Post-glacial habitat release and incipient speciation in the genus Delphinus

Genomics reveals a genetically isolated population of the Pacific white-sided dolphin (Lagenorhynchus obliquidens) distributed in the Sea of Japan

The processes by which animals become genetically isolated in an open environment such as the ocean have not yet been fully elucidated. Morphologically different populations of Pacific white-sided dolphin Lagenorhynchus obliquidens are observed sympatrically in the Sea of Japan. However, genetic studies that exclusively used limited mitochondrial loci or microsatellite DNA have failed to demonstrate the existence of genetically distinct populations. Here, to reveal the population structure, we analysed genome-wide population genetic data using single nucleotide polymorphisms (SNPs) gathered in 2018-2021 from all domestic captive individuals in aquaria, the majority of which originated from the wild, as well as from some stranded individuals, together covering a wide area of coastal water around Japan (n = 123). Multiplexed intersimple sequence repeat genotyping-by-sequencing analysis was performed to obtain the SNP data. Principal coordinate analysis and the clustering method structure indicated that two genetically-distinct populations exist, with little interpopulation gene flow revealed. In addition, the genotypic segregation was reflected in differences in external morphotype. Furthermore, a population demographic analysis based on the whole-genome sequences of an individual from each population indicated that sea-level changes during the Last Glacial Period probably led to allopatric divergence of this species in a limited area of the Sea of Japan, with that group subsequently sharing a distribution area with the other population. These findings yield insights into the formation of genetically isolated sympatric populations in the ocean.

Eco-Evolutionary Processes Generating Diversity Among Bottlenose Dolphin, Tursiops truncatus, Populations off Baja California, Mexico

For highly mobile species that nevertheless show fine-scale patterns of population genetic structure, the relevant evolutionary mechanisms determining structure remain poorly understood. The bottlenose dolphin (Tursiops truncatus) is one such species, exhibiting complex patterns of genetic structure associated with local habitat dependence in various geographic regions. Here we studied bottlenose dolphin populations in the Gulf of California and Pacific Ocean off Baja California where habitat is highly structured to test associations between ecology, habitat dependence and genetic differentiation. We investigated population structure at a fine geographic scale using both stable isotope analysis (to assess feeding ecology) and molecular genetic markers (to assess population structure). Our results show that there are at least two factors affecting population structure for both genetics and feeding ecology (as indicated by stable isotope profiles). On the one hand there is a signal for the differentiation of individuals by ecotype, one foraging more offshore than the other. At the same time, there is differentiation between the Gulf of California and the west coast of Baja California, meaning that for example, nearshore ecotypes were both genetically and isotopically differentiated either side of the peninsula. We discuss these data in the context of similar studies showing fine-scale population structure for delphinid species in coastal waters, and consider possible evolutionary mechanisms.

Ancient female philopatry, asymmetric male gene flow, and synchronous population expansion support the influence of climatic oscillations on the evolution of South American sea lion (Otaria flavescens)

The South American sea lion (Otaria flavescens) is widely distributed along the southern Atlantic and Pacific coasts of South America with a history of significant commercial exploitation. We aimed to evaluate the population genetic structure and the evolutionary history of South American sea lion along its distribution by analyses of mitochondrial DNA (mtDNA) and 10 nuclear microsatellites loci. We analyzed 147 sequences of mtDNA control region and genotyped 111 individuals of South American sea lion for 10 microsatellite loci, representing six populations (Peru, Northern Chile, Southern Chile, Uruguay (Brazil), Argentina and Falkland (Malvinas) Islands) and covering the entire distribution of the species. The mtDNA phylogeny shows that haplotypes from the two oceans comprise two very divergent clades as observed in previous studies, suggesting a long period (>1 million years) of low inter-oceanic female gene flow. Bayesian analysis of bi-parental genetic diversity supports significant (but less pronounced than mitochondrial) genetic structure between Pacific and Atlantic populations, although also suggested some inter-oceanic gene flow mediated by males. Higher male migration rates were found in the intra-oceanic population comparisons, supporting very high female philopatry in the species. Demographic analyses showed that populations from both oceans went through a large population expansion ~10,000 years ago, suggesting a very similar influence of historical environmental factors, such as the last glacial cycle, on both regions. Our results support the proposition that the Pacific and Atlantic populations of the South American sea lion should be considered distinct evolutionarily significant units, with at least two managements units in each ocean.