Moby Dick is possibly the most famous whale ever. Moby was a sperm whale, Physeter microcephalus, are the largest of the toothed whales and also the largest toothed predator on the planet. You have to be big and full of teeth to hunt colossal squid at 1000 meters deep.
Very neat ROV footage at 598 meters (1,962 ft) below the Gulf of Mexico off the coast of Louisiana.
These pelagic mammals are found worldwide in every ocean and migrate seasonally to breed and feed. Given their global distribution one would be inclined to assume they would show high genetic diversity. However, sperm whales actually show limited phylogeographic structure, meaning they have little genetic differentiation between geographically distinct areas. Traditionally, mitochondrial DNA (DNA inherited maternally only) has been heavily used in phylogeography studies, but sometimes patterns of demography and phylogeography can be misrepresented by underlying processes. These processes can include selection, variation in mutation rates, coalescentstochasticityand cultural hitchhiking. As a highly social species, cultural hitchhiking, the linkage of genetic variation of culturally-transmitted traits affecting fitness, has been suggested as a possible reason for the low genetic diversity in sperm whales.
Recent research published in Molecular Ecology has used mtDNA, complemented with three nuclear genomes, to ask questions surrounding why sperm whales exhibit such low mtDNA genetic differentiation. Specifically, could it have been a population bottleneck/expansion in the past versus a selective sweep due to cultural hitchhiking or selection on mtDNA?
The researchers analyzed the mitochondrial genomes of 175 samples from live and dead stranded whales from all over the world and three nuclear genomes. Together the low mtDNA control region data, commonly used for population level-studies as it is non-coding and highly conserved, and the demographic analyses of the nuclear genome were consistent with a global reduction in population size that ended approximately 125,000 years ago. This period correlated with the Eemain interglacial, where the sperm whale population had been reduced to a small population of around 10,000 individuals. During this time the worldwide freeze would have excluded the whales from all oceans apart from the Pacific.
The phylogeographic analysis suggests that extant sperm whale populations (~360,000 animals) descended from this Pacific maternal lineages after this period of low abundance. These results suggest a role for past climate change and that selection and cultural hitchhiking are not the only processes responsible from the current low mtDNA diversity in this highly social species.
For this fascinating publication please see the full text:
https://onlinelibrary.wiley.com/doi/full/10.1111/mec.14698
Morin, Phillip A., Andrew D. Foote, Charles Scott Baker, Brittany L. Hancock‐Hanser, Kristin Kaschner, Bruce R. Mate, Sarah L. Mesnick, Victoria L. Pease, Patricia E. Rosel, and Alana Alexander (2018) Demography or selection on linked cultural traits or genes? Investigating the driver of low mtDNA diversity in the sperm whale using complementary mitochondrial and nuclear genome analyses. Molecular ecology 27. 11: 2604-2619.
