STAMEN ELONGATION, POLLEN SIZE, AND SIRING ABILITY IN TRISTYLOUS EICHHORNIA-PANICULATA (PONTEDERIACEAE)

Selfing variants in tristylous Eichhornia paniculata (Pontederiaceae) possess an elongated, short-level stamen adjacent to mid-level stigmas , which causes autonomous selfing. The Variants commonly spread in dim orphic, but not trimorphic populations in northeast Brazil. We investi gated the effect of stamen elongation on pollen size and siring abilit y. Competition experiments using controlled hand-pollination and alloz yme markers were used to compare different pollen types. Pollen from t he elongated stamen was significantly larger in size than pollen from unmodified short-level stamens. In mixed pollinations of mid-level sti gmas, pollen from the elongated stamen sired significantly more seed t han pollen from unmodified short-level stamens. Despite these differen ces, the size and compatibility of pollen from the elongated stamen we re more similar to short- than mid-level pollen, indicating that alter ations to stamen level were not associated with major changes in polle n characteristics. The results suggest that the advantage of selfing v ariants in dimorphic populations is mainly due to efficient pollen tra nsfer to mid-level stigmas rather than increased postpollination sirin g success of pollen from modified stamens. In addition, the absence of major changes in pollen size and compatibility associated with stamen elongation support other lines of evidence indicating that selfing Va riants are not the result of recombination in the putative heterostyly supergene.