Evidence for reductions in floral attractants with increased selfing rates in two heterandrous species

Although theoretical models predict low allocation to attractive structures with increased selfing in animal-pollinated plants, empirical measurement of the reproductive costs and benefits is complicated. Here, floral sex allocation was compared in two nectarless heterandrous species with different mating systems: Monochoria korsakowii (Pontederiaceae), which has moderate outcrossing rates, and Monochoria vaginalis, a predominant selfer. In both species, mirror-image flowers have one large dark-purple anther and five small yellow anthers. Experimental evidence is provided for functional differences between the two sets of anthers using data on pollinator visitation, pollen removal and deposition, and seed set after hand pollinations. Flower manipulations in bee-pollinated M. korsakowii demonstrated different functions of the two sets of anthers: the yellow (feeding) anthers function to attract pollinators, but have similar pollen performance to the purple (pollinating) anthers. Furthermore, a disproportional reduction in pollen production of the feeding anthers in the selfing species was found. This differential allocation between feeding and pollinating anthers in heterandrous species has not been recognized before. The finding of reduced allocation to attractive structures with an increase in the rate of self-fertilization supports the theory of sex allocation.