BackgroundEvaluating the limits of adaptation to temperature is important given the IPCC-predicted rise in global temperatures. The rate and scope of evolutionary adaptation can be limited by low genetic diversity, gene flow, and costs associated with adaptive change. Freshwater organisms are physically confined to lakes and rivers, and must therefore deal directly with climate variation and change. In this study, we take advantage of a system characterised by low genetic variation, small population size, gene flow and between-trait trade-offs to study how such conditions affect the ability of a freshwater fish to adapt to climate change. We test for genetically-based differences in developmental traits indicating local adaptation, by conducting a common-garden experiment using embryos and larvae from replicate pairs of sympatric grayling demes that spawn and develop in natural cold and warm water, respectively. These demes have common ancestors from a colonization event 22 generations ago. Consequently, we explore if diversification may occur under severely constraining conditions.ResultsWe found evidence for divergence in ontogenetic rates. The divergence pattern followed adaptation predictions as cold-deme individuals displayed higher growth rates and yolk conversion efficiency than warm-deme individuals at the same temperature. The cold-deme embryos had a higher rate of muscle mass development. Most of the growth- and development differences occurred prior to hatch. The divergence was probably not caused by genetic drift as there was a strong degree of parallelism in the divergence pattern and because phenotypic differentiation (QST) was larger than estimated genetic drift levels (microsatellite FST) between demes from different temperature groups. We also document that these particular grayling populations cannot develop successfully at temperatures above 12°C, whereas other European populations can, and that increasing the muscle mass development rate comes at the cost of some skeletal trait development rates.ConclusionsThis study shows that genetically based phenotypic divergence can prevail even under conditions of low genetic variation and ongoing gene flow. Furthermore, population-specific maximum development temperatures along with musculoskeletal developmental trade-offs may constrain adaptation.

The evolution of threespine sticklebacks in freshwater lakes constitutes a well-studied example of a phenotypic radiation that has produced numerous instances of parallel evolution, but the exact selective agents that drive these changes are not yet fully understood. We present a comparative study across 74 freshwater populations of threespine stickleback in Norway to test whether evolutionary changes in stickleback morphology are consistent with adaptations to physical parameters such as lake depth, lake area, lake perimeter and shoreline complexity, variables thought to reflect different habitats and feeding niches. Only weak indications of adaptation were found. Instead, populations seem to have diversified in phenotypic directions consistent with allometric scaling relationships. This indicates that evolutionary constraints may have played a role in structuring phenotypic variation across freshwater populations of stickleback. We also tested whether the number of lateral plates evolved in response to lake calcium levels, but found no evidence for this hypothesis.

Olfactory sensory neurons of vertebrates regenerate throughout the life of the animal. In fishes, crypt cells are a type of olfactory sensory neurons thought to respond to sex pheromones. Here, we demonstrate that the number of crypt cells in the olfactory epithelium of the crucian carp varies dramatically throughout the year. During winter, few crypt cells are observed at any location within the sensory epithelium. In spring, the majority of crypt cells are located deep in the epithelium not yet exposed to the environment. However, during the summer spawning season, crypt cells are positioned at the epithelial surface. These findings may explain previous studies demonstrating a relationship between circulating androgen and olfactory sensitivity to sex pheromones.

Gregersen F, Haugen TO, Larsen ØN. Egg size differentiation among sympatric demes of brown trout: possible effects of density-dependent interactions among fry.Abstract -This study investigates differentiation in egg size among five sympatric brown trout (Salmo trutta (L.)) demes. We explore a hypothesis predicting high density-dependent interactions among juveniles to favour large eggs by sampling closely located (<100 m) deme pairs with low and high fry abundances. A mancova model fitted the egg size versus egg number relationship as a function of large-scale spatial habitat heterogeneity (basin) and maternal phenotype revealed that demes have significantly different egg size versus fecundity relationships and that the differentiation is mainly due to interdeme variation in egg size. Fry density was significantly and positively associated with egg size and a post-hoc test indicated egg size to be significantly greater in high-density than lowdensity tributaries. The data is consistent with the density-dependent hypothesis and suggest that reproductive investment can diverge over small geographic distances, potentially in response to environments favouring greater investment in offspring quality.

The diet of Arctic char, Salvelinus alpinus (L.), and brown trout, Salmo trutta L., was studied in a large Norwegian reservoir over the first four decades of regulation. Significant changes in diet occurred concurrent with changes in fish density and prey availability. Reduced biomass of fish due to the effect of hydropower development probably led to reduced inter-and intra-specific competition. Consumption of zoobenthos increased for both species during the first decade, fish then became an important food item concurrent with an increase in small-sized Arctic char in the littoral zone. The diet of Arctic char, but not brown trout, then changed radically as the introduced mysid, Mysis relicta Love´n, became an important food item. K E Y W O R D S : char, diet, hydropower development, long-term, Mysis relicta, trout.