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Hybrid ants evolve rapidly to remove deleterious genes

Despite the existence of many mechanisms that prevent reproduction between two different species in nature, hybridization is a widespread phenomenon that has shaped the genomes many of today’s species, humans included. When the genomes of two different species are combined through successful reproduction, this provides a source of genetic novelties that can be crucial in helping populations adapt to changing environments. Understanding the evolution of hybrid genomes is therefore important because it can shed light on how species barriers become established, on the costs and benefits of hybridization, and on the function of genes and their interactions. 

Researchers at the University of Helsinki were aware that two species of wood ants, Formica aquilonia and Fpolyctena have bred and formed populations of hybrids in several different locations in Finland. The researchers took advantage of these known hybrid populations to measure how rapid and predictable the evolution of genomes is in the wild, after hybridization has taken place. In this case, predictability applies to the genetic outcome after hybridization, and the question of whether, if one could redo the same hybridization, the genetic results would be similar. 

The scientists collected 39 ants from the three different hybrid populations in the forests in southern Finland, close to the Tvärminne Zoological Station. They then generated whole-genome DNA sequences from these individuals and compared them with genomes from 10 individual ants of both parent species, found within and outside the ranges where the hybrids and parental species overlap. In order to do this, they collaborated with scientists from Scotland and Portugal, and used supercomputers from the Finnish IT center for science (CSC). In total, the researchers analyzed about 1.6 million single-nucleotide polymorphisms from locations throughout the ant genomes.

The results, published in the journal PLOS Biology, showed that, for the three different hybrid populations, hybridization between the two ant species occurred only about 125 years ago, which equates to 50 ant generations. After hybridization, the three distinct hybrid populations evolved independently towards the same genetic outcome. This means that, today, the genomes of ants from the hybrid populations are remarkably similar to one another in terms of their genetic composition, even though they started out being dissimilar at the time of hybridization. 

To explain this unexpected finding, the researchers tested for evidence of gene flow between the hybrid populations, and between hybrids and parental species. They found no evidence of gene flow and concluded that other mechanisms must be involved to explain this rather rapid evolution of the three hybrid populations towards a similar genome. They suggest that natural selection against deleterious genes in the hybridizing species with the lowest effective population size (and presumably higher load of disadvantageous alleles) could explain how the hybrid populations became genetically similar over such a short time.

When asked whether these results represented a small step or giant leap in knowledge, researcher Pierre Nouhaud, from the Faculty of Biological and Environmental Sciences at the University of Helsinki responded, “A bit of both! Finnish wood ants give the opportunity to observe multiple, very recent hybridization 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.” 

“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,” adds Nouhaud. “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.”

The researchers emphasize that hybrid genomes provide powerful insights into evolution because they are exposed to strong, and often opposing, selective forces. In this way, evolution has shaped the hybrid genomes in a predictable manner, such that the genomes of the different hybrid populations are now similar. This indicates that selective pressures were similar for each hybrid population and this led to the removal of deleterious genes from the genomes.

“Since hybridization is also frequent in many other species than ants, our results could help understand its consequences more generally,” concludes docent Jonna Kulmuni, from the Faculty of Biological and Environmental Sciences, University of Helsinki. “On the long term, our work would help better apprehend the impact of hybridization in the wild and assess whether it could help species cope with changing environments,” she says. 

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, and hunt many other insects. Without them, Finnish forests would certainly look quite different.

By Alison Bosman, Staff Writer

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