Frequency-dependent variation in reproductive success in Narcissus: implications for the maintenance of stigma-height dimorphism.
Negative frequency-dependent selection is a major selective force maintaining sexual polymorphisms. However, empirical demonstrations of frequency-dependent reproductive success are rare, particularly in plants. We investigate this problem by manipulating the frequencies of style morphs in a natural population of Narcissus assoanus, a self-incompatible herb with style-length dimorphism and intra-morph compatibility. We predicted that the reproductive success of morphs would vary negatively with their frequency because of the effects of morph-specific differences in sex-organ position on patterns of pollen transfer. This prediction was generally supported. The fruit and seed set of the two morphs did not differ significantly in plots with 1 : 1 morph ratios. However, short-styled plants produced significantly fewer seeds than long-styled plants in monomorphic plots, and significantly more seeds than long-styled plants in plots with 'long-biased' morph ratios. These patterns indicate that in the absence of physiological barriers to intra-morph mating, negative frequency-dependent selection contributes to the maintenance of stylar polymorphism through inter-morph pollen transfer. Our experimental results also provide insights into the mechanisms governing the biased style-morph ratios in populations of Narcissus species.
Thompson
John D
JD
Centre d'Ecologie Fonctionnelle et Evolutive, CNRS, 1919 Route de Mende, 34293 Montpellier Cedex 5, France. john.thompson@cefe.cnrs-mop.fr
Barrett
Spencer C H
SC
Baker
Angela M
AM
D005075:Biological Evolution; D014644:Genetic Variation; D031427:Narcissus / Q000033:anatomy & histology / Q000235:genetics* / Q000502:physiology; D011058:Pollen / Q000235:genetics / Q000502:physiology; D011110:Polymorphism, Genetic / Q000502:physiology; D012098:Reproduction / Q000235:genetics; D012641:Selection, Genetic; D012727:Sex Characteristics