Genetic diversity and population structure of Oreochromis mossambicus in the Lower Shire river system of Malawi

The Lower Shire is renowned for large biomasses of O. mossambicus, O. placidus, Clarias spp and Bargus spp. The study investigated the genetic variability and population structure of O. mossambicus populations in three strata at six microsatellite loci. The populations exhibited considerable allelic variability with a mean of 21 alleles per locus, a total allele number range of 4–40 and allele size range of 68–103 base pairs. Three diversity indices (observed number of alleles (na), effective number of alleles (ne) and Shannon Information Index (I), revealed that Lisuli and Magamba were the least and most genetically diverse populations respectively. Based on the three diversity indices, the three strata exhibited similar allelic diversity. Most populations (71%) were not in HWE probably due to sampling error caused by Wahlund effect. This assertion is supported by 73% homozygosity excess and high gene flow observed among the populations (Nm=3). Mantels test showed that genetic relationships among the populations are less influenced by geographical distance (r = 0.23; p=0.9410) implying that the populations do not fit into the isolation by distance model. Nevertheless, the populations are significantly differentiated (p≤0.05) and moderately structured (FST = 0.08). Conservation efforts, therefore, need to concentrate on each population since the populations are still distinct though going toward homogeneity given the high gene flow.