Biological concept put to the test: Until now, scientists assumed that two populations had to isolate themselves completely so that new species could be formed from them. However, a current genome analysis of two mosquito species suggests that genetic isolation is not necessarily necessary for speciation. According to the study, the mosquito species continue to mate and rather form stable intermediate stages in which the genome is about 70 percent similar.
Biologists have been discussing the process of speciation for centuries: the prevailing theory suggests that two populations can split into two new species through spatial separation. But genetic factors or differences in reproductive behavior can also lead to speciation. The prerequisite, however, is usually that there is no longer any gene exchange between the populations.
Mosquitoes as a case study
However, new studies on stick insects and rare bird species have shown that, under certain circumstances, new species formation is possible despite a constant exchange of genetic information. How widespread the phenomenon of speciation is and in what period of time a new species can develop despite gene flow is still unclear. A research team led by Dennis Schreiber from the Senckenberg Biodiversity and Climate Research Center in Frankfurt has now got to the bottom of these questions.
To do this, the researchers examined the genomes of 36 individuals of two species of mosquito – Chironomus riparius and Chironomus piger – from five locations across Europe. Both species are found throughout the northern hemisphere and have similar distribution areas, as they are predominantly found in freshwater. Nevertheless, they prefer ecologically different habitats, because Chironomus piger is known, for example, to tolerate higher nitrite and salt concentrations and therefore usually predominates in stagnant waters near agricultural landscapes, while the sister species predominates in slow-flowing streams. The individuals of these mosquito species have been shown to be able to mate with one another.
70 percent of the genome in exchange
The genome analysis showed that there is actually still a gene flow between the two mosquito species. “We were able to show that species can gain and maintain their diversity, even though they share most of their genome with one another,” says Schreiber. According to this, individuals regularly exchange a good 70 percent of their genome – this proportion accounts for around half of all genes. According to the researchers, the other half of the genes are scattered across just under 30 percent of their genome, which is found in only one species. “The species identity therefore seems to be based on half of the genes,” explains the researcher.
The species-specific genes therefore comprise comparatively small areas of the genome that the mosquito species claim exclusively for themselves: “On the one hand, this concerns genes that are related to the known ecological differences between the species,” explains Schreiber. They make Chironomus piger less sensitive to the nitrite from the manure. “However, there are also differences between genes that work closely with other genes at the molecular level, for example in the respiratory chain, protein production, or in sluices through membranes,” adds Schreiber’s colleague Markus Pfenninger. “At these points incompatibilities between the different species would obviously be too disturbing, so that they remain isolated here.”
Gene flow between species has been around for centuries
The study thus proves that a gene flow between two populations is possible and still two species arise from it. But could this splitting process at least lead to a complete genomic isolation of the two species in the future? To find out, the researchers used the genomes to place the evolution of the two species in chronological order: The mosquito species probably formed more than three million generations ago – that is only about 700,000 years. But the splitting process apparently ended again before the entire genome was mutually isolated. Since then there has been regular hybridization, i.e. mating and gene exchange between the individuals of both species – and so far there has been no reproductive isolation.
“These new findings change our concept of a biological kind,” summarizes Pfenninger. “Obviously, a completely independent genome is not a prerequisite, but there are stable intermediate stages in which related, well-characterized species maintain their independence despite large genomic overlaps. We now want to continue researching this, ”concludes the researcher.
Source: Senckenberg Research Institute and Nature Museums, Article: Evolution Letters, doi: 10.1002 / possibly 3.204