Abstract:
The genus Oreochromis represents a radiation of mouth-breeding Tilapiine fish inhabiting lakes and rivers throughout Africa. Due to incomplete reproductive isolation, most of the 31 species within this radiation can interbreed giving rise to fertile F1 hybrids. Oreochromis mossambicus is endemic to southern Africa but is now coming under threat throughout much of the sub-region because of the introduction of invasive Oreochromis species, which typically inhibit other parts of Africa. Due to their exceptional growth rates, invasive species were brought to South Africa for the aquaculture industry, and it is feared that they may have hybridized with or displaced O. mossambicus. This study aims to determine, using genetics, the extent of invasion of non-native Oreochromis species into Nandoni and Albasini dams of the upper Levubu in Limpopo South Africa. It is suspected that O. niloticus entered the River after the creation of Albasini Dam, which is upstream of Nandoni Dam. Therefore, it is predicted that typically O. niloticus mtDNA haplotypes will be observable in Nandoni, but that their frequency should be much lower upstream in Albasini Dam. I collected 141 samples from both dams, amplified and sequenced the control region of mitochondrial DNA. I then reconstructed networks and phylogenies with our samples combined with the downloaded samples from which I was able to determine the magnitude of Oreochromis invasion into the upper Levubu. Surprisingly, not one of the sequenced samples possessed a haplotype that clustered with O. mossambicus reference samples. However, I was able to identify two invasive species within the upper Levubu: O. andersonii in both Albasini and Nandoni dams and O. niloticus in Nandoni Dam. Oreochromis andersonii has high genetic diversity and with evidence of demographic expansion based on results from its mismatch distribution and Bayesian skyline plot. These results provide insights into the events that led to the invasion of foreign Oreochromis species to the Levubu system. A genetic signal for a demographic
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expansion might have been caused by O. andersonii haplotypes being in the system before the “big flood” in the year 2000, with a re-introduction into Nandoni after the flood from a different source. This could explain why some haplotypes of O. andersonii are present in both dams and some are only present in Nandoni Dam. Oreochromis niloticus on the other hand, has low genetic diversity in Nandoni Dam compared to downloaded samples, and was probably introduced only once, and may have undergone a demographic bottleneck.
From these results, it is clear that O. mossambicus has been all but replaced by non-native Oreochromis in the upper Levubu. Hybridization or total replacement of O. mossambicus may have also occurred in another river system across its native range. O. mossambicus is better adapted to poorer eutrophic conditions and, most importantly it is well adapted to high salinity. Therefore, a strategy conserving genetically diverse O. mossambicus population in the lower reach of the river system, where there is higher salinity, like the lower Changane river, could be most appropriate for this species. The estuarine swamps could then become a refuge for O. mossambicus within its native range.
