Inferences about the origin of a field cricket hybrid zone from a mitochondrial DNA phylogeny

Two closely related eastern North American field crickets, Gryllus firmus and G. pennsylvanicus, hybridize along a zone that extends from Connecticut and the Hudson River Valley, south along the eastern front of the Appalachian Mountains to at least Virginia. Here we use mitochondrial DNA (mtDNA) sequences to construct a population phylogeny for this 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 COI-COII region of the mtDNA genome, reveals four exclusive groups, which correspond to regional grouping of conspecific crickets. Surprisingly, the mtDNA sequence data do not reveal any synapomorphies for either 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 biogeographical history of the north-south divergence events remains a puzzle. Observed gene genealogies support a model of secondary contact for the southern part of the hybrid zone. Sequence divergence data argue that lineages currently found in New York and New England were already distinct when this region became habitable following the most recent glaciation.