Genetic structure of olive ridley sea turtles (Lepidochelys olivacea) in Ghana, West Africa

Sea turtles migrate across ocean basins to reach foraging and breeding areas but still exhibit significant population structure across nesting beaches. Although five species of sea turtles nest in West Africa, the genetic structure of many species in this region has not been adequately explored. We investigated the diversity of mitochondrial and nuclear DNA for nesting olive ridley (Lepidochelys olivacea) turtles in two coastal communities (Mankoadze and Ada Foah) in Ghana that are approximately 160 km apart. Samples were collected during four nesting seasons: 2006, 2014, 2015 and 2016. Analysis of the control region of mtDNA (n = 45) revealed six variable positions defining five haplotypes, of which one was previously undescribed (Lo91). These findings are indicative of population expansion from a small ancestral population and support the scenario of colonization of the Atlantic via founder effect. Eleven microsatellite loci were used to analyze two years (2015 and 2016) from Mankoadze. We determined these nesting cohorts were not genetically distinct (F_ST = 0.025, P = 0.286) and can therefore be considered a single population. This population was compared to six females from Ada Foah across seven loci. Individuals from the two locations were less related than expected under a model of random chance, and showed signs of outbreeding, and reflected moderate variation between the two (F_IS = -0.160, P = 1.00; F_ST = 0.104, P = 0.002). The structuring between locations indicate nesting individuals may utilize different migration routes, foraging grounds, or breeding grounds and that Ghana may serve as a transitional area between central and west African rookeries.