CONTRIBUTION OF CRYPTIC INCOMPATIBILITY TO THE MATING SYSTEM OF EICHHORNIA-PANICULATA (PONTEDERIACEAE)

Tristylous populations of the annual aquatic Eichhornia paniculata hav e high levels of outcrossing and intermorph mating despite being fully self- and intramorph compatible. Experimental studies of pollen germi nation, pollen-tube growth, and pollinations with mixtures of genetica lly marked pollen were used to determine whether postpollination proce sses contribute to the observed mating patterns. Differences in pollen germination were small and did not contribute to differences in polle n siring ability. The fraction of pollen tubes first entering the ovar y, however, was greater for legitimate outcross pollen than for either of the other two pollen types (self or outcross illegitimate pollen) in all recipient morphs. Moreover, legitimate pollen had higher siring success when in competition with illegitimate pollen types (self or o utcross) in each recipient style morph. The ranking of pollen performa nce for different pollen-style combinations was the same for both the pollen-tube growth and marker-gene experiments indicating that differe nces in pollen-tube growth rate are the principal cause of differences in pollen siring ability. Cryptic incompatibility in E. paniculata ma y represent a weak heteromorphic incompatibility system because the ob served patterns of pollen-tube growth parallel pollen-tube growth and seed-set patterns that occur in related species with strong trimorphic incompatibility. The ability to produce mostly outcrossed progeny whe n pollinators are abundant, but to reliably produce seed under a varie ty of environmental and demographic conditions may be favored in E. pa niculata because of its colonizing life history and occurrence in ephe meral habitats. Cryptic incompatibility may be more likely to occur in species subject to wide fluctuations in population size and levels of pollinator service.