title: High outcrossing in the annual colonizing species Ambrosia artemisiifolia (Asteraceae).
abstract: OBJECTIVE
  Variation in mating patterns may be particularly evident in colonizing species because they commonly experience wide variation in plant density. Here, the role of density for the mating system of Ambrosia artemisiifolia (common ragweed), a wind-pollinated annual colonizing species previously reported as self-compatible, is explored.
  
  
  METHODS
  The effect of population density on the proportion of self- and cross-fertilized seeds was examined using allozyme markers and experimental arrays conducted over two seasons in the field. Also the reproductive success of isolated plants located in diverse habitats was measured. The potential occurrence of a physiological mechanism preventing self-fertilization, i.e. self-incompatibility, following controlled self- and cross-pollinations in the glasshouse was examined.
  
  
  RESULTS
  Outcrossing rates estimated using allozyme markers were uniformly high, regardless of the spacing between plants. However, when single plants were isolated from congeners they set few seeds. Observations of pollen-tube growth and seed set following controlled pollinations demonstrated that plants of A. artemisiifolia possess a strong self-incompatibility mechanism, contrary to earlier reports and assumptions.
  
  
  CONCLUSIONS
  The maintenance of high outcrossing rates in colonizing populations of A. artemisiifolia is likely to be facilitated by the prodigious production of wind-borne pollen, high seed production and extended seed dormancy.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks St, Toronto, Ontario, Canada. friedman@eeb.utoronto.ca
authors.forename: Jannice
authors.initials: J
authors.lastname: Friedman
authors.forename: Spencer C H
authors.initials: SC
authors.lastname: Barrett
mesh_terms: D031164:Ambrosia / Q000254:growth & development / Q000502:physiology*; D054817:Pollination / Q000502:physiology*; D011156:Population Density; D012098:Reproduction / Q000502:physiology; D012639:Seeds / Q000254:growth & development