Influence de la strategie migratrice sur l'investissement reproducteur et les traits physiologiques lies a la migration chez l'omble de fontaine (Salvelinus fontinalis)

The aim of this study was to determine whether 1+ years old anadromous and freshwater resident brook charr reared under the same controlled experimental conditions, showed differences for traits associated with growth, sexual maturation, accumulation et utilization of hepatic and muscle energetic reserves, not only at the physiological level, but also at the gene expression level. This project also aimed to determine to which extent the studied physiological traits could be transmitted genetically, through heritability calculations. Physiological responses of reciprocal hybrids were also studied to determine the genetic basis of the studied traits. Anadromous (A) and resident (R) brook charr from the same river system were mated and four cross-types ([female]A[male]A, [female]A[male]R, [female]R[male]A, and [female]R[male]R) produced. Rearing conditions were identical from egg incubation to the age of 22 months. When fish reached 17 months of age, they were gradually (2‰ day-1 ) transferred to estuarine sea water (final salinity 20‰) for the summer (June to August). The first objective of the study was to compare expression profiles of the IGF-I gene in anadromous and residents, and to determine if the responses of the hybrids were additive (or intermediate) relatively to these of pure cross-types. An increase in relative expression of the IGF-I gene occurred in residents, but not in anadromous, after saltwater transfer, which suggests the occurrence of quantitative genetic differences between both cross-types in the regulation of IGF-I expression. The responses of hybrids were similar to these of their paternal lines. The second objective was to study physiological responses of charr for traits related to sexual maturation, as well as utilization of energetic reserves, such as vitellogenin, glycogen, lipid and protein hepatic content, GSI and HSI, and to determine if those traits had a significant genetic basis, through heritability and EBV (estimated breeding values) variance calculations. QST (quantitative genetic differentiation of phenotypic traits between populations) calculations on the studied traits were also made and compared to FST values calculated by Perry et al. (2005), to determine to which extent the observed physiological responses could be the result of selection or neutral genetic processes. In females, differences between anadromous and residents for hepatic reserves accumulation (mainly glycogen), HSI and hepatic vitellogenin content from June to November suggest different maturation rates and reproductive investment in both pure cross-types. In males, no difference in hepatic glycogen accumulation was observed in November, probably because of the relatively low cost of reproduction in males, regardless of life history strategy. Except for KF, QST values were high (0.31 to 0.82) and generally higher than the estimate of neutral genetic variance (FST = 0.15), which suggests that divergent selective pressures contributed to maintain local adaptations associated with the anadromous and resident strategies in the Laval River. The third objective was to measure cathepsin L enzymatic activity and gene expression and to measure utilization of energetic reserves in the epaxial muscle of charr. There was no cross-type effect for the relative expression of the cathepsin L gene. As expected, hepatic-specific and total cathepsin L enzymatic activities were higher in anadromous than in residents in August. Such differences in cathepsin L activity could reflect energetic needs associated with the anadromous and resident strategies during upstream migration. Indeed, in the Laval River, anadromous fish experience most variable environmental conditions and must travel on longer distances than their anadromous counterparts to reach their spawning grounds. Overall, both hybrid cross-types showed few additive physiological responses, contrary to what was expected. Several traits were heritable in at least one pure strain. In anadromous fish, hepatic glycogen and expression of the cathepsin L gene had high heritabilities (h2 = 0.87 and 0.72 respectively). Heritability estimates for hepatic-specific and total cathepsin L enzymatic activities were moderate (h2 = 0.35 and 0.44, respectively). Heritabilities in residents were low for expression and specific and total cathepsin L activities (h2 = 0.06, 0.08 and 0.04) and null for all hepatic energetic reserves (h2 = 0.00). Variances of EBV were high for hepatic glycogen in anadromous (sigma 2 = 242.16), but low for all other traits in both pure cross-types. Sympatric anadromous and resident brook charr from the Laval River showed phenotypic differences and contrasting variances of EBV and heritabilities for physiological traits related to migration and reproduction. Responses of both hybrids seemed to be related to nonadditive rather than additive genetic effects. Overall, these results indicate a significant genetic basis for some of the traits related to migration and reproduction, and various potential of response to selection for pure cross types.;Keywords : anadromous, residents, cathepsin, glycogen, proteins, IGF-I, heritability. (Abstract shortened by UMI.).