We studied two Corbicula morphotypes in a syntopic population in the Rhine River in order to reveal their taxonomic, reproductive and phylogenetic relationship, using morphometrics, DAF-fingerprinting, mitochondrial COI and nuclear ITS1 sequence variation. Morphometric analysis showed that two statistically distinguishable morphotypes with few intermediates were present. Mitochondrial sequence analysis detected two divergent clades. DAF-fingerprinting revealed three highly distinctive multilocus genotypes. Two of the ultilocus genotypes were significantly associated with different morphotypes and mitochondrial lineages. The third genotype B, however, was found in both morphotypes, intermediates and mitochondrial lineages. Conclusive evidence for hybridization came from RFLP analysis of the nuclear ITS1 locus. We interpret the hybrids as F1 hybrids between different evolutionary lineages. Integration of Corbicula sequences from all over the world into Maximum Parsimony analysis suggested a simultaneous radiation resulting in several evolutionary lineages whose species status remained doubtful. An unequivocal taxonomic assignment of the two evolutionary lineages in the Rhine population was therefore not possible.
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