GLOBAL PHYLOGEOGRAPHY OF THE RIDLEY SEA-TURTLES (LEPIDOCHELYS SPP.) AS INFERRED FROM MITOCHONDRIAL-DNA SEQUENCES

The Kemp's ridley sea turtle (Lepidochelys kempi) is restricted to the warm temperate zone of the North Atlantic Ocean, whereas the olive ri dley turtle (L. olivacea) is globally distributed in warm-temperate an d tropical seas, including nesting colonies in the North Atlantic that nearly overlap the range oft. kempi. To explain this lopsided distrib ution, Pritchard (1969) proposed a scenario in which an ancestral taxo n was divided into Atlantic and Pacific forms (L, kempi and L. olivace a, respectively) by the Central American land bridge. According to thi s model, the olive ridley subsequently occupied the Pacific and Indian Oceans and recently colonized the Atlantic Ocean via southern Africa. To assess this biogeographic model, a 470 bp sequence of the mtDNA co ntrol region was compared among 89 ridley turtles, including the sole L, kempi nesting population and 7 nesting locations across the range o f L, olivacea. These data confirm a fundamental partition between L, o livacea and L. kempi (p=0.052-0.069), shallow separations within L. ol ivacea (p=0.002-0.031), and strong geographic partitioning of mtDNA li neages. The most divergent L. olivacea haplotype is observed in the In do-West Pacific region, as are the central haplotypes in a parsimony n etwork, implicating this region as the source of the most recent radia tion of olive ridley lineages. The most common olive ridley haplotype in Atlantic samples is distinguished from an Indo-West Pacific haploty pe by a single nucleotide substitution, and East Pacific samples are d istingushed from the same haplotype by two nucleotide substitutions. T hese shallow separations are consistent with the recent invasion of th e Atlantic postulated by Pritchard (1969), and indicate that the East Pacific nesting colonies were also recently colonized from the Indo-We st Pacific region. Molecular clock estimates place these invasions wit hin the last 300,000 years.