Whale Louse (Cyamid)

population genetics

Right whales carry large populations of three species of ‘whale lice’ (Cyamus ovalis, C. gracilis, C. erraticus).  Dr. Jon Seger and I used sequence variation in the mitochondrial COI gene to ask (1) whether cyamid population structures might reveal associations among right whale individuals and subpopulations, (2) whether the divergences of the three nominally conspecific cyamid species on North Atlantic, North Pacific, and southern right whales (Eubalaena glacialis, E. japonica, E. australis) might indicate their times of separation, and (3) whether the shapes of cyamid gene trees might contain information about changes in the population sizes of right whales.

We found high levels of nucleotide diversity but almost no population structure within oceans, indicating large effective population sizes and high rates of transfer between whales and whale subpopulations. North Atlantic and Southern Ocean populations of all three species are reciprocally monophyletic, and North Pacific C. erraticus is well separated from North Atlantic and southern C. erraticus. Mitochondrial clock calibrations suggest that these divergences occurred around 6 million years ago, and that the Eubalaena mitochondrial clock is very slow. Low-frequency polymorphisms are more common than expected under neutrality for populations of constant size, but there is no obvious signal of rapid, interspecifically congruent expansion of the kind that would be expected if North Atlantic or southern right whales had experienced a prolonged population bottleneck within the last 0.5 Myr.

                                                                                      Cyamus ovalis

                                                               photo by Ada Kaliszewska