Genetic evidence of the unique identity of the West African Mangrove Oyster (Crassostrea tulipa) from the Gulf of Guinea

Abstract

This research enabled the genetic identification of the West African mangrove oysters, Crassostrea tulipa, as well as establishing the evolutionary relationship between it and other Crassostrea species. Essentially, the study assisted in clearing up a long-standing confusion over this species’ synonymy with C. gasar. Also, the population structure of 60 C. tulipa individuals, from three different ecotypes, was analyzed using the mitochondrial cytochrome oxidase I (COI) genes as a marker. Results provided the first genetic sequences for C. tulipa and deposited in the GeneBank. Optimal and consensus bootstrap Neighbor-Joining trees distinctively differentiated C. tulipa from other Crassostrea species and consistently formed a different clade with C. gasar, with no bootstrap value from either NJ, MPT, or UPG trees supporting their similarity. C. tulipa sequences occurred as different haplotypes from other Crassostrea sp, with a mutation value as high as 288 and a haplotype diversity of 0.893 between C. tulipa and C. gasar sequences. High estimates of genetic distance (1.40–1.55) and patristic divergence were recorded between C. tulipa and C. gasar, in the same range as with seven other species. The study thus reveals the unique identity of C. tulipa as genetically distinct from C. gasar and other Crassostrea species. Based on the population structure analyses from the neutrality test, a low to high haplotype diversity h (0.000–0.963) and low nucleotide diversity π (0.00–0.378) were obtained. A negative mean Tajima’s D (−0.65247), and a positive Fu’s Fs (3.194), suggest rare variations or low-frequency polymorphisms. In addition to serving as the basis for phylogeny, the identification of C. tulipa and the recent data on its population structure also serve as the basis for conservation efforts and hatchery-based aquaculture.