Allozymic evidence of parapatric differentiation of brown trout (Salmo trutta L.) within an Atlantic river basin of the Iberian Peninsula

The genetic variation of brown trout from Duero, one of the main Atlantic I berian river basins, was assessed at 34 enzymatic loci in 62 native populat ions. A strong intrabasin differentiation was detected (G(ST) = 0.46; range D:0-0.066), mainly attributable to the existence of two divergent groups o f populations within Duero: southern and northern groups. This divergence w as mainly a consequence of the unequal distribution of *75 and *100 alleles at sMDH-B1,2* isoloci, which were correlated with substantial differences in genetic diversity among regions. The Lower Course region (nearly fixed f or the *100 allele) and Pisuerga River (nearly fixed for the *75 allele) sh owed lower heterozygosities (H approximate to 0.8%) in contrast with adjace nt areas, which evidenced intermediate frequencies for both alleles and hig her heterozygosities (H: 2.2-3.1%). Vicariance appeared as the more probabl e explanation for the significant positive correlation detected between gen etic and geographical distances in Duero Basin. Genetic relationships with adjacent Iberian drainages indicate a close similarity between the southern group and Cantabric trout, whereas the northern group constitutes an ancie nt form from this basin. This study confirmed complex genetic relationships in brown trout from northwest Iberia, reasserting the existence of dines a t several loci and for genetic diversity. The interaction between Cantabric and Duero trout, as well as the location of the limit of the anadromous fo rm around the 42 degrees N parallel, are both required to understand the ge netic characteristics of brown trout from this area.