Geographical ancestry of Lake Malawi's cichlid fish diversity

Abstract

The Lake Malawi haplochromine cichlid flock is one of the largest vertebrate adaptive radiations. The geographical source of the radiation has been assumed to be rivers to the south and east of Lake Malawi, where extant representatives of the flock are now present. Here, we provide mitochondrial DNA evidence suggesting the sister taxon to the Lake Malawi radiation is within the Great Ruaha river in Tanzania, north of Lake Malawi. Estimates of the time of divergence between the Lake Malawi flock and this riverine sister taxon range from 2.13 to 6.76 Ma, prior to origins of the current radiation 1.20-4.06 Ma. These results are congruent with evaluations of 2-3.75 Ma fossil material that suggest past faunal connections between Lake Malawi and the Ruaha. We propose that ancestors of the Malawi radiation became isolated within the catchment during Pliocene rifting that formed both Lake Malawi and the Kipengere/Livingstone mountain range, before colonizing rivers to the south and east of the lake region and radiating within the lake basin. Identification of this sister taxon allows tests of whether standing genetic diversity has predisposed Lake Malawi cichlids to rapid speciation and adaptive radiation.

Environmental context for understanding the iconic adaptive radiation of cichlid fishes in Lake Malawi

“…The authors further suggest that, by this time, the Ruhuhu River functioned as outlet, through a connection with the Ruvuma or Rufiji river systems that both flow into the Indian Ocean. This finding is consistent with previous molecular data suggesting a close relationship between cichlids from these watersheds and Lake Malawi (9,10).…”

Ancestral hybridisation facilitated species diversification in the Lake Malawi cichlid fish adaptive radiation

“…1; supplementary table 1) (Brawand et al 2014;McGee et al 2016;Malinsky et al 2018) . We find strong evidence for ancient hybridisation at the base of the Malawi cichlid radiation, between a lineage related to the LVRS and a riverine lineage previously shown to carry a Malawi-like mitochondrial haplotype (Genner et al 2015) . Furthermore, we show that the majority of the genetic variation produced by this hybridisation event has been lost in Malawi by the relatively rapid fixation at most loci of one or the other parental contribution in the ancestral population of Malawi cichlids.…”

Preface: Advances in cichlid research II: behavior, ecology and evolutionary biology

“…While the cichlid species flocks of the East African Great Lakes are famous both among scientists and aquarists, the cichlid fauna of African (and Middle Eastern) rivers and small lakes is much less known, even though their species richness is, although admittedly less diverse than that of the Great Lakes, high and dramatically underestimated, with numerous recent phylogenetic and phylogeographic studies reporting a large number of hitherto undescribed species (e.g., Schwarzer et al, 2011;Koblmüller et al, 2012;Egger et al, 2015;Genner et al, 2015). Two contributions to this special issue target riverine cichlids in Africa and the Middle East.…”