INTERSPECIFIC POLLEN COMPETITION AND REPRODUCTIVE ISOLATION IN IRIS

Crossing experiments, success of fruit formation from a mixed population, and genetic analyses of progeny from the same mixed population were used to measure reproductive isolation between Iris fulva and I. hexagona. The frequency of fruit formation in a natural population of approximately 600 I. fulva plants, into which 200 I. hexagona plants had been introduced, was 0.34 and 0.49 for I. fulva and I. hexagona, respectively. Only seven of 710 seeds from this mixed population were hybrids. Experimental crosses using a 1:1 mixture of pollen from I. fulva and I. hexagona were attempted to examine the role of differential fertilization by intraspecific pollen as a reproductive barrier between these two species. None of the 201 progeny (118 from I. fulva maternal plants and 83 from I. hexagona maternal plants) were hybrids. The lack of hybrid progeny from these experiments and the extremely low frequency of hybrid seed produced in the mixed population of I. fulva and I. hexagona suggest that the production of hybrids is a relatively rare event. These findings also suggest that interspecific hybridization would be more likely when flowering plants of one species are in a quantitative minority, or possibly when the majority of the pollen delivered to a flower is from the alternate species. Either condition could result in the establishment of hybrid populations by a small number of individuals.