Genetic divergence of insular marsh rice rats in subtropical Florida

Three island populations of the marsh rice rat (Oryzomys palustris) found in Florida have been described as taxonomically unique; the Pine Island rice rat (O. p. planirostris) and Sanibel Island rice rat (O. p. sanibeli), originally described as subspecies, and the endangered silver rice rat (O. argentatus), originally described as a separate species. We aimed to resolve the phylogenetic relationships among these island populations and investigate their genetic distinctness with DNA sequence data from the mitochondrial control region and cytochrome-b gene. Using parsimony, maximum-likelihood, and Bayesian analyses, we estimated the phylogenetic relationships and genetic distances among the 3 island populations and 4 other described subspecies. Our mitochondrial DNA analyses indicate that the silver rice rat and Sanibel Island rice rat are genetically divergent from mainland populations, but the Pine Island rice rat is not. The silver rice rat and Sanibel Island rice rat may be genetically isolated due to geographic separation resulting in reduced gene flow. They have the potential to become distinct phylogenetic groups and eventually species.