Population structure of a migratory small coastal shark, the blacknose shark Carcharhinus acronotus, across cryptic barriers to gene flow
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A recent study using mitochondria DNA and microsatellites identified genetic discontinuity between blacknose sharks (Carcharhinus acronotus) caught along the U.S. South Atlantic coast (Atlantic), the Gulf of Mexico (Gulf), and the Bahamas. The analytical resolution of these markers was unable to assign individuals from the Florida Keys to any regional population with any significance. Since this area is at the boundary of the identified Atlantic and Gulf of Mexico populations, higher resolution markers were necessary to understand patterns of gene flow in the area and gauge if current management policy is satisfactory for this species. Patterns of population structure in the blacknose shark were assessed using double digest Restriction Associated DNA methods and analyses were performed on 2,178 nuclear encoded single nucleotide polymorphism containing loci scored for 249 individual sharks sampled from the U.S. South Atlantic, eastern and western Gulf of Mexico. Results concurred with findings from the previous study that utilized microsatellites and mtDNA, supporting divergence between the Gulf and Atlantic and weak structure between the eastern and western Gulf. Individuals from the Florida Keys, an area which could potentially be a seasonal mixing zone for Atlantic and eastern Gulf populations, largely assigned to the eastern Gulf population. Of 79 Keys individuals analyzed, 50 were assigned to the Gulf, and four to the Atlantic with greater than 80% membership probability. Seven potential Gulf migrants were identified in the Atlantic and three potential Atlantic migrants were identified in the Gulf using the whole data set. When a reduced data set consisting only of markers that were significantly divergent between the eastern Gulf and Atlantic was used for assignment only five migrants were detected (four in the Atlantic and one in the Gulf). The results presented here indicate that the Straits of Florida do not act as a hard barrier that prevents movement between the western Atlantic and eastern Gulf. Despite the number of migrants detected exceeding that which is theoretical required to cause gene pools to homogenize, the western Atlantic and Gulf remain distinct; suggesting differences in potential and realized dispersal over generational scales. Additionally, the presence of migrants between the populations but absence of gene flow brings to question if genetic, behavioral, or physiological (olfactory, etc.) factors can explain this behavior.