Is the evolutionary breakdown of tristyly a common process?

Abstract

Tristyly is a floral polymorphism only reported in seven angiosperm families. It is characterized by the presence of three floral morphs differing in the height of sexual organs. In general, the equilibrium morph ratios in tristylous populations are regulated by frequency-dependent selection through the legitimate pollen flow, the incompatibility system and the lack of variation infitness among different morphs. Although tristyly has theoretically been considered as an evolutionarily stable polymorphism, it has repeatedly been demonstrated that its evolutionary breakdown is very common. Evidence provided by several studies shows the presence of distylous and monomorphic populations in tristylous species. Through an indirect approach based on the analysis of morphratios variation reported in populations of tristylous species, this paper evaluates and discusses: 1) how unstable the tristyly is, 2) whether the pattern of variation in morph ratios in populations is consistent with a hypothesis of selection or random processes, and 3) the possible mechanisms involved in the breakdown of the predicted equilibrium. The results suggest that the evolutionary breakdown of tristyly is a common process that could be explained by natural selection and/or random processes depending on the pattern of morph-ratios variation detected between populations of a species. However, more experimental and comparative studies are needed to identify the mechanisms responsible for the dissolution of this floral polymorphism.