Interspecific differentiation and intraspecific substructure in two closely related clupeids with extensive hybridization,Alosa alosaandAlosa fallax

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Abstract
In this study, the evolutionary relationships within and among populations of European shads,Alosa alosaandAlosa fallax, was investigated. Screening of allelic variation across eight allozyme loci and sequencing 448 bp of the mtDNA cytochromebgene in 14 rivers throughout the range of the species supported that the two taxa were independent lineages (1·3% net nucleotide divergence) despite extensive hybridization. Genetic diversity and structure was considerably higher inA. fallaxthanA. alosaand the former species revealed evidence of distinct lineages in the Mediterranean and Atlantic basins. A Bayesian clustering approach combined with gill raker counts verified that individuals of the two species could be assigned to their parent group with relatively high confidence. Evaluation of hybridization in the Lima and Mondego Rivers in Portugal provided evidence that introgression is extensive but is not currently obscuring (through hybrid swarming) the diagnosability of the two species.