Mating system variation in hybridizing irises: effects of phenology and floral densities on family outcrossing rates

The frequency of outcrossing in two hybridizing species of Iris was estimated for populations and for individual fruits. Effects of floral phenology and the local densities of flowers on outcrossing rates were examined and the potential for hybrid seed formation under different pollen environments was assessed. The populations examined differed with respect to the spatial distribution of plants and the level of genetic structure; the I. fulva population consisted of a number of low density patches and appeared to have some genetic differentiation whereas the I. hexagona population consisted of a single high-density experimental plot of randomly distributed genotypes. Population outcrossing rate estimates were relatively high (0.67-0.90) for both species. The distribution of family outcrossing rates tended to be bimodal for both species with individual fruits either having the majority of seeds fertilized by outcrossed donors or being almost entirely selfed. The frequency of outcrossed fruits increased with the number of flowers open at other plants and decreased when more flowers were open on the same stem in I. hexagona. In I. fulva the opposite trends were apparent; outcrossing decreased when more flowers were open on other stems and increased when more flowers were open on the same stem. The unexpected responses of outcrossing frequency in I. fulva may have been a consequence of higher levels of vegetative reproduction and genetic structure and the behaviour of pollen vectors. Differences in pollen prepotency and the higher selfing rates observed at low floral densities in I. hexagona may have contributed to the observed patterns of hybrid seed formation. The analyses of family outcrossing rates provide important information on factors responsible for mating system variation and evolution.