Timing the tides: Genetic control of diurnal and lunar emergence times is correlated in the marine midge Clunio marinus

We crossed two strains of Clunio marinus that differ in the timing of the diurnal and lunar rhythms of emergence. The phenotype distribution of the segregating backcross progeny indicates polygenic control of the lunar emergence rhythm. Diurnal timing of emergence is also under genetic control, and is influenced by two unlinked genes with major effects. Furthermore, the lunar and diurnal timing of emergence is correlated in the backcross generation. We show that both the lunar emergence time and its correlation to the diurnal emergence time are adaptive for the species in its natural environment.The correlation implies that the unlinked genes affecting lunar timing and the two unlinked genes affecting diurnal timing could be the same, providing an unexpectedly close interaction of the two clocks. Alternatively, the genes could be genetically linked in a two-by-two fashion, suggesting that evolution has shaped the genetic architecture to stabilize adaptive combinations of lunar and diurnal emergence times by tightening linkage. Our results, the first on genetic control of lunar rhythms, offer a new perspective to explore their molecular clockwork.Lunar rhythms of reproduction are found in a number of species, especially from the intertidal zone [1-7], where the recurrent alternation between marine and terrestrial conditions results in huge fluctuations in abiotic factors. Certain tidal conditions re-occur predictably during the lunar cycle and the adaptive value of lunar rhythms can likely be attributed to restricting the delicate events of reproduction to suitable, narrow windows of time [7,8]. Lunar rhythms in reproduction are also reported for other marine species that do not experience tidal fluctuations, one of the most famous examples being the swarming of the Palolo Worm Eunice viridis in Samoa [9,10]. They are also reported for plankton and insects of tropical lakes [11,12]. In these cases the likely adaptive value of the lunar rhythm is to synchronize reprod