The role of a circalunar clock in speciation: Investigations of the marine midge Clunio marinus

Once thought impossible, many contemporary studies now suggest that speciation with gene flow is not only possible, but perhaps more common than previously assumed. Phenotypes influencing both mating and ecology (magic traits) lie at the center of many simple models of sympatric speciation and may play important roles in empirical systems. However, few examples have been fully studied in terms of ecology, reproductive isolation, and genetic basis. Reproductive timing is one such trait, as organisms often synchronize reproduction to environmental cycles such as seasons. This assortative mating in time isolates reproduction between individuals with different timing phenotypes (chronotypes). Often, reproductive timing is controlled by biological clocks; endogenous molecular mechanisms that track environmental cycles. Given their prevalence, ecological relevance, and potential to isolate reproduction, biological clocks may play an important role in sympatric speciation. In my thesis, I explore these ideas by investigating the role of biological clocks in the diversification of Clunio marinus chronotypes. C. marinus is a marine midge inhabiting rocky intertidal zones along the European coast, notable for timing its development and reproduction to the lunar cycle via an endogenous clock. On the northern coast of Brittany, France populations have recently diverged into several sympatric chronotypes, though the mechanisms remain unclear. By combining modeling, field studies, laboratory experiments, and molecular techniques, my thesis examines the mechanistic basis of chronotype diversification and provides a comprehensive example of how magic traits may facilitate divergence. It highlights the role of biological clocks in speciation and raises new questions about context dependence in magic traits, trait polygenicity, and complexity.