Inter-specific hybridization in the fish family Cyprinidae
The breakdown of reproductive isolation leading to inter-specific hybridization is a widespread phenomenon amongst cyprinid fishes. There are seventeen cyprinid species occcurring in the British Isles, within five sub-families, giving rise to some ten different types of hybrid. Most of these belong to the sub-family Leuciscinae. The most commonly occurring hybrids are probably those between roach, Rutilus rutilus (L.), and common bream, Abramis brama (L.). In this study the phenomenon of hybridization amongst species of the Cyprinidae was investigated through: an experimental breeding programme to investigate the nature of inter-species and hybrid gamete compatibilities; the identification of species and hybrids from the experimental breeding programme through the analysis of morphometric, meristic and genetic characters (genetic characters were analysed using enzyme electrophoresis); the comparison of morphmetric, meristic and genetic information of natural fish with similar features of fish from the experimental breeding programme to identify the occurrence of post Fl hybridization in natural populations; the use of restriction enzyme analysis of mitochondrial DNA to elucidate the importance of maternal ancestry in a natural hybrid population. The breeding programme found, for the species in this study, that there was no success in cross-fertilization of taxa between different sub-families. Interspecific gamete compatibility was only found within the leuciscine sub-family. In cases where a hybrid cross produced progeny it was also noted that the reciprocal cross was successful. This suggests that there is not a genetic barrier to gamete compatibility resulting from the sexual directionality of a hybrid cross. Female roach/common bream hybrids also produced progeny when crossed with males of leuciscine species. Identification of the progeny of the experimental breeding programme showed that the genetic techniques of enzyme electrophoresis was more reliable than the statistical analysis of meristic and morphometric traits in the identification of species and their Fl hybrids. However, genetic information alone cannot establish precisely the nature of post Fl hybrids and in the identification of backcrossed roach/common bream hybrids it was noted that meristic information was needed to support genetic data. In the two natural hybrid populations of roach/common bream and rudd/common bream, from the Forty Foot Drain and Essex University Lake respectively, the analysis of morphometric, meristic and genetic characters found no evidence of post Fl hybridization in these waters. It is suggested that absence is due to either the limitations of the sampling methods or biological processes. Possible biological processes include factors such as the inappropriate mating behaviour of Fl hybrids or the inferior fitness of post Fl hybrids. The analysis of mitochondrial DNA did not yield sufficient results to elucidate the importance of maternal ancestry in hybridization. It is suggested that this aspect of hybridization is of such critical importance that it must become the subject of a future research programme. The importance of the causes and consequences of inter-specfic hybridization in fishes are discussed. It is suggested that, because they are rarely investigated in hybrid studies, these become incorporated into research programmes in the future. These areas of investigation will have implications for fisheries management, freshwater ecology, genetic conservation and species integrity.