Ants shed light to predicting evolution after hybridization

Understanding the consequences of hybridisation, i.e. mating between two different species, giving hybrid offspring, is important because it is a widespread phenomenon in nature. By transferring genetic material between species, it can also help populations adapt to new challenging environments, such as warming climate. One key aspect of hybridisation is predictability, which is also an important research question: if we could redo the same hybridisation event several times, how often would we get similar results?

Populations evolve both quickly and predictably because of natural selection

Researchers at the University of Helsinki took advantage of distinct hybrid wood ant populations identified in Southern Finland to study whether hybridisation was predictable. They found that after multiple hybridisation events between two wood ant species, distinct hybrid populations evolved independently towards the same direction. They are nowadays highly similar to one another in terms of genetic composition, suggesting that the outcome of hybridisation is predictable. Researchers also determined that hybridisation occurred less than 50 ant generations ago, approximately 125 years ago, making it a fairly rapid case of evolution in the wild.

"Since hybridisation is also frequent in many other species than ants, our results could help understand its consequences more generally. On the long term, our work would help better apprehend the impact of hybridisation in the wild and assess whether it could help species cope with changing environments, says Docent Jonna Kulmuni, from the Faculty of Biological and Environmental Sciences, University of Helsinki. She has been leading ant research for 15 years at this point.

A small step or giant leap in knowledge?

"A bit of both! Finnish wood ants give the opportunity to observe multiple, very recent hybridisation events, and the amount of predictability we found despite this recency is remarkably high, which is quite novel. In the meantime, our study also confirms previous results obtained in a handful of species, including humans, suggesting the patterns we see in wood ants are quite general," explains researcher Pierre Nouhaud, from the Faculty of Biological and Environmental Sciences, University of Helsinki.

DNA contains information about the evolutionary history of populations

For this study the research group collected hybrid individuals in Finnish forests in Southern Finland, close to the Tvärminne Zoological Station. DNA accumulates mutations as it is transmitted from parents to offspring, as such it contains information about the evolutionary history of populations. Accessing this history required researchers to reconstruct the first reference genome for red wood ants. Researchers then sequenced and analysed hybrid genomes using supercomputers from the Finnish IT center for science (CSC). Some analyses were carried out in collaboration with scientists from Scotland and Portugal.

Reconstructing past evolutionary histories using computer simulations

"On an evolutionary timescale, we are dealing with recent events, less than 50 ant generations, which had very little time to leave footprints in DNA sequences. This means it can be hard to distinguish between competing hypotheses. In our study we performed computer simulations considering different evolutionary scenarios to take this uncertainty into account and ensure our results are robust," says Nouhaud.

Next the ant research group will monitor the genetic composition of hybrid populations over multiple years to find evidence for adaptation and test whether hybrids can combine temperature ranges from both species, which might help wood ants deal with warming climate. According to Nouhaud this could be quite important because wood ants are key species of boreal forests: they provide food for many species, contribute to nutrient cycling, hunt many other insects, among other things. Without them, Finnish forests would certainly look quite different!