You are probably familiar with the idea of ​​the sterile mule: a hybrid animal born of a horse and a donkey that cannot reproduce.

But what about a fertile mule whose teeth are simply not suited to chewing the grass around it?

Hybrid animals are often sterile or unable to live because their genes can be incompatible due to natural selection, in which species evolve to adapt. These incompatibilities are a by-product of the genes the parent populations used to adapt to their environment, and arise due to “mismatched” genes that may affect things like cellular processes. These incompatibilities are an important mechanism in speciation, as different populations or species cannot collapse into one larger population, but can remain separate.

However, research at the University of British Columbia on hybrid three-spined sticklebacks suggests that hybrid intolerances may be dependent on their environment, says lead author Dr. Ken Thompson, a PhD student at the UBC Department of Zoology and Biodiversity Research Center at the time of graduation.

It is almost always assumed that hybrid incompatibilities as a genetic mechanism affect hybrid organisms in all habitats they encounter. But some hybrids can be perfectly viable and fertile in the laboratory – only when they are exposed to ecological stresses, such as from predators or foraging, do they die or do not mate as successfully as other fish.

Dr. Ken Thompson, Postdoc in the Biology Department at Stanford University

Previous research has shown that there appears to be a “mismatch” component in this ecological species, for example when some hybrid fish have mismatched jaws that reduce their ability to feed. What was not clear was whether the genetics of the “ecological” mismatch were related to the genetics of the “intrinsic” mismatch that resulted in infertile or non-viable individuals. “Although the genetics of ‘intrinsic’ incompatibilities has been relatively well studied for decades, we knew almost nothing about the genetics of ‘ecological’ incompatibilities that only occur in certain environments. However, these ecological incompatibilities are likely to be widespread in nature. ”« Says co-author Dr. Catherine Peichel, professor at the Institute for Ecology and Evolution at the University of Bern.

Dr. Thompson and colleagues compiled genetic data from experiments conducted at UBC between 2003 and 2013 on two types of hybrid three-spined sticklebacks, with approximately 3,300 fish raised in large artificial ponds at UBC and 550 in a laboratory. The researchers found that fish raised in the pond had a three percent higher heterozygosity, suggesting that they had a lower incidence of “mismatched” genes. The researchers suspect that fish with “mismatched” genes only died in the ponds, which means that the surviving fish have a greater average heterozygosity. In the laboratory, with no ecological pressure or influence, these “mismatched” fish survived, and the heterozygosity did not differ from 50 percent, the expected value according to the classical rules of genetics.

“This is the first paper to show that a genetic signature of hybrid incompatibilities – the death of individuals with more ‘mismatched genes’ – can be caused by ecology,” says Dr. Thompson. “However, analyzes of average ‘ancestral heterozygosity’ are very crude and more research is needed to identify exactly which genes and traits are affected.”

The research shows how theoretical predictions about selection for mismatched genes, such as heterozygosity of ancestry, could be applied to real world data from experiments, he says. “I think a lot of people will have this kind of genetic data and maybe they just never thought about how to use it to test predictions about hybrid incompatibilities.”


University of British Columbia

Journal reference:

Thompson, KA, et al. (2022) Analysis of parentage heterozygosity suggests that hybrid incompatibilities in the three-pronged stickleback are environmentally dependent. PLOS biology.


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