Dobzhansky Reloaded: Eco-Evo-Devo of a Putatively Overdominant Colour Polymorphism in the Alpine Orchid Gymnadenia (Nigritella) Rhellicani

Understanding the emergence of stable phenotypic differences in and between natural populations, eventually leading to novel species, is a central aspect of evolutionary biology. Systems with polymorphisms in fitness-relevant traits such as coloration allow studying the molecular mechanisms underlying speciation. In 1955, Dobzhansky stated that polymorphisms evolve mainly due to overdominance, i.e. a higher fitness of the intermediate morph. However, half a decade later, there is still little empirical evidence for the action of overdominance in colour polymorphic systems. We are currently assessing the molecular and ecological background of a floral colour polymorphism with three morphs in the rewarding Alpine orchid Gymnadenia (Nigritella) rhellicani. Metabolomic analysis of floral pigments show, that the colour differences between the morphs are caused by different concentrations of two anthocyanins, cyanidin 3-O-glucoside and (likely) cyanidin 3-O-oxalylglucoside. An additional high-throughput transcriptomic screen based on a Lexogen QuantSeq kit further suggests that these pigmentation differences are linked to differential expression of an anthocyanidin synthase (ANS) gene. Pollinator observation data from time-lapse cameras set up in the population show that the intermediate morph has both a higher pollinator visitation rate and a higher seed set than either the darker or the brighter morph, suggesting the action of overdominance in this system.