GENETIC-POLYMORPHISM OF A HEMOGLOBIN CHAIN AND ADENOSINE-DEAMINASE INEUROPEAN SHADS - EVIDENCE FOR THE EXISTENCE OF 2 DISTINCT GENETIC ENTITIES WITH NATURAL HYBRIDIZATION

The degree of genetic differentiation between European shads, Alosa al osa and A. fallax, was studied in populations from different Portugues e hydrographic basins. Using isoelectric focusing, polymorphic variati on was detected in a haemoglobin (HE) chain and in adenosine deaminase (ADA). These polymorphisms are shared by the two species but show sign ificant differences in their gene frequency distributions. Individuals with intermediate morphological characteristics were found to exhibit intermediate allele frequencies. While not completely excluding intro gression, our results strongly support the existence of two distinct s pecies that can hybridize. (C) 1996 The Fisheries Society of the Briti sh Isles