REPRODUCTIVE INTERACTIONS BETWEEN HYBRIDIZING IRISES - ANALYSES OF POLLEN-TUBE GROWTH AND FERTILIZATION SUCCESS

Pollen-tube growth and seed siring ability were measured in crosses be tween the Louisiana iris species Iris fulva and Iris hexagona and thei r F-1 and F-2 hybrids. Flowers of the parental species were pollinated with self, outcross intraspecific, and interspecific pollen. Pollen-t ube lengths were similar for all three pollen types in I. fulva, but i n I. hexagona interspecific pollen tubes were longer than intraspecifi c pollen tubes. Pollen-tube lengths also differed for F-1 and F-2 flow ers pollinated with I. fulva, I. hexagona, and hybrid pollen. For both hybrid classes I. fulva pollen tubes were the shortest white pollen t ubes from I. hexagona and hybrids grew the furthest. Mixtures of genet ically marked pollen were used to assess the seed siring ability of in tra- and interspecific pollen in the parental species by varying the p roportion of each pollen type in a replacement series design. For both species, the observed proportions of hybrid seeds were lower than the expected based on the frequency of each pollen type in the mixtures a cross all treatments. Flowers of I. fulva produced less than 10% hybri d progeny even when 75% of the pollen applied to stigmas was derived f rom interspecific flowers. The frequency of hybrid seed formation was somewhat greater in I. hexagona, but was still significantly lower tha n expected across all mixture treatments. Seed set per fruit remained constant across the mixture treatments for both species, but in I. ful va fruit set decreased with an increase in the proportion of interspec ific pollen. The data indicate that both pre- and postfertilization pr ocesses contribute to discrimination against hybrid seed formation.