A Narrow Hybrid Zone between Closely Related Cricket Species

Harrison, Richard; Arnold, Jonathan

SUMMARY: There is still considerable uncertainty about the distributional and evolutionary relationships of the closely related North American field crickets, Gryllus pennsylvanicus and G. firmus. Although the two species differ in morphology and electromorph frequency, no single character is diagnostic. Here we present data on morphological and allozymic variation in 51 collections of fall adult field crickets from the eastern United States. It is clear from these data that two forms of fall adult crickets occur in the northeastern United States. One of the two forms, found primarily in inland or upland localities, is G. pennsylvanicus. A coasta1llowland form is similar in all respects to G. firmus from the southeastern United States. We suggest that the range of G. firmus extends considerably further north and inland than has previously been reported. Our data reveal an extensive zone of interaction between G. pennsylvanicus and G. firmus in the eastern United States. This zone appears to follow the easternmost ridge of mountains (the Blue Ridge in Virginia, the Appalachians in Pennsylvania, New Jersey and New York). Observations of morphology and Est genotypes of individual crickets from mixed or intermediate populations suggest that little gene exchange occurs where the species are found together in Virginia. However, significant gene exchange may occur in areas of the northeast. Possible origins for the current distributions of G. pennsylvanicus and G. firmus are discussed.