Effects of temperature during embryogenesis on life history traits of a partial migratory species

Environmental conditions experienced early in life can have far-reaching consequences for organisms. Through developmental plasticity, early influences can be carried over to later life stages, and understanding the underlying mechanisms of this plasticity is particularly important in the context of rapidly changing environments. In this project, we studied knock-on effects on later emerging traits of the poikilothermic brown trout as a result of differences that the trout experienced as embryos. We studied the effects of thermal conditions during embryogenesis on the metabolic rates, somatic growth, body size, personality and proclivity to migrate of juveniles and if this early embryonic experience is expressed epigenetically, which was measured in terms of the incidence of DNA methylation. Methylation can change the activity of a DNA segment without changing the genetic code and typically acts to repress gene transcription. Brown trout is a partly migratory species, i.e. some individuals migrate to sea and become sea trout and others remain resident in the home stream. By raising eggs of sea trout parents, resident parents and crosses between the two, we demonstrated that a 3°C increase in temperature during embryo development led to earlier hatching and lower metabolic rates and aerobic scopes during the juvenile stage. Thus, in a warmer climate, brown trout should expend less energy, but it also implies that their ability as a top predator and in escaping predators may be impaired due to a reduced energy available for activities. Thus, the metabolic rate of juveniles is phenotypically plastic and respond to differences in water temperature during embryogenesis. This result provides an explanation for the commonly reported finding that populations of poikilotherms originating from different altitudes or latitudes and then raised at the same temperature differ in their metabolic rates. These kinds of studies have used these findings as evidence of population-specific genetic adaptation, but here, we show that this can be a phenotypically plastic response, without invoking any influence on genetic structure or composition. The difference in the metabolic rate of juveniles in relation to incubation temperature did not translate into differences in growth rate. There was, however, an effect of incubation temperature on body size, with juveniles that experienced elevated incubation temperatures being larger than juveniles that experienced ambient incubation temperatures. This difference in body size was due to an earlier hatching and therefore longer growth season for the warm-incubated trout. The larger body size corresponded to an earlier outmigration to sea, but to our surprise, this occurred to a greater extent in offspring of freshwater resident than sea-run migratory parents. Today, one talks about fish as having personalities. Based on the differences in metabolic rates between cold-incubated and warm-incubated trout, we had expected there to be differences in personality, with cold-incubated fish being bolder and more aggressive than warm-incubated fish. We had also expected that juveniles with sea trout parents would be bolder and more aggressive than juveniles with resident parents. Indeed, we found support for our hypothesis regarding incubation temperature as cold-incubated trout were bolder and more aggressive than warm-incubated trout. The support for our hypothesis about parental origin was weaker but we did find that juveniles with resident parents were less aggressive than juveniles with sea trout parents or of mixed (one resident and one sea trout parent) origin in some of our studies but not in others. Did elevated temperature lead to differential methylation of genes? Yes, incubation at 3°C higher temperatures resulted in epigenetic imprints that persisted later in life. Specifically, trout showed different methylation responses to elevated temperature. Furthermore, resident and anadromous trout represent different life histories within a population, and to date no convincing results have demonstrated genetic differences separating them. Yet in our study, offspring of the two life history types showed differential methylation at a number of genes, suggesting trans-generational effects of the environments experienced by the parents. The most likely explanation involves maternal effects, e.g. differences in nutrients transferred from sea trout and resident mothers to their offspring. The results show that incubation at higher temperatures, as would be expected under future climate change, does indeed lead to epigenetic imprints that persist later in life and are likely to have important consequences for ecology, physiology and behaviour.

The main finding is that physiological, ecological and behavioural traits are influenced by egg incubation temperature, and that these changes parallel differences in gene methylation that typically repress gene transcription. This is a new finding with relevance for the understanding of effects of temperature during egg incubation of later performance of brown trout, but may also hold for other species. Our results are consistent with the counter-gradient variation (CGV) hypothesis explaining variation in metabolic rates at the same temperature of individuals from populations naturally living at different altitudes or latitudes. Earlier, however, this difference has been used as evidence of thermal adaptation in poikilotherms. Here, we demonstrate that this is no valid evidence, because metabolic rates is inversely related to thermal conditions at the embryo stage within the same offspring groups. These are important findings for the research community.

Epigenetic effects, where phenotypically plastic attributes change as a result of genes being activated or silenced under environmental influence, have recently challenged the traditional view of evolution based on DNA-sequence variation. Epigenetic effects are potentially heritable and can have dramatic and persistent effects on how organisms adapt to changing environments, highly relevant in today's rapidly changing climate. This project will reveal how the expression of phenotypic plasticity depends on embryonic thermal experience, and if it can be linked to epigenetic effects, as shown by variation in DNA methylation. Our research team will, in a series of laboratory experiments, study effects of 2 embryonic temperatures on offspring behaviour (personality), physiology (ventilation rates, food assimilation efficiency), growth (5 temperatures), and early maturation versus migratory propensity of offspring of anadromous brown trout Salmo trutta and freshwater resident brown trout, and anadromous x resident crosses from the same river. The phenotypic variation will be linked to studies of DNA methylation of tissues in the brain, gonads and liver of these offspring, targeting a wide range of genes and their regulatory regions. Linking phenotypic plasticity and epigenesis is novel, and the results from this 4 year project will shed light on the development of population-specific thermal adaptations, regarded as cornerstones in ecology and evolution as well as provide insight as to how poikilotherms adapt to the warmer temperatures predicted by climate models.