| dc.description.abstract |
Recent decline in Nile perch abundance in Lake Victoria as deduced from decreasing catches is a concern to the fishing industry and conservation biology, as population on the decline is expected to lose most of its genetic variation and because diversity and survivorship correlate positively, this may threaten its survival and evolutionary potential. To determine the level to which this decline threatens the persistence of this species, a study was designed to quantify the level of genetic variation and population structure (existence of mating subgroups, diagnosed by allele frequency differences) in Lake Victoria. Further, the number of species of Nile perch in Lake Victoria was investigated, as previous morphometric and allozyme studies lacked adequate power to clearly identify species. To identify species differences, variation at mitochondrial Cytochrome C oxidase 1 (CO1), a gene used as a basis for a global identification system for animals (barcode), was sequenced in a panel of 163 samples comprising individuals from five locations transecting the shoreline of the Kenyan gulf of Lake Victoria: Sio Port, Dunga beach, Mbita point, Mfangano Island, and Karungu bay, and 21 individuals from Lakes Albert (Uganda) and Turkana (Kenya). Sequences were compared among samples in Victoria and reference samples from Albert and Turkana, and data from a public sequence repository (NCBI) of other Lates species in the Asian seas. Results showed the species to harbor no sequence variation at all within Lake Victoria at the CO1 gene. All samples from Victoria were characterized by a single invariable haplotype, the same haplotype fixed for Lake Turkana and matched the only available database sequences of Lates niloticus from Tanzania. Lake Albert, however, exhibited two haplotypes varying in a single T-C transition, one unique while the other found in Victoria and Turkana. These results suggested that Nile perch of Lake Victoria comprised one species, Lates niloticus. Genetic diversity was quantified using heterozygosity and mean number of alleles, while genetic structure was analyzed using both differentiation at allele frequencies and multilocus genotypes from nuclear microsatellites to delineate genetic populations using an individual-based approach that apportions individuals to their source populations using genotypes and Bayesian modeling. Genetic variation across the seven microsatellites was low with average number of alleles per locus = 5. Observed heterozygosity (HO) for Victoria populations was 0.848, while the rarefacted mean number of alleles (allelic richness) ranged from 2.6 to 3.3. HO was consistently and significantly higher than HE for all populations and overall (p < 0.05). Patterns of allelic distribution showing fewer alleles relative to a wider allele size range suggested a recent population bottleneck, apparently from recent introduction of few founders, perhaps confounded by a limited sampling of a narrow geographic range for this study. Genetic differentiation was low (FST = 0.054; p ≥ 0.05) indicating an absence of genetic structure. Similarly, Bayesian analysis of genetic structure showed all samples to belong to one population. In order to ascertain whether variation resided within quantitative traits previously shown to have adaptive significance in other aquatic species, population divergence and differentiation was analyzed at meristic and morphometric characters. The number of gill rakers, counts of rays in pectoral fins and a suite of morphometric characters showed some level of divergence between two populations delineated from a pool of samples from Lake Victoria and two reference lakes, Albert and Turkana. Despite appearing to diverge at certain morphological traits, the evidence of low genetic diversity and a lack of detectible genetic differentiation among populations is a theoretical pointer to potential loss of alleles, usually affecting the long term persistence. However, rangewide sampling will be required to confirm these conclusions. This work was motivated by the need to quantify the extent to which reduction in census has impacted on its genetic variability. In view of current findings, measures aimed at protecting the perch should consider the heterozgygote excess as a healthy outcome, but the low sub-population differentiation suggests to a single species within the lake, and that all geographic regions can be considered under the same management regime. |
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