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 es timated for populations and for individual fruits. Effects of floral p henology and the local densities of flowers on outcrossing rates were examined and the potential for hybrid seed formation under different p ollen environments was assessed. The populations examined differed wit h respect to the spatial distribution of plants and the level of genet ic structure; the I. fulva population consisted of a number of low den sity patches and appeared to have some genetic differentiation whereas the I hexagona population consisted of a single high-density experime ntal plot of randomly distributed genotypes. Population outcrossing ra te estimates were relatively high (0.67-0.90) for both species. The di stribution of family outcrossing rates tended to be bimodal for both s pecies with individual fruits either having the majority of seeds fert ilized by outcrossed donors or being almost entirely selfed. The frequ ency of outcrossed fruits increased with the number of flowers open at other plants and decreased when more flowers were open on the same st em in I. hexagona. In I. fulva the opposite trends were apparent; outc rossing decreased when more flowers were open on other stems and incre ased when more flowers were open on the same stem. The unexpected resp onses 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. hexag ona may have contributed to the observed patterns of hybrid seed forma tion. The analyses of family outcrossing rates provide important infor mation on factors responsible for mating system variation and evolutio n.