Blue whales breed with other species of whales, including fin whales, more often than you might think. Examination of their genome revealed that it contained unexpected amounts of fin whale DNA, suggesting frequent mating between the two species as well as with their hybrid offspring.
“Size isn’t everything,” said American columnist Bill Vaughan about the blue whale (Balaenoptera musculusBalaenoptera musculus). According to the International Union for Conservation of Nature (IUCN), it is the largest mammal currently living on Earth, but its status is critically endangered.
One factor that could likely hinder population recovery is inbreeding, which was studied in a population of Atlantic blue whales by scientists at the journal Conservation Genetics. The results are unexpected, to say the least: 100% of the 31 individuals examined possessed fin whale DNA (Balaenoptera physalusBalaenoptera physalus), accounting for an average of 3.5% of the entire genome.
Hybrid DNA in unexpected quantities
We have known for several years that these two species can hybridize, a 2018 study even demonstrated the fertility of their hybrid offspring. The latter, in particular, is able to reproduce with blue whales and this genetic process has a name: it is introgression. So-called “backcross” individuals arise, which result from the transfer of genes from one species to another after repeated hybridizations. What surprised scientists, however, was the amount of hybrid DNA found in the sequenced genomes. “The level of interspecies introgression we found was unexpected and much higher than previously reported,” notes study co-author Mark Engstrom, a geneticist and ecologist at the University of Toronto.
A unidirectional and localized phenomenon
In contrast, scientists have found no evidence that fin whales inherited blue whale DNA through introgression. It appears that “only blue whales are able, or perhaps willing, to breed with these hybrids,” says Mark Engstrom. This could be because there are many more fin whales than blue whales. » Furthermore, introgression between fin whales and blue whales appears to be a phenomenon that, for no explicit reason, only affects the North Atlantic.
Then how about inbreeding?
Although there is no evidence of negative effects on the blue whale, the researcher fears that in the long term this introgression will reduce the amount of specific DNA of these whales in the population, which risks making them less resilient to changes in their environment, in particular caused by humans. The good news is that the originally targeted inbreeding rates are much lower than expected because of gene flow connecting them to other populations elsewhere in the world, facilitated by ocean currents. This would be a sign of better genetic diversity and therefore greater resilience to obstacles that hinder the recovery of this population. The genetic dynamics discovered are insights that could shape the future of conservation strategies for iconic marine species.