Genetic polymorphism of a haemoglobin chain and adenosine deaminase in European shads: evidence for the existence of two distinct genetic entities with natural hybridization

The degree of genetic differentiation between European shads, Alosa alosa and A. fallax , was studied in populations from different Portuguese hydrographic basins. Using isoelectric focusing, polymorphic variation was detected in a haemoglobin (HB) chain and in adenosine deaminase (ADA). These polymorphisms are shared by the two species but show significant differences in their gene frequency distributions. Individuals with intermediate morphological characteristics were found to exhibit intermediate allele frequencies. While not completely excluding introgression, our results strongly support the existence of two distinct species that can hybridize.