Stamen elongation, pollen size, and siring ability in tristylous elchhornia paniculata (Pontederiaceae)

Abstract

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 dimorphic, but not trimorphic populations in northeast Brazil. We investigated the effect of stamen elongation on pollen size and siring ability. Competition experiments using controlled hand‐pollination and allozyme markers were used to compare different pollen types. Pollen from the elongated stamen was significantly larger in size than pollen from unmodified short‐level stamens. In mixed pollinations of mid‐level stigmas, pollen from the elongated stamen sired significantly more seed than pollen from unmodified short‐level stamens. Despite these differences, the size and compatibility of pollen from the elongated stamen were more similar to short‐ than mid‐level pollen, indicating that alterations to stamen level were not associated with major changes in pollen characteristics. The results suggest that the advantage of selfing variants in dimorphic populations is mainly due to efficient pollen transfer to mid‐level stigmas rather than increased postpollination siring 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 variants are not the result of recombination in the putative heterostyly supergene.