown of reproductive isolation via host choice following the introduction of an invasive plant.

Methods: We combined males and females of both species in a multi-choice experiment in the laboratory and recorded mating events.

Conclusion: Without host plant cues, mate choice is an incomplete barrier to hybridization. Reproductive isolation between host-specific parasites can be influenced by environmental disturbance because a non-ecological barrier (mate choice) alone is too weak to maintain reproductive isolation.

Keywords: hybridization, invasives, mating behaviour, reproductive barrier, sexual isolation, speciation.

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