DIFFERING LEVELS OF AMONG-POPULATION DIVERGENCE IN THE MITOCHONDRIAL DNA OF PERIODICAL CICADAS RELATED TO HISTORICAL BIOGEOGRAPHY.

Mitochondrial DNA (mtDNA) haplotypes were determined for 118 individuals of 13-and 17-year periodical cicadas (genus Magicicada) collected from 16 localities throughout the Midwest and eastern United States. Two distinct mtDNA lineages, identified as A and B, differ by 2.5% based on analysis of fragment patterns and restriction maps. Observed levels of mtDNA diversity within each lineage are low compared to estimates for other taxa. The two lineages are regionally segregated, with the boundary line occurring at a latitude of approximately 33° North. The levels of mtDNA diversity and population genetic structure differ within the two lineages. There is a remarkably low level of mean mtDNA divergence and no genetic structure in lineage A, whereas lineage B exhibits an order of magnitude higher level of mtDNA diversity and significant genetic structure among sampled populations. The low level of mtDNA diversity in cicadas may be attributed to (1) a population bottleneck that most likely occurred during the Pleistocene, (2) recent colonization following the retreat of the glaciers and the expansion of deciduous forests, and/or (3) high among-family reproductive variance (as a consequence of large population size, high fecundity, aggregative behavior of adults, and clumping of eggs). The difference in mtDNA diversity and population genetic structure between the lineages suggests that they experienced different biogeographic histories; we relate this to Pleistocene changes.