INTERSPECIFIC POLLEN COMPETITION AS A REPRODUCTIVE BARRIER BETWEEN SYMPATRIC SPECIES OF HELIANTHUS (ASTERACEAE)

Artificial crosses between Helianthus annuus and H. petiolaris using 1 :9, 1:1, and 9:1 mixtures of intraspecific : interspecific pollen were conducted to determine the role of interspecific pollen competition a s a reproductive barrier in Helianthus. Of 1,245 achenes analyzed from the pollen competition experiments, only 49 were hybrids. The number of hybrids observed was significantly less than expectations for all t hree pollen mixtures, regardless of the identity of maternal parent (P < 0.01). Stigma age and pollen ratio had no significant impact on hyb rid frequency. However, hybrids were significantly more frequent with H. annuus than with H. petiolaris as the maternal parent (P < 0.01). A nalysis of pollen tube growth rates revealed no differences in the rat e of growth of intraspecific vs. interspecific pollen. Likewise, polli nations with either intraspecific or interspecific pollen or with diff erent pollen ratios did not affect the percentage of filled achenes. T hus, the mechanism responsible for selective fertilization by intraspe cific pollen in mixed pollen loads remains unclear. Nonetheless, these findings suggest that interspecific pollen competition plays an impor tant role in controlling the formation of hybrids between H. annuus an d H. petiolaris and may partially account for patterns or differential cytoplasmic vs. nuclear introgression in Helianthus.