INFERENCES ABOUT THE ORIGIN OF A FIELD CRICKET HYBRID ZONE FROM A MITOCHONDRIAL-DNA PHYLOGENY

Two closely related eastern North American field crickets, Gryllus fir mus and G. pennsylvanicus, hybridize along a zone that extends from Co nnecticut and the Hudson River Valley, south along the eastern front o f the Appalachian Mountains to at least Virginia. Here we use mitochon drial DNA (mtDNA) sequences to construct a population phylogeny for th is pair of hybridizing cricket species. Using a phylogenetic approach, we attempt to discriminate between alternative population histories ( secondary contact vs. primary intergradation) leading to formation of the hybrid zone. A strict consensus tree, based on > 1600 bp of the CO I-COII region of the mtDNA genome, reveals four exclusive groups, whic h correspond to regional groupings of conspecific crickets. Surprising ly, the mtDNA sequence data do not reveal any synapomorphies for eithe r G. pennsylvanicus or G. firmus. However, the mtDNA data do reveal a clear north-south split within each of the cricket species, a pattern not seen for morphological or other molecular characters. The biogeogr aphical history of the north-south divergence events remains a puzzle. Observed gene genealogies support a model of secondary contact for th e southern part of the hybrid zone. Sequence divergence data argue tha t lineages currently found in New York and New England were already di stinct when this region became habitable following the most recent gla ciation.