Genetic variation revealed by mitochondrial and microsatellite analysis in Heaviside’s dolphins (Cephalorhynchus heavisidii)

Abstract

Heaviside’s dolphins are endemic to southwest Africa; their population ecology remains little known, although individual ranging pattern is thought to be limited. In this study, using mtDNA control region (580 base pairs) and 14 microsatellite loci (n=97 biopsy samples), we examined the genetic structure of this species at the southern edge of their geographic distribution in two proximate sites, Table Bay and St. Helena Bay; 170km apart. The null hypothesis was that these two sites represent different populations. We identified 18 mitochondrial haplotypes, each sex had 13 of these haplotypes. Sex specific analysis of molecular variance showed significant differentiation for both sexes for mitochondrial (♂ FST=0.22, ΦST=0.30, P< 0.001, ♀ FST=0.12, ΦST=0.24, P< 0.001) and nuclear markers (♂ FST=0.013, RST=0.028, ♀ FST=0.024, RST=0.024), although the result was not significant for RST. Levels of gene flow estimated using an isolation with migration model for mtDNA were asymmetrical with gene flow higher in the southward direction (towards the perimeter of the species' geographic range) for both sexes. Gene flow was uncharacteristically high for females (Table Bay, m=0.005 (0.12–9.6); St. Helena Bay, m=6.0 (0.84–9.7) as compared to males (Table Bay, m=0.005 (0.01–8.4); St. Helena Bay, m=0.995 (0.32–7.0), which contradicts what is currently thought of the ranging pattern and dispersal of females. The 95% posterior density distributions for these estimates were large and overlapping, and we suggest that additional data is required to confirm given the unusual nature of these results. The level of genetic differentiation found on this scale is similar to some other coastal dolphins (e.g. Tursiops sp. in Shark Bay, Australia) which are separated by similar distances, but is surprising considering the lack of obvious oceanographic barriers. More work is currently underway to further investigate the sex-biased dispersal pattern and population structure across a substantially larger range.