Assessment of Genetic Diversity, Gene Flow and Demographic History of Frigate Tuna (Auxis thazard) Populations in Tanzanian Marine Waters Using Mitochondrial DNA Control Region

Aim: To investigate genetic diversity, gene flow and demographic history of frigate tuna (Auxis thazard) populations found in Tanzanian marine waters.

Study Design: The study used a descriptive research design whereby fish samples were randomly collected from four locations and the genetic variation within and among the four populations analyzed using mitochondrial DNA (mtDNA) control region.

Place and Duration of the Study: Fish samples were collected from landing sites in Tanga, Dar es Salaam, Mtwara and Zanzibar, Tanzania. The study was conducted between July 2020 and June 2021.

Methodology: A total of 100 frigate tuna were randomly sampled from small-scale fishermen at the landing sites of Dar-es-Salaam (20), Tanga (30), Mtwara (30), and Zanzibar (20). For each fish, 50 g muscle tissue was obtained and put in a vial containing 95% ethanol. DNA was extracted from the muscle using a commercial DNA Kit (Quick-DNATM Miniprep Plus Kit, Zymo Research Corp.) according to the instructions of the manufacturer and a fragment of 432 bp of the mtDNA control region was amplified and sequenced using ABI PRISIM™ 3100 Genetic Analyzer (Applied Biosystems). Haplotype and nucleotide diversity, gene flow and historic demographic were estimated from 92 fish samples.

Results: A total of 88 haplotypes were identified in all fish samples. The highest haplotype diversity was found in Zanzibar (1.000 ± 0.017) and Mtwara (1.000 ± 0.010) populations while the lowest was observed in Tanga population (0.992 ± 0.012). Tanga population had the highest nucleotide diversity (0.078 ± 0.018) while Dar es Salaam had the lowest (0.016 ± 0.009). The highest genetic differentiation (FST) was found between Tanga and Dar-es-Salaam (0.178) and the lowest was observed between Mtwara and Zanzibar (0.016). The genetic distances between pairs of populations were small. The phylogenetic tree revealed two main clusters. Cluster 1 which consisted of nine haplotypes was dominated by Tanga population with seven haplotypes while the remaining two haplotypes were from Mtwara and the reference sequences of Euthynnus affinis (Kawakawa tuna). Cluster II had 84 haplotypes of individuals from all four populations, with no population specific subcluster. The number of immigrants per generation was highest between Mtwara and Zanzibar (Nm=18.310) and lowest between Tanga and Dar-es-Salaam (Nm=1.180). The neutrality test indicated negative values, suggesting a recent population expansion.

Conclusion: There is high genetic diversity within the populations, but there is no significant genetic differentiation among the four frigate tuna populations, suggesting that the four populations comprise a single panmictic population.