Exploration de la variabilite genetique presente dans cinq peuplements naturels de pin Laricio de Corse (Pinus nigra ssp. laricio var. corsicana Loud.)

La variabilité génétique présente dans cinq peuplements naturels de pin laricio de Corse a été étudiée pour quelques prédicteurs de la croissance en volume et de la forme, à partir d’un test de descendances maternelles âgé de 11 ans, situé en forêt d’Orléans. L’analyse factorielle discriminante met en évidence une variation clinale des caractères de croissance suivant un gradient altitudinal. Les peuplements représentant la tranche 700-1000 m se révèlent plus vigoureux que ceux situés entre 950 et 1200 m d’altitude. La comparaison des cinq estimations de variances-covariances interfamilles, réalisée à l’aide du test de Kullback, conduit à rejeter l’hypothèse d’homogénéité des structures génétiques intrapeuplement. La forte différenciation observée entre ces cinq populations naturelles voisines est discutée en faisant référence aux rôles potentiels des pressions de sélection naturelle et de la dérive génétique.Studies on genetic variabitity existing in natural populations of Corsican pine are of major concem in developing an optimum tree breeding program. A total of 97 wind-pollinated families grouped into 5 natural stands from the northern part of the Corsican island (Fig. 1, Table I), were raised in a progeny test in Orleans forest and assessed for growth characteristics from age 2 till age 11 after plantation. Measurements of forking defects were also made for the years 1983, 1984 and 1985. Two models of analysis of variance were used : the first one to compare stand means, the second one to compute for each stand estimates of genetic parameters (between-family variances, individual heritabitities, genetic correlations). Phenotypical differences among population means, indicated by discriminant functions, were mainly explained by juvenile growth performances (Table II, Fig. 2, Table III). For these traits, a pattern of clinal variation might exist, depending on an elevational gradient. Genetic parameters varied widely from one stand to another (Table IV, Table V). For growth characteristics, a negative relationship appeared between performance mean and between-family variance value. Nowever, because of a lack of precision in variance components estimafion, thehypothesis of homogeneity based on beriveen-family variances, checked by Kullback test could not be rejected for a single given trait (Table VI). Kullback test values computed with special groups of traits showea’ large discrepancies of genetic structure among the 5 natural stands (Table VII, Fig. 3). Results were discussed refering to two main causes of genetic differentiation of isolated populations : - natural selection pressures, essentially rainfall conditions for Corsican pine in its native range; - genetic drift and factors liable to reduce mean consanguinity rate as could probably be, fire, colonisation of neighbouring areas and human activity