The homeostatic regulation that controls total thymocyte and peripheral T-cell numbers is not clearly understood. We describe here a direct hormonal influence of endogenous levels of glucocorticoids (GCs) on thymocyte and peripheral T-cell homeostasis independent of indirect systemic effects of GCs. The results were obtained by generating transgenic mice with an altered GC sensitivity targeted to thymocytes and peripheral T cells by increasing or decreasing glucocorticoid receptor (GR) expression specifically in thymocytes and peripheral T cells. A twofold increase in GC sensitivity resulted in a major decrease in thymocyte number, affecting all subpopulations, although single-positive CD8+ cells were less influenced. In the thymus, this was due to increased apoptosis in the organ, whereas proliferation of thymocyte populations was unaffected. In the periphery, a pronounced reduction in T-cell number was seen, demonstrating an effect of endogenous GCs also on T-cell homeostasis. The effects were confirmed in transgenic mice with reduced GR expression, which showed increased thymocyte and T-cell numbers. Thus, our data demonstrate that physiological GC levels are directly involved in controlling the size of both thymocyte and T-cell pools.

Previous studies of genetic structure in the European eel have resulted in seemingly conflicting results, ranging from no detectable heterogeneity to small but statistically significant differences and isolation by distance patterns among eels sampled across the continental range. Differences with respect to sampling design and choice of molecular markers, combined with a lack of power estimates, complicate comparisons of existing results. In this study we have used six microsatellite markers and, for the first time, compared maturing silver eels of known age from southern and northern Europe (Italy and Baltic Sea). In comparison with previous studies, our data may give a better representation of potential spawning stocks because eels were sampled when having begun their migration toward the presumed spawning area in the Sargasso Sea. Despite large sample sizes (404 and 806 individuals) we could not observe any signs of genetic differentiation (average F ST ¼ À0.00003, P ¼ 0.61), and a power analysis showed that the true level of heterogeneity (if existing) must be exceedingly small to have remained undetected (say, F ST o0.0004). A tendency for slightly increased genetic differences between cohorts over time could be seen, but the amount of temporal change was minor and not statistically significant. Our findings reiterate the notion that previous reports of continental genetic differentiation in the European eel may be largely explained by uncontrolled temporal variation between juvenile glass eel samples.

Summary 1.Although releases of hatchery-produced salmonids to support conspecific wild populations have increased dramatically during recent decades, little information is available about the performance in the wild of hatchery fish and their offspring. Important factors determining the success and genetic outcomes of supportive breeding programmes include (i) the relative reproductive success of released hatchery fish in the wild, and (ii) the extent to which the propagation affects the variance in reproductive success in the population as a whole. 2. We performed two field experiments on brown trout Salmo trutta from the River Dalälven in Sweden, where we examined reproductive success in an experimental stream. In experiment 1 we compared reproductive success between trout from a seventhgeneration hatchery stock of native origin and wild-born trout from the river. In experiment 2, we compared reproductive success between seventh-generation hatchery trout and hatchery-reared trout derived from wild-born parents. Individual reproductive success, based on the number of offspring assigned using microsatellite markers, was assessed on three occasions after reproduction: immediately after hatching and after the first and second growth seasons. 3. In experiment 1 there were no significant differences in reproductive success between seventh-generation hatchery trout and wild-born trout. In experiment 2, males from wildborn parents were more successful than males from the seventh-generation hatchery stock, but this difference was not observed among females. 4. There was some evidence for a positive association between body size and reproductive success among females but not males. For males, the number of mates was significantly associated with reproductive success, but this relationship was not evident among females. 5. The variance in reproductive success was pronounced in both experiments, yielding estimates of the ratio between the genetically effective size and the census size of our experimental populations ranging from 0·12 to 0·59. 6. Synthesis and applications . Our results suggest that the reproductive success in the wild of hatchery-produced and wild-born trout with a common genetic background may be rather similar. These findings, in combination with the pronounced variance in reproductive success observed among breeders, indicate that supportive breeding can be managed to increase not only the census but also the genetically effective size of small, endangered salmonid populations. However, to minimize negative effects of hatchery selection, it is important to give priority to the restoration of natural habitats and thereby increase the reproductive output from individuals in the wild.

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