Partial autonomy: how species separate, but not completely

The prevailing theory so far suggests that regular gene flow through successful matings between populations is the main barrier to their segregation in species. In recent years, however, theoretical and empirical work has shown that speciation is possible under certain circumstances despite this ongoing exchange of genetic information. To date, few research results are available that show the true extent of the speciation phenomenon despite the documented gene flow and the time span it takes for a new species to arise. In particular, it is not clear whether the speciation process, once initiated, will inevitably lead, sooner or later, to complete genomic and reproductive isolation. This would mean that divergent populations are developing into new species in the sense of the widely recognized concept of biological species.

The results of a new study at the LOEWE Center for Translational Biodiversity Genomics (TBG) and the Senckenberg Biodiversity and Climate Research Center question these definitions. “We were able to show that species can achieve and maintain their differences despite still sharing most of their genome with each other,” says Dennis Schreiber, Senckenberg Biodiversity and Climate Research Center and Johannes Gutenberg University in Mainz.

Schreiber analyzed the genome of individuals of two chironomid species, Chironomus riparius and Chironomus piger. Both species are present throughout the northern hemisphere and share similar areas; however, they prefer ecologically diverse habitats. Their genome reveals that these species have rapidly diverged from several million generations (i.e. around 700,000 years ago). Yet the speciation process apparently stopped completely before the entire genome of each species was completely isolated from the other. Since then, hybridization, that is, the crossing between individuals of both species, has taken place regularly.

During the process, about 70% of their genome is exchanged; this involves about half of all genes. However, the species remain ecologically recognizable, even though each can exclusively claim 30% of its genome (although it includes the other half of the genes), and these areas are relatively small and scattered throughout the genome. “On the one hand, this concerns genes that are related to known ecological differences between species, for example their susceptibility to nitrogen compounds such as nitrites from liquid manure,” Schreiber explains.

“At the same time, however, genes that cooperate closely with other genes at the molecular level, for example in the respiratory chain, in protein production or in membrane transfer, also show differences. Here, the incompatibility between species it would apparently be too disruptive: therefore, species retain their individuality in these areas, “says study leader Prof. Markus Pfenninger of the LOEWE Center for Translational Biodiversity Genomics and the Senckenberg Biodiversity and Climate Research Center.

“These new discoveries change our concept of a biological species. Apparently, this does not require a completely independent genome; instead, there are stable intermediate stages in which related and well-characterized species maintain their autonomy despite a large genomic overlap. This will be the subject of research. our future research, ”Pfenninger adds in conclusion.