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University of Basel

18 November 2020

Cichlid fishes from African Lake Tanganyika shed light on how organismal diversity arises

A cichlid from Lake Tanganyika
Ophthalmotilapia ventralis, a cichlid from Lake Tanganyika. (Photo: Adrian Indermaur, University of Basel/Zoology)

Lake Tanganyika in Africa is a true hotspot of organismal diversity. Approximately 240 species of cichlid fishes have evolved in this lake in less than 10 million years. A research team from the University of Basel has investigated this phenomenon of “explosive speciation” and provides new insights into the origins of biological diversity, as they report in the journal “Nature”.

3D-structure of a lower pharyngeal jaw bone of a cichlid from Lake Tanganyika.
3D-structure of a lower pharyngeal jaw bone of a cichlid from Lake Tanganyika. (Image: Fabrizia Ronco/Halil Kesselring, University of Basel/Zoology)

Based on this material, they compiled a comprehensive dataset covering information on morphology, ecology, and genetics. For example, the team analyzed body shape and jaw morphology of all species using X-ray imaging and high-resolution computed tomography. The zoologists were particularly interested in the three-dimensional structure of the pharyngeal jaw. This second set of jaws is situated in the pharynx of cichlid fishes and is used to masticate food, allowing the fish to specialize in very specific nutritional niches.

Because adaptation to different environments is a central component of adaptive radiation, the researchers also quantified the “ecological niche” used by each species. In collaboration with the Botanical Institute of the University of Basel, they measured the stable carbon and nitrogen isotope composition in the muscle tissue of the fish. These measurements allow to determine in what habitat the fishes lived and what food resources they used. Further, the team sequenced two complete genomes for each and every cichlid species from Lake Tanganyika. Based on this molecular information they were able to reconstruct the complete phylogeny of the cichlids in that lake.

Based on their analyses, the scientists could demonstrate that the evolution of cichlid fishes in Lake Tanganyika was not a gradual process, but rather occurred in three discrete pulse-like stages of rapid morphological evolution. “Each of these stages was characterized by specialization to a different aspect of the habitat provided by the lake,” says lead author Dr. Fabrizia Ronco. The first pulse involved diversification in body shape followed by a pulse in mouth morphology and a final pulse in pharyngeal jaw shape. Especially the pharyngeal jaw has played a key role in the radiation, since its rapid morphological evolution coincided with a high number of speciation events.

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