Rapid isolation and characterization of polymorphic microsatellite loci in the mud crab, Scylla paramamosain (Portunidae)

Genetic diversity and connectivity in the East African giant mud crab Scylla serrata: Implications for fisheries management

The giant mud crab Scylla serrata provides an important source of income and food to coastal communities in East Africa. However, increasing demand and exploitation due to the growing coastal population, export trade, and tourism industry are threatening the sustainability of the wild stock of this species. Because effective management requires a clear understanding of the connectivity among populations, this study was conducted to assess the genetic diversity and connectivity in the East African mangrove crab S. serrata. A section of 535 base pairs of the cytochrome oxidase subunit I (COI) gene and eight microsatellite loci were analysed from 230 tissue samples of giant mud crabs collected from Kenya, Tanzania, Mozambique, Madagascar, and South Africa. Microsatellite genetic diversity (H_e) ranged between 0.56 and 0.6. The COI sequences showed 57 different haplotypes associated with low nucleotide diversity (current nucleotide diversity = 0.29%). In addition, the current nucleotide diversity was lower than the historical nucleotide diversity, indicating overexploitation or historical bottlenecks in the recent history of the studied population. Considering that the coastal population is growing rapidly, East African countries should promote sustainable fishing practices and sustainable use of mangrove resources to protect mud crabs and other marine fauna from the increasing pressure of exploitation. While microsatellite loci did not show significant genetic differentiation (p > 0.05), COI sequences revealed significant genetic divergence between sites on the East coast of Madagascar (ECM) and sites on the West coast of Madagascar, mainland East Africa, as well as the Seychelles. Since East African countries agreed to achieve the Convention on Biological Diversity (CBD) target to protect over 10% of their marine areas by 2020, the observed pattern of connectivity and the measured genetic diversity can serve to provide useful information for designing networks of marine protected areas.

Transcriptome analysis of the mud crab (Scylla paramamosain) by 454 deep sequencing: assembly, annotation, and marker discovery

In this study, we reported the characterization of the first transcriptome of the mud crab (Scylla paramamosain). Pooled cDNAs of four tissue types from twelve wild individuals were sequenced using the Roche 454 FLX platform. Analysis performed included de novo assembly of transcriptome sequences, functional annotation, and molecular marker discovery. A total of 1,314,101 high quality reads with an average length of 411 bp were generated by 454 sequencing on a mixed cDNA library. De novo assembly of these 1,314,101 reads produced 76,778 contigs (consisting of 818,154 reads) with 5.4-fold average sequencing coverage. The remaining 495,947 reads were singletons. A total of 78,268 unigenes were identified based on sequence similarity with known proteins (E≤0.00001) in UniProt and non-redundant protein databases. Meanwhile, 44,433 sequences were identified (E≤0.00001) using a BLASTN search against the NCBI nucleotide database. Gene Ontology (GO) analysis indicated that biosynthetic process, cell part, and ion binding were the most abundant terms in biological process, cellular component, and molecular function categories, respectively. Kyoto Encyclopedia of Genes and Genome (KEGG) pathway analysis revealed that 4,878 unigenes distributed in 281 different pathways. In addition, 19,011 microsatellites and 37,063 potential single nucleotide polymorphisms were detected from the transcriptome of S. paramamosain. Finally, thirty polymorphic microsatellite markers were developed and used to assess genetic diversity of a wild population of S. paramamosain. So far, existing sequence resources for S. paramamosain are extremely limited. The present study provides a characterization of transcriptome from multiple tissues and individuals, as well as an assessment of genetic diversity of a wild population. These sequence resources will facilitate the investigation of population genetic diversity, the development of genetic maps, and the conduct of molecular marker-assisted breeding in S. paramamosain and related crab species.

Characterization of 40 single nucleotide polymorphism (SNP) via Tm-shift assay in the mud crab (Scylla paramamosain)

In this study, single nucleotide polymorphism (SNP) were identified, confirmed and genotyped in the mud crab (Scylla paramamosain) using Tm-shift assay. High quality sequences (13, 311 bp long) were obtained by re-sequencing that contained 91 SNPs, with a density of one SNP every 146 bp. Of all 91 SNPs, 40 were successfully genotyped and characterized using 30 wild specimens by Tm-shift assay. The minor allele frequency per locus ranged from 0.017 to 0.500. The observed and expected heterozygosity, and polymorphism information content (PIC) ranged from 0.000 to 0.600, from 0.033 to 0.509, and from 0.033 to 0.375, respectively, with an average of 0.142, 0.239 and 0.198 per locus. Seventeen SNPs were significantly deviated from Hardy-Weinberg equilibrium. No significant linkage disequilibrium between pairs of loci was detected after sequential Bonferroni correction (P > 0.00125). Seventeen SNPs were related with known function genes. This study provided new molecular markers for investigation of population genetic diversity, construction of genetic linkage maps and molecular marker-assisted selection in this important crustacean species.