Correlated evolution of floral morphology and mating-type frequencies in a sexually polymorphic plant

Abstract

In sexually polymorphic species, reproductive morphology governs mating patterns and the character of negative frequency-dependent selection. If local environmental conditions cause sexual morphs to differ between populations, then frequency-dependent selection should create corresponding geographic variation in morph frequencies. We investigate this relation with a model of morph-ratio evolution and analysis of geographic variation in the heterostylous plant Narcissus triandrus. Unlike other tristylous species, N. triandrus possesses both imperfect reciprocity among morphs in sex-organ position and a self-incompatibility system that permits outcrossing within and between morphs. We sampled 137 populations throughout the Iberian Peninsula for floral-morph ratios, and measured floral morphology in 31 populations. Morph ratios exhibited three atypical features: (1) predominance of the long-styled (L) morph; (2) absence of the mid-styled (M) morph from 17.5% of populations; and (3) a negative relation between the frequencies of the L and M morphs among populations. Morph ratios varied geographically, with decreasing frequency of the M morph from the southeast to the northwest of the species' range. Much of this variation accompanied allometric change in the positions of sex organs, especially the mid-level organs, with the M morph declining in frequency and ultimately being lost in large-flowered populations. Using multivariate multiple regression, we demonstrate that variation in floral morphology among populations predicts this geographic variation in morph frequencies. Our theoretical analysis illustrates that patterns of pollen transfer governed by imperfect sex-organ reciprocity can select for unequal equilibrium morph ratios like those observed for N. triandrus. We interpret the L-biased morph ratios and the unusual morphology of N. triandrus as a consequence of its atypical intramorph compatibility system.