Phylogeography and Population Structure in Highly Mobile Marine Taxa in the Western Indian Ocean: Bottlenose Dolphins (Tursiops spp.) and Common Dolphins (Delphinus sp.)

Abstract

In the marine environment, where barriers to dispersal are limited, taxa normally exhibit genetic homogeneity across large spatial scales. Extraordinarily, marine mammals regularly exhibit genetic differentiation within their cruising range. Furthermore, recent radiation in Delphininae has resulted in several closely related species that remain taxonomically unresolved, particularly bottlenose dolphins (BND) Tursiops spp. and common dolphins (CD)
Delphinus spp., making these taxa interesting for studying evolutionary processes.

Using mitogenomes and a multi-locus dataset, BNDs from the northwest Indian Ocean (IO) were compared with other recognized species/ecotypes around the world. A new (third) lineage of Indo-Pacific BND, T. aduncus, was identified from the region. Reconstructions of ancestral biogeography and divergence date estimates, suggest a divergence mechanism within T. aduncus that coincides with climate change over the Pleistocene. Reconstructions of ancestral morphology suggest a coastal ancestry for BNDs.

Significant population structure was exhibited between T. aduncus populations in the western IO based on mtDNA control region sequences and 14 microsatellite loci. Genetic subdivision appears to correlate with habitat heterogeneity across the study area, which may be driving differentiation through local adaption.

Traditional and geometric morphometric techniques were used to investigate congruency between genetic and phenotypic differentiation of three BND lineages in the northwest IO. Strong differences were exhibited in morphology between common BNDs, T. truncatus, and T. aduncus. The T. aduncus lineages were similar, however significant differences in morphology were evident.

Significant genetic structure was evident between CD populations off Portugal, South Africa and Oman, based on mtDNA sequences and 14 microsatellites. Further analyses support the taxonomic designation of D. capensis tropicalis in the northwest IO.

Both genera exhibit significant population structure over spatial scales outdistanced by their dispersal abilities. Contemporary and historic environmental heterogeneity are suggested as drivers for this structure. Further evidence is provided for the northwest/northern IO as a region of evolutionary endemism, which will inform regional conservation initiatives.