Diversification and gene flow of tilapia species driven by ecological changes in lowland and mountain areas of southern Mauritania

Abstract

Haplotilapiine are members of Cichlidae (cichlid fishes), one of the most species-rich vertebrate families. Many haplotilapiines diversified via allopatric divergence, sexual selection, hybridization and ecological adaptation, making them excellent models for evolutionary research. One extraordinary example of how haplotilapiine diversified are the species found within the Sahara desert, surviving in isolated wetlands for thousands of years. Seasonal, and longer, climate cycles in these areas have resulted in periods of connectivity and isolation within and between lowland and mountain regions via ephemeral rivers, which highly impacts the potential for migration of water-species. Here we studied how ecological variability and secondary contact have affected the population genetics of haplotilapiine cichlid fishes (Sarotherodon galilaeus), in the lowland Karakoro sub-basin and highland Afollé mountain regions of Mauritania. We used DNA-sequence data of mitochondrial (ND2, N\u2009=\u200959) and nuclear (S7 1st intron, N\u2009=\u200932) genes, and microsatellite markers data (13 novel loci developed for Sarotherodon, N\u2009=\u200961). Our results based on microsatellite data showed two genetically differentiated lowland groups of S. galilaeus that exist in sympatry. Absence of one of these groups in mountain areas can be due to small sample size or local extinction. We found no significant genetic differentiation between lowland and mountain based on microsatellite and mtDNA data, supporting our hypothesis of recent, downstream gene flow. As expected, genetic diversity was significantly lower in mountain population, which can be due to different factors, including stochastic effects or downstream migration increasing diversity via gene flow. Ecological changes (seasonal and long-term) have likely driven divergence and posterior secondary contacts on the studied Sarotherodon lineages at multiple times, leaving open questions for future studies about the specifics of these evolutionary processes. Moreover, the pattern of genetic diversity in lowland and mountain populations highlights the importance of protecting geographically isolated areas for long-term persistence of tilapia species.