First generation tetraploids were produced by hydrostatic pressure treatment before the first cleavage and raised until the adult stage. Their survival and growth were severely depressed when compared to the diploid control: after two years, no ovulated females were found although males produced sperm at 1 and 2 years of age and were mated individually with diploid females. The progenies were consistently normal with high survival rates. They were found to be almost all triploids by karyology, which failed to detect a significant rate of aneuploidies. However, the fertilizing ability of tetraploid males was always low (0 to 97% of the control; average 40%). Several arguments presented here support the hypothesis that diploid spermatozoas, which are wider than haploid ones, would be frequently blocked during their penetration through the micropyle canal. Second generation tetraploids were produced after such matings by heat shocks, causing the retention of the second polar body. Their survival and growth were much more satisfactory than in the first generation, although still lower than in diploid and triploid controls issuing from diploid parents. Performances of second generation triploids were comparable to those of diploids, and slightly better than those of conventional triploids issuing from diploid parents. 94.5% of the second generation tetraploids were male.

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RÉSUMÉL'étude comparée de différentes populations de truites arc-en-ciel, introduites et élevées dans un même milieu, a révélé l'existence d'une importante variabilité intergroupes pour l'âge à la première reproduction : de 6 à98% d'animaux matures à 2 ans chez la femelle et de44 à 99% chez les mâles.Des croisements factoriels entre des géniteurs ayant maturé pour la première fois à des âges variés (de 1 à 3 ans pour les mâles, de 2 à 4 ans pour les femelles) ont permis de préciser les possibilités de sélection individuelle et les effets respectifs des parents mâle et femelle. L'effet du parent mâle sur la fréquence de mâles et de femelles précoces dans la descendance est net ; l'effet du parent femelle apparaît plus limité. Une estimation de l'héritabilité de la précocité sexuelle est proposée (h 2 = 0,3) et les possibilités d'amélioration génétique de ce caractère sont présentées et discutées. POTENTIAL OF SELECTION FOR AGE AT FIRST SEXUAL MATURATION IN RAINBOW TROUT (SALMO GAIRDNERI R.) SUMMARYComparison of différent populations of rainbow trout reared in the same environment revealed important between strains variation for the âge at first sexual maturation : proportion of mature fish at 2 years ranged from 6 to 98% in females, and from 44 to 99% in maies.Réponse to mass sélection for âge at first maturation and spécifie effect of both maie and female parents were screened across factorial crosses between parents with différent âges attheir first maturation (1 to 3 years for maie parents, 2 to 4 years for female parents). There is an important maie effect on f requencies of early maturing maies and females in the progeny ; influence of female parent is less important. Heritability estimate for sexual precocity is provided (h 2 = 0,3), and feasability of genetic improvement of this character is discussed.

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