A new way to make new species

The evolution of a new species by hybridization from two already described species without a change in chromosome number is very rare in the animal kingdom. So far, only a few accepted empirical examples exist for this spontaneous way of evolution (from one generation to the next), known as homoploid hybridization.

The research team of Axel Meyer, Professor of Zoology and Evolutionary Biology at the University of Konstanz, has succeeded in demonstrating the emergence of a new hybrid species in cichlid fishes, probably the first example of this genetic speciation method in vertebrates, which is described in the current issue of Nature Communications.

The researchers used whole genome sequencing of more than 120 individuals and a variety of other methods to show that a new hybrid species has evolved from cichlid A. sagittae and A. xiloaensis in the crater lake Xiloá in Nicaragua.

Back in 2018, the research team came across fishes in the crater lake Xiloá that looked like a cross between the two cichlid species. Genetically, too, depending on the marker, it was detectable that portions of both species were present in the genome of these fishes. "We can now sequence the complete genomes of the fishes and look much more closely at how the genome of the hybrids is composed. In fact, it was possible to identify on the chromosome which part of the hybrid originated from A. sagittae and which from A. xiloaensis," says Axel Meyer.

The majority of fishes reproduce among themselves

The detail of the markings also allowed the research team to determine that the majority of individuals of the new species reproduced only among themselves, an indication that it is indeed a new species. Also conceivable would be that hybrids have emerged due to a "mistake" in mate selection, which is why their offspring might turn out to be non-fertile, or hybrid animals that mate again with one of the two parent species ("backcrossing").

The new, very young species, emerging within a few hundred generations, is not directly intermediate between the two parent species, A. sagittae and A. xiloaensis, neither morphologically, physiologically, nor ecologically. Instead, the hybrids show aspects of a transgressive phenotype with traits not found in either parent species. As a result, they occupy a different ecological niche than their two parent species, allowing them to coexist in the lake.

Ecological consequences from the physique

The fishes differ from their ancestors in the shape of their caudal root—the part of the body where the tail fin attaches. "Possibly that is why they are better swimmers. You find this type of body proportion often in fish that can accelerate very quickly," Meyer explains. This allows the hybrids to roam different feeding grounds than the other four species in lake Xiloá, including both parental species, of which one is an elongate species living in open water, while the other has a deeper-bodied shape and lives close to the shore.

With stable isotope analysis of the animals, the researchers were able to show that the prey of the new species consists of other fish, crabs and shrimp—prey that is already very high on the food chain. Probably the individuals of the new species are the most successful predators of the lake.

Unique ecological niche

The new hybrid species occupies a unique ecological niche, which is very important in a small ecosystem like Lake Xiloá, whose diameter is only a little more than one kilometer. "The prerequisite for individual species coexisting for long periods of time in such a limited habitat is that they are no competition for each other," says Axel Meyer. Especially since the new speciation does not occur over a large geographic distance, but under sympatric conditions within the same small habitat as that of the original species.

Genome sequencing, morphometrics, stable isotope analysis—with this combination of different data sets, the researchers were able to understand how the new species has evolved. In a new study, the researchers examine how often errors occur when hybrid fishes are given the choice of reproducing with each other or with individuals of their parent species.

Despite the research, one question remains: How is mate choice controlled genetically?