The neglected floral polymorphism: mirror-image flowers emerge from the shadow of heterostyly

Abstract

Abstract Morphological asymmetries in plants and animals raise intriguing questions concerning their function and how they have evolved. One of the most conspicuous asymmetries in plants involves mirror-image flowers (enantiostyly) in which styles are deflected to either the left or right sides (L or R, respectively) of the flower. Species with this floral polymorphism often possess two types of stamens (heteranthery): centrally located feeding anthers and a pollinating anther orientated in the opposite direction to the style (reciprocal enantiostyly). However, some species lack heteranthery and sex-organ reciprocity can be partial or absent (non-reciprocal enantiostyly). Many enantiostylous species have nectarless flowers and are ‘buzz-pollinated’ by pollen-collecting bees. In contrast to other stylar polymorphisms such as heterostyly, enantiostyly exists as either monomorphic or dimorphic conditions, with L and R flowers on the same plant in the former, and genetically determined floral morphs with either L or R flowers in the latter. Enantiostyly has been reliably reported from 11 angiosperm families, but in only two is there convincing evidence that dimorphic enantiostyly occurs. Various hypotheses concerning developmental or selective constraints attempt to explain the rarity of this genetic polymorphism. Experimental studies on the function of enantiostyly indicate that the reciprocity of stigmas and pollinating anthers promotes pollinator-mediated cross-pollination and limits geitonogamous selfing. Insufficient or inferior pollinator service can result in the evolutionary breakdown of enantiostyly, including reduced stigma–anther separation, increased selfing, and dissolution of heteranthery. In this article we review recent advances and knowledge gaps in understanding these curious asymmetries and discuss why they have received less attention than heterostyly.