On Evolution

9. Conservation Genetics and Sea Turtles

Marine turtles are wonderful subjects for genetic analysis because molecular markers can uncover many aspects of their behaviors and evolutionary histories that are otherwise difficult or impossible to address due to the animals’ oceanic habits and long life spans. All living species of sea turtle are endangered or threatened, so these charismatic reptiles are also of special conservation concern. In , Avise was asked to deliver the Wilhemine E. Key Invitational Lecture of the American Genetic Association to kick off a special symposium devoted to conservation genetics in the sea. For several years, students and collaborators in his laboratory (Brian Bowen, Steve Karl, and others) had been studying sea turtles using various classes of molecular marker. So, the Key Lecture gave the author an opportunity to review genetic findings on sea turtles, and to place these discoveries in the broader context of conservation efforts in the world’s oceans. This chapter presents relevant excerpts from that review. If not for the fact that about  of the earth’s surface is covered by oceans, the long-term prospects might be even dimmer for the biosphere’s eventual recovery from global environmental crises precipitated by human overexploitation. The oceans have resisted permanent human settlement, and their vast size and composition provide some buffer against global environmental insults by man. Yet, even in the relatively untouched marine realm, human impacts on biodiversity have been profound. Populations of many of the world’s largest and most spectacular marine mammals, reptiles, birds, fishes, and invertebrates have been depleted severely or forced to extinction by human harvesting, by human activities that pollute or otherwise Conservation Genetics and Sea Turtles 9 degrade saltwater environments, or from the effects of human-mediated introductions of alien species. Conservation issues for marine organisms have attracted the attention of many geneticists. The explosion of interest in conservation genetics was made possible by the deployment in the past three decades of usable laboratory techniques for the direct assay of DNA and proteins. Prominent among these have been mitochondrial DNA (mtDNA) assays that permit a characterization of matrilineages within and among species, various nuclear assays (e.g., of allozymes or microsatellite DNAs) that yield genotypic descriptions for particular Mendelian loci, direct nucleotide-sequencing methods that in principle can be applied to any nuclear or cytoplasmic genes, and polymerase chain reaction (PCR) procedures that permit recovery of DNA from even tiny amounts of tissue. These molecular procedures have opened the entire biological world for genetic scrutiny. Molecular techniques permit genetic analyses that were unimaginable earlier in the century, when the primary access to information on particular genetic traits came either from captive pedigrees or (indirectly and insecurely ) from morphological and other organismal appraisals. Sea turtles represent many of the traits present in species requiring conservation efforts in the marine realm. During its lifetime a sea turtle experiences dramatic growth, covers vast geographic areas, and is extremely vulnerable to human impacts. A long-standing question in marine turtle research has been whether females, after a sexual maturation process measured in tens of years and oceanic movements often measured in thousands of kilometers, return to nest at or near their natal beaches. Decades of field observations and physical tagging experiments have failed to answer this question conclusively. However, several pioneering molecular surveys of mtDNA have recently shown that conspecific rookeries of green turtles (Chelonia mydas ), loggerheads (Caretta caretta), and hawksbills (Eretmochelys imbricate ) within an ocean basin commonly display large or nearly fixed differences in matriline frequencies, a result that strongly supports natal homing scenarios for adult females. Because females ultimately  On Evolution govern the reproductive output of a rookery, this natal-philopatric behavior signifies a considerable demographic autonomy of each turtle rookery with regard to its reproduction. Thus, natural recruitment from foreign rookeries is unlikely to compensate for mortality in heavily exploited rookeries or to reestablish (over ecological timescales ) rookeries that have been extirpated by human activities or other causes. Molecular genetic markers also have been used to decipher movement and association patterns of marine turtles at other stages of the life cycle. Marine turtles spend most of their lives on oceanic journeys or on feeding grounds that may be far removed (hundreds or even thousands of kilometers) from rookery sites. Several studies have employed rookery-characteristic mtDNA markers to assign individuals captured on feeding grounds or during migration to rookeries of origin . An emerging generality is...