The hybridization of two divergent cichlid species enabled genetic variants to be recombined on a scale which would not otherwise be possible in a single population. According to Dr Joana Meier, first author of the study: “It’s similar to the way the recombination of parts from Lego tractor and aeroplane kits could generate a wide variety of vehicles.”
The hybridization event probably took place around 150,000 years ago, when – during a wet period – a Congolese lineage colonized the Lake Victoria region and encountered representatives of the Upper Nile lineage. Across the large lakes of this region, the hybrid population then diversified in a process known as “adaptive radiation” (evolution of multiple new species adapted to different ecological niches).
The study involved sequencing over 3 million sites in the genome of 100 cichlid species – a task which until recently would not have been feasible. This allowed the group led by Ole Seehausen (head of the Fish Ecology and Evolution department at Eawag and Professor of Aquatic Ecology at Bern University) to provide strong evidence for his theory that hybridization between divergent species, in conjunction with ecological opportunity, can facilitate rapid adaptive radiation. Over a few thousand years, this process gave rise to a complex food web in Lake Victoria, as the new species, in turn, influenced their environment.
Over the past 50 years, changes in land use and wastewater inputs have led to eutrophication of the lake, with increased turbidity and oxygen depletion in deeper waters. Consequently, various species have merged into hybrid populations, as the male nuptial coloration which attracts females of the same species has become less effective; certain deep-water habitats are no longer viable. Some of the lake’s biodiversity and ecological diversity has thus been lost.
A long way from the Great Lakes of East Africa, the diversity of Swiss whitefish species also arose from a hybrid population, emerging in the relatively short period since the last ice age (Hudson et al., 2010). In Swiss stickleback, hybridization has also driven adaptive variation (Lucek et al. 2010, http://dx.doi.org/10.1111/j.1365-294X.2010.04781.x).
Contacts and sources:
Joana I. Meier, David A. Marques, Salome Mwaiko, Catherine E. Wagner, Laurent Excoffier, Ole Seehausen;
Nature Communications, 8: http://dx.doi.org/10.1038/ncomms14363 (2017)