Abstract
Cockles are highly appreciated mollusks and provide important services in coastal areas. The two European species, edible (Cerastoderma edule) and lagoon (Cerastoderma glaucum) cockles, are not easily distinguishable, especially when young. Interestingly, the species show different resistance to Marteilia cochillia, the parasite responsible for marteiliosis outbreaks, which is devastating cockle production in some areas. C. edule is severely affected by the parasite, while C. glaucum seems to be resistant, although underlying reasons are still unknown. Hybrids between both species might be interesting to introgress allelic variants responsible for tolerance, either naturally or through artificial selection, from lagoon into edible cockle. Here, we used 2b restriction site-associated DNA sequencing (2b–RAD) to identify single nucleotide polymorphisms (SNP) diagnostic for cockle discrimination (fixed for alternative allelic variants). Among the nine diagnostic SNPs selected, seven were validated using a SNaPshot assay in samples covering most of the distribution range of both species. The validated SNPs were used to check cockles that were suggested to be hybrids by a claimed diagnostic tool based on the internal transcribed spacers of the ribosomal RNA. Although these were shown to be false positives, we cannot rule out the fact that hybrids can occur and be viable. The SNP tool here developed will be valuable for their identification and management.
Highlights
True cockles are members of the genus Cerastoderma, a group of bivalve mollusks widely distributed in the Mediterranean and Atlantic coasts of Europe [1]
This parasite was first recorded in that area in 2012 [4], but its first description was reported in Catalonia (NE Spain) from cockles identified as C. edule [5], despite the typical cockle species occurring in the Mediterranean coasts is the lagoon cockle (Cerastoderma glaucum (Poiret, 1789)) [1]
The known geographic ranges of both Cerastoderma species [1] supports the fact that cockles occurring in the Mediterranean Sea are C. glaucum, and the cockles infected with M. cochillia in Catalonia should be C. glaucum
Summary
True cockles are members of the genus Cerastoderma, a group of bivalve mollusks widely distributed in the Mediterranean and Atlantic coasts of Europe [1]. Edible cockle production has been severely affected by marteiliosis outbreaks caused by Martelia cochillia, especially in Galicia (NW Spain), where the parasite has devastated its production in the Southern estuaries (Rias) This parasite was first recorded in that area in 2012 [4], but its first description was reported in Catalonia (NE Spain) from cockles identified as C. edule [5], despite the typical cockle species occurring in the Mediterranean coasts is the lagoon cockle (Cerastoderma glaucum (Poiret, 1789)) [1]. Nine markers fixed for alternative allelic variants identified in “pure” individuals of the two species were validated in samples covering a wide distribution range and tuned up in a SNaPshot assay designed to provide a cheap and straightforward diagnostic tool This SNP panel will be useful for management of extensive and intensive production aGreenaess2w01h9,e1r0e, 7b6o0th species co-occur, and to address more sophisticated tools for studying the resist4aonfc1e1 of cockles to marteiliosis. Figure 11..GGeeooggrarapphhiciacal lsistuitautaiotinonofothf ethneatnuaratul rbaeldbs eadnsalaynzeadlyiznedtheinprtehseenptrsetsuednitess.tuCdoidese.s Caroedsehsowarne oshnoTwabnleon.Table 1
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