Mitochondrial DNA Assessment of the Phylogeography of the Gopher Tortoise
Identifying geographic barriers that define genetic structure within a species is crucial in formulating an effective conservation plan. The identification of appropriate management units is critical for the protection and recovery of the gopher tortoise Gopherus polyphemus, which have declined across their entire range. Previous molecular work at various spatial scales has identified distinct population assemblages of the gopher tortoise. The goal of this study was to assess the genetic structure in gopher tortoises through a more complete sampling of the federally listed as threatened portion of the range and evaluate the extent of genetic isolation imposed by several potential geographic barriers. We sequenced a 712-base-pair portion of a mitochondrial gene (NADH dehydrogenase 4) for 322 individuals from 42 sites across the range. We found two major assemblages of haplotypes separated by a modest phylogenetic break (average uncorrected p distance = 0.015). The biogeographic barrier that best explained the geographic partitioning of genetic variation was the Apalachicola-Chattahoochee rivers and not the one used to delimit the federally listed as threatened portion of the range (Tombigbee-Mobile). However, the presence of distinct (group 1 and 2) haplotypes on either side of Apalachicola-Chattahoochee rivers indicates that the two lineages experienced historical isolation and divergence, after which they came back into contact. If one were to define genetic units of conservation for gopher tortoises, then the Apalachicola-Chattahoochee rivers delineation would be the most appropriate based on the analysis of molecular variance of the mitochondrial sequence data. However, a model that combines the Apalachicola-Chattahoochee and Tombigbee-Mobile rivers as geographic breaks was the second-best model in this analysis, which suggests that the federally listed as threatened portion of the range also contains important geographic structure. Thus, we recommend that making management decisions on the basis of mitochondrial data alone is premature, and that prior to any status review additional work that examines finer scale patterns of genetic structure by using microsatellite loci is required.