The comparative biology of mirror-image flowers

Enantiostyly, the deflection of the style to the left or right side of the floral axis, is a sexual polymorphism reported from at least 10 angiosperm families in both monocotyledons and dicotyledons. To investigate variation in the enantiostylous condition, we examined sex- organ reciprocity and the arrangements of left- and right- styled flowers in 15 enantiostylous species from eight genera in six families. Enantiostyly can be expressed at three distinct levels of structural organization. In monomorphic enantiostyly, individual plants produce both floral forms, either mixed within an inflorescence or segregated between left- and right- styled inflorescences. In dimorphic enantiostyly, plants are exclusively left- or right- styled and this condition is a genetic polymorphism. Enantiostyly is most commonly associated with the reciprocal deflection of a pollinating anther ( reciprocal enantiostyly), although there is a continuum in stigma- anther position, ranging from no reciprocal anther to complete reciprocity. Comparative analyses of a composite phylogeny of the monocotyledons indicate that in this group enantiostyly has evolved at least 11 times, often simultaneously with the gain of heteranthery. Both concentrated changes and maximum likelihood tests indicated that heteranthery and the loss of nectaries are significantly associated with the presence of enantiostyly, indicating a syndrome of traits where pollen is the main reward and anthers are divided into two functions: pollination and nutrition. The association of enantiostyly with heteranthery, the loss of nectaries, and the reciprocal placement of a pollinating anther suggest that the polymorphism functions to increase the precision of cross- pollen transfer and to reduce interference of stigmas and anthers within or between flowers on the same plant.