A Novel Interpretation Of Speciation, Hybridization, And Genetic Population Structure Of The Stone Crabs Menippe Mercenaria (Say, 1817-1818) And M. Adina Williams & Felder, 1986 (Decapoda: Brachyura: Menippidae)

Mitochondrial DNA cytochrome oxidase (COI) and microsatellite DNA loci were used to better explain the speciation events, history of hybridization, and genetic population structure of the stone crabs Menippe adina Williams & Felder, 1986 and M. mercenaria (Say, 1817-1818). A survey of a 670-bp COI sequence of 711 stone crabs collected from Texas to North Carolina and from Belize, Central America showed the presence of two well-defined mtDNA lineages, Lin-A and Lin-B. Based on molecular clock calibrations of a published study these lineages shared a common ancestor between 1.5 to 1.8 million years. Lin-A was evident throughout the range of the two Menippe species, but Lin-B was found only in the eastern Gulf of Mexico and does not represent any of the two species. Our findings refute the claim that the eastern Gulf hybrid zone is the product of hybridization between M. adina and M. mercenaria. We propose that the hybrid zone is instead a relic of introgressive hybridization between the M. mercenaria of eastern Gulf stone crab nDNA Lin-AA that carries Lin-A and the new nDNA Lin-BB that carries LinB, forming the hybrid nDNA Lin-AB. Lin-B emerged from a founder effect in the Okefenokee Trough and introgressed into the eastern Gulf. There is no Lin-B in the Atlantic, hence no hybridization in that region. Using stone crab-specific microsatellite markers we found no evidence of an ongoing hybridization in the eastern Gulf and this region cannot be called a hybrid zone since hybridization here ceased probably soon after the last glacial episode. The microsatellitemarker analyses also suggested that the stone crab was geographically divided into four genetic clusters by three barriers to gene flow, the strongest of which is the one between the two species, M. adina and M. mercenaria, around Cape San Blas, Apalachicola Bay, Florida.