These Female Fish Are Creating New Hybrids by Mating With Attractive Males From Other Species

Scientists have uncovered evidence of fish mating with species different from their own—a process that can lead to the creation of several new species.

For a study published in the journal Nature Communications, a team of scientists studied cichlid fish in two African freshwater lakes—Lake Bangweulu and Lake Mweru—over a period of 10 years.

Cichlids are a large and diverse family of primarily freshwater fish which include more than 1,300 known species. Scientists often find new cichlid species and there are thought to be many more that are yet to be discovered.

In fact, the scientists, led by Joana Meier from the University of Cambridge in the U.K., identified more than 40 new cichlid species over the course of the study in Lake Mweru—which is located on the longest arm of the Congo River, straddling the border between Zambia and the Democratic Republic of the Congo.

"We found a dazzling variety of ecologically diverse new species—called radiations—that were previously unknown," Meier said in a statement.

In Lake Bangweulu meanwhile, the researchers did not find even a single new species.

According to the researchers, the diverse array of different cichlid species in Mweru can be explained by a process known as hybridization. This is when two different species breed, leading to the creation of a new species.

"We wanted to find out what explains this huge discrepancy between species richness between the lakes," Meier told Newsweek. "Over 40 new species in Lake Mweru and no new species in Lake Bangweulu! We found out with DNA analyses—sequencing the genomes—that the species which evolved in Lake Mweru are all of hybrid origin between Congolese and Zambezian cichlid lineages."

"The few cichlid species we found in Lake Bangweulu are all widespread in the Zambezi drainage system," Meier said. "Lake Mweru was originally part of the Congolese drainage system and got colonized by Congolese cichlids. However, about one million years ago, there was a river capture event diverting the outflow of Lake Bangweulu into Lake Mweru. This allowed the Zambezian lineages from Lake Bangweulu to colonize Lake Mweru. Lake Mweru cichlids could not swim into Lake Bangweulu because large rapids and waterfalls prevent it."

According to Meier, the Zambezian lineages that arrived in Lake Mweru hybridized—i.e. interbred—with the Congolese lineages in Lake Mweru and the resulting hybrid populations diversified into many different species adapted to feed on different things or live in different parts of the lake to their parents.

In lab tests, the scientists determined the circumstances under which this hybridization could happen.

When cichlid fish mate, the females choose their partners. However, the team found that the females would sometimes choose males from a different cichlid species if they displayed similar coloring to the males from their own species. Furthermore, they noticed that the females would also choose males of another species when light conditions were poor because they couldn't see properly.

cichlid fish
One of the new predator cichlid fish species that evolved in Lake Mweru. Ole Seehausen

"When Lake Mweru was formed it combined cichlid lineages from the Congo and the Zambezi. The cichlids from these different drainage systems then mated with each other. This could have been because when the lake formed, the water was very cloudy and they couldn't see colours properly so the females were not being as choosy about selecting a mate in their new environment," Meier said in a statement.

The researchers say that the latest findings cast new light on our understanding of species hybridization.

"Hybridization is generally believed to be a bad thing for biodiversity as it can lead to species fusion and the loss of local adaptation," Meier told Newsweek. "However, our study shows that hybridization can also facilitate the rapid evolution of new species. When species interbreed, they generate large genetic variation that speeds up the evolution of new species. The Lake Mweru cichlids are a prime example that when you have new ecological opportunity—for example, when a new lake forms—many new species can evolve."

"However, usually, this takes many million years. Here, it was much faster because the new species evolved from hybrid ancestry. Hybrids are genetically very variable as each of them is a different combination of the genes of the two parental species. Two species combined their genes and selection could then act on this large diversity and select out those individuals that are better at eating a specific food resource or living in a specific habitat," she said.