Name: Dilyte J
Co-supervisors: Alexandrino P, Sabatino S
Title: Population structure and gene flow in desert environment: an application of molecular tools to isolated fish populations in West Africa
Institution: University of Porto
The Sahara desert fluctuated between desert and tropical conditions over the past seven million years, which has had a profound effect on the evolution of its flora and fauna. Desertification of the Sahara appears to have promoted species diversification in some cases. This diversification is probably due to the repeated isolation of lineages in extreme environmental conditions, which can promote ecological adaptation and divergence.
The diversification of species via ecological adaptation seems to be particularly important in aquatic taxa that are distributed in lowland and mountain areas, as they are often isolated and connected through changes in stream and river drainages. Changes in precipitation in desert areas are often extreme and occur seasonally as well as decadal and much longer scales (e.g., the Pleistocene glaciations). Consequently, many relict fish populations survived in those habitats with extreme climates.
The main goal of this research was to measure population structure and genetic diversity in Tilapia fishes from the mountain and lowland regions of Mauritania. We chose to study Tilapias to help to determine if the desertification of the Sahara has impacted rates of divergence of aquatic species by altering patterns of dispersal and variance and promoting ecological adaptation. The Tilapia specimens collected for this research were obtained from rivers and streams across North-West Africa, including both coastal and inlands areas with a focus on locations within Mauritania. Molecular phylogenetic analysis was used to help determine the species to which each of our samples was a member. Specifically, were conducted a phylogenetic analyses of collected specimens using mitochondrial DNA (16S and ND2) and nuclear DNA (nDNA) (1st intron of S7) sequence data. The results showed that Tilapia samples consisted of individuals from two tribes: Oreochromini and Coptodonini. Each tribe contained two genera: Sarotherodon (A and B lineages) and Coptodon (A and B lineages), with their distribution in coastal (A) and inland (B) waters. Mauritania inland waters contained the members of Sarotherodon (B) and Coptodon (B) genera, while Sarotherodon B was predominant in mountains and lowlands. The population-level analyses were focused on the individuals of Sarotherodon A and B lineages, due to their close genetic relationship based on the nuclear locus used for phylogenetic analyses.
Thirteen novel microsatellite loci were developed to answer the main study question. Six statistically significant genetic groups (FST range: from 0.1209 to 0.6413, P<0.005 after Bonferroni correction) were found, two of which were genetically differentiated populations: Morocco and coastal Mauritania. While the rest four groups were identified within Mauritania inland. Two groups out of four had broad and sympatric distribution, and were distinct from the rest of Mauritanian populations at different genetic levels.
The main study finding was the Affolé mountain population isolation from the western Mauritania populations (FST 0.1209). The Affolé population did not exhibit any private alleles, which could be a signal of the recent bottleneck. Tilapia population dynamics appear to generally be related to the hydraulic network in each sub-basin but still sample size disallowed us to formerly test this hypotheses. Besides, in Karakoro sub-basin were found individuals from different populations. The results of this study help to understand the phylogenetic relationships among Tilapias in North and West Africa. The diversification of Tilapias appears to be related to their isolation in extreme environments. The isolation of lineages of Tilapia may be due to their dispersal abilities and patterns of connectivity in regions where they are found, which are sensitive to changes in precipitation on yearly and longer time scales.