Riverscape genomics of cichlid fishes in the lower Congo: Uncovering
mechanisms of diversification in an extreme hydrological regime
Rivers provide excellent models to understand how species diversity is
generated and maintained across heterogeneous habitats. The lower Congo
River (LCR) consists of a dynamic hydroscape exhibiting extraordinary
aquatic biodiversity, endemicity, and ecological specialization.
Previous studies have suggested that the numerous high-energy rapids
throughout the LCR form physical barriers to gene flow, thus
facilitating diversification and speciation, and generating
ichthyofaunal diversity. However, this hypothesis has not been fully
explored using genome-wide SNPs for fish species distributed across the
LCR. In this study, we examined four species of lamprologine cichlids
endemic to the LCR, of which three are sequentially distributed along
the LCR without range overlap. Using genome-wide SNP data, we tested the
hypotheses that high-energy rapids serve as physical barriers to gene
flow that generate genetic divergence at inter- and intraspecific
levels, and that gene flow occurs primarily in a downstream direction.
Our results are consistent with the prediction that the rapids sometimes
serve to reduce gene flow, but also suggest that at certain temporal and
spatial scales, they may also act as promoters of gene flow.
Furthermore, we detected both upstream and downstream gene flow between
some populations of Lamprologus tigripictilis as well as hybridization
between congeneric species. These results suggest that powerful
high-energy rapids may therefore provide occasional multidirectional
dispersal opportunities for riverine cichlid fishes, highlighting the
complexity of factors driving evolutionary processes in the LCR.