Rare crossing generated hybrid lineage of fur seals – 05/05/2023 – Science

An unlikely courtship between members of two distinct species of fur seals off the coast of Peru eventually gave rise to a third species, which carries the DNA of both ancestral lineages.

The phenomenon, which for the time being seems to be unique among mammals like us, was unveiled by an international team of scientists, whose leaders are Brazilians from PUC-RS (Pontifical Catholic University of Rio Grande do Sul). In a study that has just been published in the specialized journal Science Advances, they estimate that the hybrid fur seals in Peru began their independent evolutionary path around 400,000 years ago, during the Ice Age.

“We still have no way of estimating the exact number of crossings, but everything indicates that there were few individuals in a fortuitous event, a very rare situation”, explains geneticist Sandro Bonatto, one of the main authors of the research alongside his colleague Fernando Lopes, both linked to the Gaucho institution.

Elementary and high school textbooks often teach that a species is characterized by the fact that its members are only able to generate fertile offspring with partners of the same species. However, the real situation is far more complex. Plants and animals that belong to species that are relatively closely related to each other can end up generating fertile hybrids, although, in general, they live in different environments and have very different characteristics.

However, the most common thing is that these cases of hybridization only lead to the incorporation of some stretches of DNA from one species into the genome of the other. This is what happened when some humans with modern anatomy joined archaic humans, such as Neanderthals, at the end of the Ice Age, for example.

The situation caught by the PUC-RS researchers is much more intriguing and rare. Bonatto recalls that there were already morphological and behavioral indications to consider that the Peruvian fur seals (and their neighbors in the north of Chile) were different from their kin.

In addition, they are separated from other South American fur seals by about 2,000 km of coastline on the Pacific Ocean. In the rest of the continent, in a more or less continuous strip that runs from the extreme south of the coast of Rio Grande do Sul to the south of Chile, there is another population of these marine mammals.

Until now, all these South American animals were considered members of the same species, the Arctocephalus australis. There are more distant relatives of them in places like New Zealand and Australia (the A. forsteri), as well as in the Galapagos Islands near Ecuador (the A. galapagoensis).

When investigating in detail the genome of the various populations of South American fur seals, comparing it with that of relatives in other parts of the world, the team realized that the DNA of the Peruvian animals had curious characteristics. While the fur seals of the Galápagos and those of the southern population (gauchos, Uruguayans, Argentines, etc.) formed two clearly separate groups, the marine mammals of Peru were “in the middle of the road” in genetic terms.

But this intermediate character clearly came from a long time ago, because the apparent mix of DNA from the Galápagos and the southern population in the Peruvian animals was quite homogeneous and distributed across the different regions of the genome. If it had happened recently, you’d expect different animals to carry different proportions of each species’ DNA—some with 75% and 25%, others with 20% and 80%, and so on. Instead, the most likely scenario is that the mix is ​​homogeneous and around 50% each species contributes, says Bonatto.

In other words, there was enough time for the Peruvian fur seal population, in relative isolation, to reach a unique state of complete mixing between the two “parental” populations, as the scientists say —according to the team’s calculations, the process around 400,000 years ago. More likely, events such as extreme weather conditions dragged some southern fur seals and others from the Galápagos to the Peruvian coast, and then interbreeding between these individuals took place.

Larissa Oliveira, another author of the study, is preparing a formal description of the new species, which will give it its own scientific name. Meanwhile, genomic analyzes continue. Many details of the hybridization process are still unclear, and there are even indications that, with climate change, new individuals from the Galápagos are coming to islands farther off the Peruvian coast.

“Since they are more adapted to warm waters, close to the Equator, this movement could be linked to ocean warming”, ponders Bonatto. This would open up the possibility of new genetic mixtures among fur seals, altering the population composition established in the Ice Age.