Genetic and cytological evidence of hybridization between native Ruditapes decussatus and introduced Ruditapes philippinarum (Mollusca, Bivalvia, Veneridae) in NW Spain

The introduction of foreign species in marine ecosystems is an established method to increase productivity and generate income in aquaculture. A good example is the introduction of the Japanese carpet shell, Ruditapes philippinarum, now the major contributor to clam landings in Europe. The similarity of R. philippinarum and the native grooved carpet shell, R. decussatus, the plasticity of their shell morphology and the presence of both species on the same beds make it very difficult sometimes for the morphological identification of specimens that show intermediate shell characteristics. A species identification method using length differences in the internal transcribed spacer (ITS) region of the major ribosomal RNA genes and the whole repeat sequence of the minor ribosomal RNA genes (5S rDNA) was developed for Ruditapes. Following morphological recognition, this method enabled the identification of both species of Ruditapes. However, nine of the 222 individuals morphologically identified as R. decussatus had sequences specific to both R. decussatus and R. philippinarum in both the ITS region and the 5S rDNA and were putative hybrids. Species-specific ITS and 5S rDNAs were used as probes in fluorescent in situ hybridization (FISH) experiments to map these sequences on surface spread synaptonemal complexes (SCs) of R. decussatus and R. philippinarum. Differences on the chromosomal location of those sequences between species were also detected. FISH on the SCs of two of the nine hybrid individuals also showed signal patterns corresponding to those detected in both R. decussatus and R. philippinarum thus confirming their hybrid nature. The hybrids showed 16S mitochondrial rDNA sequences of R. decussatus. In conclusion, genetic and cytological evidence of the occurrence of hybridization between female R. decussatus and male R. philippinarum was obtained.