What role do crossbreeds between different related primate species play in the fitness of the offspring and the evolution of the species involved? Researchers have now investigated this question using a combination of long-term field data and genomic information. For more than 50 years, they observed two species of wild baboons that kept interbreeding. While the hybrid offspring showed no obvious disadvantages, the genes of one species were eliminated again over time in the genome of the hybrids. The results also contribute to a better understanding of early human evolution, when our ancestors interbred with other human species such as Neanderthals.
Tens of thousands of years ago, when our ancestors lived side-by-side with Neanderthals and other now-extinct Homo lines, interbreeding between the closely related species was common. To this day, traces of Neanderthal genes can be identified in our genome. Paleontologists suspect that mechanisms of natural selection ensured that hybrid descendants of modern humans and Neanderthals were less able to reproduce, so that the species boundary was preserved despite the possibility of joint reproduction. Since today there are no other homo lines apart from Homo sapiens with which we could reproduce, this thesis cannot be tested directly experimentally.
50 years of field studies
“However, studies in living primates provide a context for understanding the mixing dynamics in our own lineage,” explains a research team led by Tauras Vilgalys from Duke University in the USA. The researchers chose baboons in the Amboseli region in Kenya as the object of study. In this region, the southern yellow baboons meet with the northern Anubis baboons. Although the species diverged around 1.4 million years ago, in regions where their ranges overlap, they can interbreed and produce fertile offspring together.
For 50 years, scientists have been observing baboons in the Amboseli region, recording interspecies interbreeding, individual migration and migration, and the appearance, behavior and reproductive success of hybrid offspring resulting from a cross between yellow and olive baboons. Vilgaly’s team combined this data with genome analyzes of over 440 baboons from the region. The genetic data included nine generations of baboons born since 1969.
Disadvantaged Genes
The long-term observations did not reveal any obvious disadvantages for hybrid individuals. Some of the hybrid males even turned out to be more successful in choosing a mate. So how is it that the species did not fully intermix into one, but continue to exist separately – with only a very narrow zone where interbreeding occurs? The genetic data provide an answer to this question. First of all, the researchers found that all baboons in the Amboseli region carry genes from both species in their genome – around one third Anubis DNA, two thirds yellow baboon DNA.
In about half of the animals whose genome Vilgalys and his team analyzed, the introgression of Anubis DNA took place no more than seven generations ago, i.e. during the time when the animals were already being observed scientifically. In the case of 214 baboons, however, according to the genetic data, the crossing must have taken place before the observations began in 1971. In the genome of these “historical” hybrids, the researchers discovered indications that the Anubis DNA is sorted out as part of natural selection: Compared to non-coding gene regions, genome sections that code for proteins or are involved in gene regulation contained significantly fewer Anubis DNA.
Parallels to human evolution
Based on the data available so far, the researchers cannot answer exactly why Anubis genes are less likely to be passed on to the offspring. “Both hybrid incompatibilities and ecological selection could play a role,” they surmise. “For example, some reports suggest that the two baboon species occupy different climatic niches.” This may have resulted in the Anubis genes bringing indirect disadvantages to the hybrid offspring, causing them to partially reappear over hundreds of generations were eliminated.
From the authors’ point of view, the combination of observational and genetic data in primates is very helpful in order to better understand our own evolutionary history. A similar pattern is also evident in our genome, with Neanderthal DNA being more prevalent in regions where it has no functional implications. As with the baboons, a hidden selection could also have taken place here – without the individual descendants of Homo sapiens and Neanderthals necessarily having had any noticeable disadvantages.
Source: Tauras Vilgalys (Duke University, Durham) et al., Science, doi: 10.1126/science.abm4917