Astatotilapia sp. 'ruaha blue'
15-Jun-2015 — The ancestor to all Malawi haplochromines
By Ad Konings
A few days ago a very important paper was published by Genner et al. (2015) who looked at the genetic relationships of haplochromines in- and outside of Lake Malawi using mitochondrial DNA. For a very long time it had been assumed that Astatotilapia calliptera would be the logical ancestor to all Malawi cichlids, as it occurs inside as well as outside the lake, mostly in southern and eastern rivers. However, Genner and colleagues found that comparing the Lake Malawi species flock with a large number of riverine Astatotilapia from various drainage systems that an as yet undescribed species (A. sp. ‘ruaha’) is sister to all Malawi haplochromines, including A. calliptera. They found this ancestral species in the Great Ruaha River which is to the northeast of Lake Malawi. Due to different sediment depositions on the lake bottom it was already known that Lake Malawi was formed starting in the north and expanded towards a southern direction. Genner et al. (2015) make it plausible that the formation of the Livingstone Mountains (Kipengere Range), located along the northeastern shore of the lake, caused the split in the Ruaha drainage system with the southern part becoming Lake Malawi. The Ruaha cichlids, then in a novel environment, i.e. a deep lake, underwent rapid speciation and eventually also produced A. calliptera. This species subsequently invaded the rivers that flowed into the lake and even got isolated in the Ruvuma River which forms the border between Tanzania and Mozambique and has no longer a connection with Lake Malawi. All A. calliptera they tested fell within the Malawi species flock, even an A. calliptera population they found in Lake Chidya which is in the Ruvuma drainage system about 30 km away from the Indian Ocean and almost 600 km in a straight line east of Lake Malawi! Now that they know this ancestral species in the Ruaha they can examine whether the genetic variability of the huge Malawi radiation was already present in the riverine ancestral form or that this came about due to a deep lake environment.
Genner, Martin & B.P. Ngatunga, S. Mzighani, A. Smith, G.F. Turner. 2015. "Geographical ancestry of Lake Malawi's cichlid fish diversity". Biology Letters. 11(6):1-12. DOI: 10.1098/rsbl.2015.0232 (crc06744) (abstract)