title: Sexual stability in the nearly dioecious Pinus johannis (Pinaceae).
abstract: OBJECTIVE
  Even though dioecy is a dominant sexual system among gymnosperms, little is known about its evolutionary history. Pinus johannis may represent a model system because unisexual and monoecious individuals compose its populations. The presence of unisexual individuals in other Pinus species is a consequence of sexual lability. Here we determined whether P. johannis represents the first example of a dioecious or nearly dioecious reproductive system in conifers by evaluating its sexual stability. •
  
  
  METHODS
  To assess the stability of sexual expression, we quantified the proportion of male vs. female reproductive structures produced by trees over multiple years and tested for the presence of sexual dimorphism. Sexual lability hypotheses were also examined by looking at the relationship between environmental factors and sexual expression and by comparing the reproductive behavior of P. johannis with its closest labile relative, P. edulis. •
  
  
  RESULTS
  Pinus johannis is nearly dioecious: ~99% of individuals are unisexual or express a low proportion of the opposite gender with few changes in sexual expression through time. We found sexual dimorphism consistent with sexual stability. Sexual expression did not vary with tree size/age, abiotic environment, or herbivore removal, providing evidence against sexual lability. Individuals of P. johannis tended to produce only male or female strobili, whereas those of P. edulis were mainly monoecious with a gradient in the female to male strobili ratio. •
  
  
  CONCLUSIONS
  This study represents the first report of a nearly stable dioecious Pinus species. The variety of sexual morphs coexisting in the same population makes P. johannis a model for studying the evolution of dioecy in gymnosperms.
authors: Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México, A. P. 70-275, México, D. F. 04510 México. lluvia.fl ores@nau.edu
authors: Lluvia
authors: L
authors: Flores-Rentería
authors: Francisco
authors: F
authors: Molina-Freaner
authors: Amy V
authors: AV
authors: Whipple
authors: Catherine A
authors: CA
authors: Gehring
authors: C A
authors: CA
authors: Domínguez
mesh_terms: D018533:Biomass; D060434:Herbivory / Q000502:physiology; D028223:Pinus / Q000502:physiology*; D018520:Plant Shoots / Q000254:growth & development; D012098:Reproduction / Q000502:physiology; D013997:Time Factors; D014197:Trees / Q000502:physiology