Evolutionary ecology of sea turtles
Summary PHD Thesis
The genetic diversity and structure of natural populations is the product of past and present ecological and evolutionary processes, which can be intrinsic, such as behavioural or ecological factors, or extrinsic, such as environmental variation and climate change. Molecular markers can be used to study the genetic diversity and structure of natural populations to gain a fundamental understanding of the extrinsic and intrinsic processes that underlie their evolutionary histories. These insights are critical to understanding the ecology and evolution of species, and can play signiticant roles in aiding conservation and management.
In the present doctoral thesis, traditional and next‑generation DNA sequencing approaches were employed to investigate the intrinsic and extrinsic processes that shaped the genetic diversity and structure of the green turtle, Chelonia mydas, and hawksbill turtle, Eretmochelys imbricata, in the Atlantic and Southwest Indian Ocean. The thesis was divided into two sections: an ecological section (Chapters 2 and 3) concerned with the intrinsic and extrinsic processes affecting dispersal and recruitment dynamics in sea turtles in the present and an evolutionary section (Chapters 4 and 5) that addressed questions regarding the intluence of past climate and environmental change on the evolution of sea turtles.
In Chapter 2, the effect of changes in population dynamics at rookeries on juvenile recruitment to feeding grounds was investigated by monitoring temporal changes in the genetic composition at a major juvenile green turtle feeding ground in the southern Caribbean using mitochondrial DNA control region sequences. Temporal changes in the frequencies of mitochondrial DNA haplotypes indicated recruitment from eastern Caribbean rookeries decreased between 2006 and 2016, whereas recruitment from northwestern Caribbean rookeries increased during this period. Changes in recruitment through time correlated with population recovery trends in the Caribbean; northwestern Caribbean rookeries showed the highest degree of recovery, whereas the lowest degree of recovery was observed in eastern Caribbean rookeries. These tindings suggested that changes in population dynamics at rookeries can affect recruitment of juveniles to feeding grounds and can intluence sea turtle meta‑population dynamics.
In Chapter 3, mitochondrial DNA control region sequences were used to investigate the intluence of ocean currents on juvenile dispersal in green turtles in the Southwest Indian Ocean. Recruitment from northern Mozambique Channel rookeries to a juvenile green turtle feeding ground located in the southern Mozambique Channel Evolutionary Ecology of Sea Turtles 175 was high, while recruitment from southern Mozambique Channel rookeries was low. The relatively high recruitment from northern rookeries to a juvenile feeding ground located in the southern Mozambique Channel supported a scenario where juvenile green turtle dispersal was mediated by southward tlowing ocean currents. These tindings suggested an important role for ocean currents in determining juvenile sea turtle dispersal patterns, though more long‑term studies are needed to further investigate the temporal stability of juvenile recruitment patterns in the face of the complex and variable oceanography of the Southwest Indian Ocean.
In Chapter 4, the intluence of past climate and environmental change on the population structure and phylogeography of green turtles was studied in the Atlantic and Southwest Indian Ocean. A large number of single nucleotide polymorphisms (SNPs) were obtained from green turtles sampled in the East Caribbean, the East Atlantic and the Southwest Indian Ocean using double‑digested restriction site associated DNA sequencing. Model‑based clustering supported the presence of three genetic clusters and revealed signatures of admixture in the East Caribbean and Southwest Indian Ocean. The last most recent common ancestor of Atlantic and Southwest Indian Ocean green turtles in the was dated to the last interglacial period (130 – 116 thousand years ago), which was a relatively warm period. The divergence of Southwest Indian Ocean and East Caribbean green turtles from the East Atlantic population was associated with the transition from the last interglacial period (130 – 116 thousand years ago) to the last glacial period (116 – 14 thousand years ago). The tindings of Chapter 4 suggested that ancestral Atlantic and Southwest Indian Ocean green turtles became isolated in three glacial refugia during the last glaciation, and subsequently expanded and admixed in the East Caribbean and Southwest Indian Ocean after the termination of the last glacial period approximately 14 thousand years ago.
In Chapter 5, the impact of past sea level tluctuations on the evolution of sea turtles was investigated in Caribbean hawksbill turtles using a moditied approach to double‑digested restriction site associated DNA sequencing to account for potential biases caused by PCR enrichment. Past tluctuations in genetic diversity were estimated from the folded site frequency spectrum and correlated with changes in shallow marine habitat area, i.e. habitat with a depth between 0 and 60 meters, during the last 125 thousand years. The tindings of Chapter 5 showed that shallow marine habitat area was severely reduced throughout the last glacial period. Furthermore, Summary 176 past changes in shallow marine habitat area correlated strongly with past changes in genetic diversity. Genetic diversity increased sharply after the end of the last glaciation, suggesting Caribbean hawksbill turtles rapidly expanded as global climate conditions warmed, continental ice sheets regressed and sea levels rose. The tindings of Chapter 5 demonstrated past sea level tluctuations had a strong impact on the past population dynamics of hawksbill turtles in the Caribbean, possibly through reduced feeding habitat availability during periods with lower sea levels.