Genetic structure of northwestern Spanish brown trout (Salmo trutta L.) populations, differences between microsatellite and allozyme loci

摘要

Conservation of biological diversity in species of ecological and economical interest has been, for the last decades, one of the main challenges to overcome for many countries which have invested an extensive effort directed towards these species. This becomes critical both in brown trout (Salmo trutta L.) and other salmonid species which have lost large parts of their intraspecific variability due to environmental degradation, harvest and stocking.In order to halt and reverse this negative trend, different organizations have started supportive programs. However in most of them, the origin and genetic composition of both the added specimen and the natural populations have not been taken into account. Many studies based on the analysis of genetic markers have shown the risk of such practices on natural populations, causing the loss of intraspecific diversity, due to hybridization and/or introgression among autochthonous (individuals from a specific area that share some genic features) and allochthonous (foreign individuals) specimen (Hansen et al. 1993; Skaala et al. 1996; Blanco et al. 1998, García-Marín et al. 1998; Cagigas et al. 1999a; Poteaux et al. 1999). Managers must recognize the biological reality of existing genetic diversity and devise appropriate management strategies on the basis of conserving genetic variability effectively (Ryman 1991). Therefore, as a previous step to any release of fishes, managers should take into account the genetic status of the species in general and the populations that will be manipulated in particular.The existence of a hierarchical populational genetic structure in brown trout has been confirmed (Ryman 1983), although the number of levels within this hierarchy and interactions among different lineages or groups still remain unsolved to a great extent (García-Marín et al. 1999; Machordom et al. 2000; Sanz et al. 2000). The influence of glaciarism in the current patterns of gene diversity has been largely recognized in brown trout (Laikre et al. 1999). Also the existence of a high level of genetic diversity has been shown among the Spanish populations, exhibiting in many cases, a close link between the genetic differentiation and the hydrographical pattern. Thus, in the Iberian Peninsula, at least two clearly distinguished lineages can be identified: populations from the Mediterranean drainage (lineage IV) and those from the Cantabric drainage (lineage II) (García-Marín and Pla 1996; Cagigas et al. 1999b; García-Marín et al. 1999). More recent studies based on the analysis of mitochondrial DNA have confirmed this dichotomy reported previously by enzymatic loci (Machordom et al. 2000; Bernatchez 2001). This high differentiation at a macrogeographic level is also kept at a microgeographic level (Estoup et al. 1998; Sanz et al. 2000; Bouza et al. 2001; Cagigas et al. 2002), and it seems to be connected with the genetic drift and the scarce gene flow that is established among natural populations.During the past few decades the development of new and highly variable genetic markers, such as microsatellite DNA, has led to new research opportunities that were not possible using allozymes only. The higher level of microsatellite variability associated with its neutral character has turned them into the favourite technique for researchers when studying the genetic population structure of brown trout (Estoup et al. 1993; Cagigas et al. 1999b; Bernatchez 2001; Fritzner et al. 2001). Most of the surveys done with such markers have been focused on the evaluation of restocking on natural populations or the genetic characterization of brown trout populations at a macrogeographical level (Hansen et al. 2000; Fritzner et al. 2001; Ruzzante et al. 2001).In this work, we studied a microgeographic region in the Province of Leon, where two hydrographic basins are located: the Sil and Duero basins. Both basins are managed by the Servicio Territorial de Medio Ambiente y Ordenación del Territorio de León and, f