Population Structure, Gene Flow, and Historical Demography of a Large Coastal Shark, the Bull Shark (Carcharhinus leucas), in the Northwestern Atlantic
Abstract
Bull Shark, Carcharhinus leucas, reproductive patterns are believed to be characterized by female philopatry towards their natal site, while males migrate longer distances. This behavior linked to sex-biased dispersal can result in differing patterns of population structure between markers that differ in the mode of inheritance. This study tested the hypothesis that Bull Shark exhibit male-biased dispersal by characterizing and comparing the maternally inherited mitochondrial DNA Control Region (mtDNA CR) to single nucleotide polymorphism (SNP) data, from the biparentally inherited nuclear DNA genome. Population structure and historical demography were assessed using representative samples throughout the entire Gulf of Mexico (GoM) and Atlantic coastline of the US (NWA), with reference sequences from the Caribbean, eastern Pacific, and western Pacific.
Here we report significant population differentiation within the mtDNA CR between the Gulf of Mexico (GoM) and Atlantic coast (NWA) of the US (FCT, p < 0.05), though no structuring was observed within either region. The Florida Keys was identified as containing significantly higher levels of haplotype diversity than both the GoM and NWA, indicating it may act as a mixing zone between the two regions. Interestingly, the Caribbean samples from the San Juan River in Nicaragua diverged as a distinct clade, suggesting Nicaragua may have a genetically unique freshwater population, or may be mixing with the southern Atlantic populations.
Historical demography estimates were calculated for both the GoM and NWA. Both Tajima’s D and R2 neutrality statistics suggest a constant population size, and lack support for recent expansion. The estimate of female effective population size was much lower in the GoM (105k) than the NWA (194.5k), though now surprising given the extremely low levels of haplotype and nucleotide diversity observed within the GoM.
STRUCTURE analyses of SNP data using 18,174 variant sites identified discrete populations between the southern GoM (Campeche, Mexico) and the rest of the GoM. Surprisingly, 50% of the samples from North Carolina were assigned to the same cluster at the southern GoM samples. Given the discrepancy in observed population structuring within the GoM between mtDNA and SNP data, we concluded the observed differences in gene flow between the southern GoM and NWA were the result of male-biased dispersal. The structuring of male dispersal between the southern GoM and NWA (North Carolina) has not yet been described in other shark species. The distinct subdivision of each sex creates uniquely restricted gene flow that should be considered and implemented in future conservation efforts to best maintain viability of the Bull Shark.
Citation
Atkins, Erica Lane (2020). Population Structure, Gene Flow, and Historical Demography of a Large Coastal Shark, the Bull Shark (Carcharhinus leucas), in the Northwestern Atlantic. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /193013.