Microsatellite DNA sequences were isolated from the cockle, Cerastoderma edule
(L.), genome by employing two separate methods of isolation allowing for the
development of suitable rnicrosatellite DNA markers for population genetic analysis.
Six microsatellites were isolated from a non-enriched C. edule partial genomic library after hybridization to specific repetitive probes using a commercial non-radioactive screening kit. Microsatellites isolated by this method were generally unsuitable as DNA markers owing to an insufficient number of repeat units or their mononucleotide structure. However, one microsatellite displayed similarity to a telomeric microsatellite and was developed as a potential DNA marker. Amplification of this microsatellite produced multiple banding patterns suggesting either multi-locus or non-specific amplification.
An alternative method, involving enrichment of C. edule insert DNA, resulted in the isolation of 17 microsatellite sequences. Nine of these microsatellites were developed as potential DNA markers. Only two primer pairs produced unambiguous banding patterns which were consequently used in a preliminary investigation to assess their applicability as genetic markers for stock structure analysis of this commercially important species. Additionally, one primer pair revealed a second region of polymorphic variation indicative of a dinucleotide repeat.
The genetic structure of C. edule populations from the UK (Wash, Thames, Dwyryd, Dovey, Traeth Melynog and Morecambe) were assessed at three polymorphic microsatellite loci. Low levels of polymorphism were observed but this was attributed mainly to one locus, Cedl 59, which displayed little variability. Mean observed heterozygosity across all populations and loci was 0.31 ± 0.11. Heterozygote deficits were observed at one locus, Ced213 b, in most populations possibly due to null alleles or inadequate sample sizes. Fsr and Rsr-based estimates provided little evidence to suggest population subdivision, however, heterogeneity in allele frequencies were detected at locus Ced213b. UPGMA cluster analysis based on Nei's genetic distance indicated that the Wash population was distinct however, cluster analysis based on delta mu indicated that the Thames population was genetically distinct. Estimations of the number of migrants per generation between populations based on pairwise FsT values suggested considerable gene flow occurs between populations.
Based on these data, little evidence exists to suggest differentiation of cockle
populations. Estimations of migration levels between populations appears high
providing little evidence to suggest the presence of isolated stocks. Recruitment of larvae from neighbouring areas is therefore proposed but this study is largely
inconclusive based on the limited number of populations and loci used.