Use of molecular tools on surveys of genetic variation and population structure in three species of sharks.

摘要

Molecular tools, such as sequencing of the mitochondrial DNA Control Region (CR) and genotyping of highly variable nuclear microsatellites were applied to survey the genetic diversity, population structure and phylogeography of three shark species: the whale shark, Rhincodon typus; the bull shark, Carcharhinus leucas; and the nurse shark, Ginglymostoma cirratum. The highly migratory and pelagic whale shark exhibited the largest length variation yet reported for an elasmobranch CR (1143--1332 bp), and high haplotype (h = 0.974 +/- 0.008) and nucleotide diversities (pi = 0.011 +/- 0.006). No geographical clustering of lineages was observed and the most common haplotype was distributed globally. The haplotype frequency, however, differed between the Atlantic and Indo-Pacific populations (AMOVA, phiST = 0.107, P 0.001). For the bull shark, both mtDNA CR and five microsatellite loci were surveyed for animals from the Gulf of Mexico, the East coast of Florida and the Brazilian coast. Strong genetic structure was observed between the Brazilian and all northern populations for the CR (phiST > 0.8, P 0.001), but not for the nuclear microsatellite. The results here presented are congruent with restricted maternal gene flow between populations as a consequence of female nursery site fidelity. The philopatric tendencies as well as the relatively low levels of genetic diversity raises concerns about the conservation of this species. Finally, for the western Atlantic nurse sharks the genetic diversity estimated in a 1,166 bp fragment of the mtDNA comprising partial cytochrome b, tRNAPro, tRNAThr, and partial CR was the second smallest ever recorded for sharks (h = 0.45 +/- 0.04; pi = 0.0004 +/- 0.0004). The data indicated moderate but significant genetic structure with the mtDNA marker (phiST = 0.22, P0.05) and no substantial structure in eight microsatellite loci analyzed. A population bottleneck as recent as the lower Pleistocene might have eroded the nurse shark genetic diversity and also contributed to its relatively lower population structure. The data also indicated that dispersal rather than vicariance better explains the Atlantic distribution of nurse shark, and that the Pacific nurse shark might be a cryptic sister species to Ginglymostoma cirratum.