Mitochondrial DNA Variation in Field Crickets across a Narrow Hybrid Zone’

We have examined mitochondrial DNA (mtDNA) restriction-site variation within and between populations of fall-adult field crickets across a narrow hybrid zone in Connecticut. Our analysis reveals two distinct mtDNA lineages, each consisting of a single common genotype together with one or more rare genotypes. Outside the hybrid zone, the association between mtDNA genotype and species designation is nearly perfect. The A lineage corresponds to Gryllus pennsylvanicus and the B lineage to G. firmus. However, two G. firmus individuals from a coastal Connecticut population carry the A mtDNA genotype. Within the hybrid zone, the association between morphological characters, enzyme genotypes, and mtDNA genotypes breaks down to varying degrees. We compare patterns of introgression for these three types of markers. Previous authors have suggested that the behavior of mtDNA in hybrid zones may be different from that of nuclear-gene markers. We argue that differences in behavior are probably quantitative, not qualitative, and depend on the genetic architecture of speciation. We also document patterns of mtDNA length variation and associated heteroplasmy in both the A and B mtDNA lineages. The existence of identical size classes in both lineages suggests that the mutation rate for generating length variants is high. Therefore, genome size may not be a reliable marker of common ancestry.