Isolated populations of fish living in ponds are often phenotypically and genetically diverged from their source populations. However, to what degree this divergence reflects maternal, genetic or environmental effects is often unclear, as is the possible adaptive nature of this differentiation due to the strong impact of random genetic drift on populations residing in isolated habitats.
The aim of my thesis was to investigate the relative influence of genetic, environmental and maternal effects on phenotypic differentiation using Fennoscandian populations of nine-spined sticklebacks, Pungitius pungitius, as a model system. I used controlled crosses of fish from an ancestral Baltic Sea and a derived Pyöreälampi pond population - known to differ both genetically in neutral marker genes (Fst = 0.46) and phenotypically in numerous traits (viz. morphology, anatomy, behaviour and life history) - to explore the relative importance of additive genetic, non-additive genetic, environmental and maternal effects as determinants of population differentiation in morphology, life history, and behaviour.
I found evidence for strong genetic contributions to divergence in all studied traits (viz. body size, growth, age at maturity, feeding behaviour), but these genetic effects were not always additive. Rather, strong non-additive (dominance) contributions were observed in some traits, such as age at maturation. Furthermore, evidence for age-dependent dominance was found in the case of feeding behaviour. These results indicate that much of the phenotypic differentiation among marine and pond populations of nine-spined sticklebacks is due to genetic, rather than to environmental or maternal effects. Nevertheless, results from feeding manipulation experiments revealed a great deal of phenotypic plasticity in growth rates and patterns, and in particular, clear evidence for recovery growth. Specifically, full recovery - and even over compensation - was observed in response to unrestricted feeding following a period of resource limitation. The results of these experiments also indicate that there are genetically based population differences in recovery growth potential, and that maternal effects play into explaining patterns of recovery growth in response to feeding treatments. Furthermore, the impact of feeding treatments experienced by mothers was found to carry-over to influence the phenotypes of their offspring.
Taken together, the results presented in this thesis demonstrate that the observed phenotypic differences among marine and pond populations of nine-spined sticklebacks are likely to be based on genetic differentiation, although the mode of gene action underlying this differentiation is not always additive. Although environmental and maternal effects were shown to be important modulators of phenotypic variation in this system, their role in explaining population differentiation appears to be secondary. Further studies utilizing an F2-backcross-design - as well as replicate populations - might shed more light on the questions that remain open. Nevertheless, these results reinforce the conjecture that Fennoscandian pond populations of nine-spined sticklebacks might, or should be viewed as a significant evolutionary and conservation unit that require special attention in regional and national management and conservation plans.Ei saatavill
