Experimental tests of the function of mirror-image flowers

Abstract

Enantiostyly, the reciprocal deflection of the style to the left or right side of the floral axis has evolved independently in at least a dozen angiosperm families. Unlike other plant sexual polymorphisms, the adaptive significance of these mirror-image flowers remains unclear. Most authors have interpreted enantiostyly as a floral mechanism that promotes cross-pollination. However, any functional interpretation is complicated by the fact that enantiostyly occurs in two forms. In monomorphic enantiostyly there are left- and right-styled flowers on the same plant, while in dimorphic enantiostyly they are on separate individuals. In this paper we develop a model of pollen transfer which indicates that monomorphic enantiostyly should reduce geitonogamous pollination compared to a non-enantiostylous condition, and that the lowest levels of geitonogamous pollination should occur in dimorphic enantiostyly. We tested these predictions using floral manipulations of bee-pollinated Solanum rostratum in garden arrays. We compared mating patterns and fertility in five array types: non-enantiostylous and straight-styled, monomorphic enantiostylous, dimorphic enantiostylous, and arrays uniform for either left or right stylar deflection. Outcrossing rates in non-enantiostylous arrays (t = 0.33 ± 0.04) were significantly lower than all other arrays, while monomorphic enantiostylous arrays (t = 0.74 ± 0.06) had significantly lower outcrossing rates than dimorphic enantiostylous arrays (t = 0.88 ± 0.04) and those uniform for stylar deflection (t = 0.84 ± 0.04). In dimorphic enantiostylous arrays, intermorph pollen transfer accounted for 75% of all outcrossing events. In pollen-limited situations, both types of enantiostylous arrays had significantly higher female fertility than arrays fixed for one direction, demonstrating that enantiostyly promotes pollen transfer between flowers of opposite stylar orientation. Our results provide support for the hypothesis that enantiostyly functions to increase the precision of cross-pollination in bee-pollinated plants by reducing geitonogamy.