VARIATION IN THE MATING SYSTEM OF EICHHORNIA PANICULATA (SPRENG.) SOLMS. (PONTEDERIACEAE)

A multilocus procedure was used to estimate outcrossing rates from allozyme data in nine populations of Eichhornia paniculata from NE Brazil and Jamaica. The populations were chosen to represent stages in a proposed model of the evolutionary breakdown of tristyly to semi-homostyly; they differed in morph structure (trimorphic, dimorphic, or monomorphic) and floral traits likely to influence the mating system. The interpopulation range in outcrossing rate, t, was 0.96–0.29. Two additional populations from Jamaica, composed exclusively of self-pollinating, semi-homostylous, mid-styled plants, were invariant at 21 isozyme loci, precluding estimates of outcrossing frequency. Trimorphic populations from Brazil had uniformly high outcrossing rates of 0.96–0.88. Values for the floral morphs within populations were not significantly different. A controlled pollination experiment, comparing the competitive ability of self and cross pollen using the Got-3 marker locus, provided evidence that the maintenance of high outcrossing rates in trimorphic populations results from the prepotency of cross pollen and/or the selective abortion of selfed zygotes. Morph-dependent variation in t was detected within a dimorphic population with the L morph outcrossing with a frequency of 0.76 in comparison with 0.36 in the M morph. The difference in the mating system of floral morphs results from modifications in position of short-level stamens in flowers of the M morph resulting in automatic self-pollination. The occurrence of E. paniculata populations composed exclusively of self-pollinating, mid-styled variants is thought to be associated with the spread of genes modifying stamen position. The high level of self-fertilization demonstrated in the M morph would allow automatic selection of these genes, augmented by fertility assurance in the absence of specialized pollinators.