Population divergence between European lake and stream threespine stickleback

During my dissertation I worked on incipient speciation in lake and stream threespine stickleback populations from Central Europe. My previous work showed prominent life history shifts in the two traits age and size at reproduction with lake fish being one year older and 30 % bigger (s.l.) than stream fish. I started my PhD by answering the two questions about the basis of these shifts: i) is it plastic or genetically based and ii) what is the underlying mechanistic basis? Two alternative ways may lead to the described shift: Assuming similar growth rates in lake and stream populations, higher maturation size thresholds in lake fish, which could only be attained in two years would lead to bigger and older fish in the lake when compared to the stream. Alternatively, maturation size thresholds may be the same with reduced growth rates in the lake, precluding them from overcoming minimal size for reproduction within one year. In this scenario, lake fish would have to invest into somatic growth for another year and become larger than stream fish at reproduction. I answered these questions using a combination of two lab experiments, testing for genetic differences in growth rates and unequal maturation size thresholds and a field experiment, testing for phenotypic plasticity in transplanted lake fish. I found evidence for a high degree of plasticity likely induced by differential feeding regimes, leading to drastically reduced growth rates in lake fish.
A high–resolution single nucleotide polymorphism (SNP) marker dataset supported the low overall genetic differentiation between the lake and the stream populations. However, it also contained SNP markers with moderate to high differentiation. Hence, in the last experiment of my PhD, I wanted to test for local adaptation, despite weak genome wide differentiation. I used a long term field experiment to let lab reared stream, lake and F1 lake–stream hybrids compete against each other. By measuring survival over time, I found strong evidence for selection against migrants and hybrids and hence gave an answer to the long standing question of the whereabouts of the barriers to gene flow in lake–stream stickleback populations.