GENOME INTEGRITY AND TROUT STOCKING IN TH E MEDITERRANEAN BASIN

The stocking practices used for brown trout (Salmo trutta) in the Sout h of France result in secondary contact and introgression between popu lations from two genetically differentiated forms of the same species (KRIEG and GUYOMARD, 1985; GUYOMARD, 1989) : domesticated stocks that originate from the Atlantic form and the wild Mediterranean population s. This paper reviews the protein data for 44 Mediterranean samples ex amined in the laboratory, with an appraisal of introgression by the do mesticated form and a description of the genetic consequences of stock ing on the existing populations. The samples were collected from sever al French departments : Pyrenees Orientates (12 stations), Herault (5 stations), Vaucluse (2 stations), and Corsica (25 stations) (see Table I). Electrophoresis on horizontal starch gel revealed 28 loci. Introg ression was quantified using three diagnostic protein loci : lactate d ehydrogenase of the eye (LDH-5), serum transferrin (TF*) and fructose biphosphatase of liver (FBP-1). The individuals identified as domest icated fish on the basis of their genotype were discarded. Frequencies at diagnostic loci, expected heterozygosity, the deviation from panmi xia in each sample, and domestic introgression are given in Table II. Heterozygote deficit was estimated using parameter f of WEIR and COCKH ERAM (1984). Likewise, parameter 8 was used to assess whether the stru cture of the different samples was statistically significant. The valu es of these two parameters were tested by permutation using the GENETI X 2.0 programme (BELKHIR et al., submitted). The populations analysed can be classified according to their degree of introgression, which ra nged from 0 to 77 %. Most introgressed populations displayed substanti al and significant heterozygote deficit. Stocking is the most likely c ause of these imbalances. However, this interpretation cannot be gener alised to all the samples because the deficit did not increase with th e severity of introgression. The results as a whole tend to show that stocking has a genetic effect on natural populations (with changes in their genotype, and imbalances in panmixia and linkages). The data als o illustrate the weak effect of stocking on population densities. This has already been reported by several authors. The various causes for this lack of success in stocking (stocking techniques, learning proble ms in the wild environment, domestication) are discussed. Introgressio n-free Mediterranean populations generally occur at high altitudes and they are practically monomorphic. The advantages of conserving geneti cally pure populations with no polymorphism are discussed. Two solutio ns that reconcile objectives as different as supporting trout populati on densities and protecting biodiversity are examined :the creation of local strains, which requires rigorous genetic management (CHEVASSUS, 1989) and the introduction of triploid individuals, which would resul t in solely ecological interactions. Finally, the question of the comp lete interruption of stocking in the whole area or in ''genetic sanctu aries'' that have been identified as genetically original is discussed . Other diagnostic markers distinguishing trout of the two origins hav e been described recently. They provide information on introgression a t levels other than that of proteins. The study of the mitochondrial g enome gives data on the maternal origin of each individual and hence o n introgression through the mother. Finally, the study of highly polym orphic microsatellite markers will make it possible to distinguish bet ween the natural and domesticated components of the populations of the Atlantic basin, a zone in which there are no diagnostic allozymic loc i for these two forms.