Population genetics and phylogeography of sea turtles

Abstract

The seven species of sea turtles occupy a diversity of niches, and have a history tracing back over 100 million years, yet all share basic life-history features, including exceptional navigation skills and periodic migrations from feeding to breeding habitats. Here, we review the biogeographic, behavioural, and ecological factors that shape the distribution of genetic diversity in sea turtles. Natal homing, wherein turtles return to their region of origin for mating and nesting, has been demonstrated with mtDNA sequences. These maternally inherited markers show strong population structure among nesting colonies while nuclear loci reveal a contrasting pattern of male-mediated gene flow, a phenomenon termed 'complex population structure'. Mixed-stock analyses indicate that multiple nesting colonies can contribute to feeding aggregates, such that exploitation of turtles in these habitats can reduce breeding populations across the region. The mtDNA data also demonstrate migrations across entire ocean basins, some of the longest movements of marine vertebrates. Multiple paternity occurs at reported rates of 0-100%, and can vary by as much as 9-100% within species. Hybridization in almost every combination among members of the Cheloniidae has been documented but the frequency and ultimate ramifications of hybridization are not clear. The global phylogeography of sea turtles reveals a gradient based on habitat preference and thermal regime. The cold-tolerant leatherback turtle (Dermochelys coriacea) shows no evolutionary partitions between Indo-Pacific and Atlantic populations, while the tropical green (Chelonia mydas), hawksbill (Eretmochelys imbricata), and ridleys (Lepidochelys olivacea vs. L. kempi) have ancient separations between oceans. Ridleys and loggerhead (Caretta caretta) also show more recent colonization between ocean basins, probably mediated by warm-water gyres that occasionally traverse the frigid upwelling zone in southern Africa. These rare events may be sufficient to prevent allopatric speciation under contemporary geographic and climatic conditions. Genetic studies have advanced our understanding of marine turtle biology and evolution, but significant gaps persist and provide challenges for the next generation of sea turtle geneticists.

Genetic fingerprinting reveals natal origins of male leatherback turtles encountered in the Atlantic Ocean and Mediterranean Sea

“…One powerful genetic tool, assignment testing, may be used to determine the source nesting populations of individual animals sampled in foraging or developmental habitats . Female sea turtles have previously been shown to have high levels of site fidelity to a beach or region (Bowen and Karl 2007;Wallace et al 2010;Stewart and Dutton 2014;Bannister et al 2016), and distinct genetic signatures of populations (or management units, MUs) reaffirm these findings (e.g., Dutton et al 2013;Jensen et al 2013;Roden et al 2013;Shamblin et al 2014). Foraging site fidelity is also observed in many sea turtle species, and most foraging areas are composed of mixed stocks (i.e., they are a mixture of individuals from various nesting populations) (James et al 2005;Velez-Zuazo et al 2008Schofield et al 2010.…”

Population structure and conservation implications for the loggerhead sea turtle of the Cape Verde Islands

“…Similarly, the nesting population from northeast Florida to North Carolina was found to be genetically homogeneous over about 1,000 km (Bowen and Karl 2007). In contrast, loggerhead nesting populations in south and northeast Florida separated on a scale of 50-100 km are distinct in their mtDNA haplotype frequencies, showing that females are capable of homing on a relatively fine scale (Bowen and Karl 2007). Similar results were obtained in the Japanese islands, where loggerhead nesting colonies separated on a scale of about 400 km were found to be genetically distinct (Hatase et al 2002).…”

“…The second explanation centers on the possibility of a stricter nest site fidelity of turtles from Sal. Such fine scale homing behaviour of sea turtles has been identified in several populations of the species elsewhere in the world (Bowen and Karl 2007). Stricter homing behaviour on Sal than on the other islands could be due to unknown natural or anthropogenic environmental conditions.…”

“…In our study, the two most distant islands are separated by more than two hundred kilometres. Similarly, the nesting population from northeast Florida to North Carolina was found to be genetically homogeneous over about 1,000 km (Bowen and Karl 2007). In contrast, loggerhead nesting populations in south and northeast Florida separated on a scale of 50-100 km are distinct in their mtDNA haplotype frequencies, showing that females are capable of homing on a relatively fine scale (Bowen and Karl 2007).…”

Geo-political genetics: Claiming the commons through species mapping

“…5 Although there is evidence of male-mediated gene flow among these different management units (Roberts et al 2004), the argument for the dominance of mtDNA data in defining management units is that natal homing exhibited by adult females implies that a rookery will not be re-established if all females are extirpated (Bowen and Karl 2007).…”