A comparison of the mating system and evolutionary dynamics of lemon sharks in the western Atlantic
DNA Punch from a Lemon Shark
Despite the fundamental importance of sharks as predators in marine ecosystems, there are very few data regarding mating systems, demographics, or population structure of any elasmobranch species. Further, there is a complete lack of knowledge regarding the evolutionary dynamics of large marine vertebrates in the wild. Therefore, I am taking part in a collaborative research effort (McGill University; Natural History Field Museum, Chicago; Bimini Biological Field Station, Bahamas) that combines long-term field (mark-recapture and morphological information) and laboratory studies (DNA samples) to examine the mating system and evolutionary dynamics of the lemon shark, Negaprion brevirostris, at two tropical nursery lagoons in the western Atlantic. At our primary study site, Bimini, Bahamas, I will examine genetic data over 13 breeding seasons based on studies beginning in 1995. I will also consider nine years of data at an additional site, Marquesas Key, Florida.
With microsatelitte markers already developed for the lemon shark, I can infer parent/offspring and sibling relationships of future sampled individuals as well as reconstruct genetic information from any unsampled parents. I can then use pedigree data to estimate quantitative genetic parameters in each population. I am particularly interested in explaining the observed morphological and early life-history trait differences (e.g., body size at age, growth rate) between juvenile sharks at Marquesas and Bimini. Thus, I hope to address the following research questions in my thesis: 1) How do lemon shark mating systems vary between populations (i.e. mating strategies, birthing cycle, number of reproducing females, philopatry to breeding grounds) and does this reflect adaptive differentiation? 2) Can life history trait differences between these populations be explained by differential selection at each nursery site? This issue will be explored by estimating the strength of selection, heritability, and breeding values for a number of traits using an "animal model" approach 3) What are the neutral and quantitative genetic impacts of on-going coastal development at Bimini? 4) Is local adaptation to each nursery site evident at the molecular level as well? This will be assessed by developing MHC markers for use in the lemon shark, which are highly variable genes with an immune system function, and conducting population-level sequence analyses. The role of sexual selection in maintaining MHC diversity will also be considered.
Redpath Museum, Department of Biology
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Mcgill University, Montreal, Canada