Evolution and maintenance of stigma-height dimorphism in Narcissus. II. Fitness comparisons between style morphs

Populations of the insect-pollinated geophytes Narcissus assoanus and N. du bius (Amaryllidaceae) are commonly dimorphic for stigma height. An extensiv e survey of populations of the two species in SW France revealed a wide ran ge of style-morph frequencies, particularly populations with significantly more long-styled than short-styled plants. Here we employ experimental and theoretical approaches to investigate potential selective mechanisms govern ing the variation in style-morph frequencies. Controlled pollination of bot h species demonstrated that N. assoanus is moderately self-sterile whereas N. dubius is highly self-compatible. Both intra- and intermorph crosses of N. assoanus were equally fertile, indicating that the species does not exhi bit heteromorphic incompatibility. Estimates of female fertility (fruit- an d seed-set) and multilocus estimates of outcrossing using allozyme markers provided no evidence of morph-specific differences in maternal components o f reproductive success in natural populations of the two species. This resu lt suggested that differences between the morphs in male fertility may be l argely responsible for the observed morph-ratio variation. To investigate t his hypothesis we developed a mating model that incorporates the genetics o f stigma-height dimorphism and contrasting rates of assortative and disasso rtative mating in the style morphs. Simulation results demonstrated that st igma-height dimorphism will always be maintained when levels of disassortat ive mating are greater than assortative mating, and that the observed L-bia sed populations in Narcissus spp. probably result from greater levels of as sortative mating in this morph in comparison with the S-morph.