POLLEN DISCOUNTING AND THE SPREAD OF A SELFING VARIANT IN TRISTYLOUS EICHHORNIA-PANICULATA - EVIDENCE FROM EXPERIMENTAL POPULATIONS

Floral traits that increase self-fertilization are expected to spread unless countered by the effects of inbreeding depression, pollen disco unting (reduced outcross pollen success by individuals with increased rates of self-fertilization), or both. Few studies have attempted to m easure pollen discounting because to do so requires estimating the mal e outcrossing success of plants that differ in selfing rate. In natura l populations of tristylous Eichhornia paniculata, selfing variants of the mid-styled morph are usually absent from populations containing a ll three style morphs but often predominate in nontrimorphic populatio ns. We used experimental garden populations of genetically marked plan ts to investigate whether the effects of population morph structure on relative gamete transmission by unmodified (M) and selfing variants ( M') of the mid-styled morph could explain their observed distribution. Transmission through ovules and self and outcross pollen by plants of the M and M' morphs were compared under trimorphic, dimorphic (S morp h absent), and monomorphic (L and S morphs absent) population structur es. Neither population structure nor floral morphology affected female reproductive success, but both had strong effects on the relative tra nsmission of male gametes. The frequency of self-fertilization in the M' morph was consistently higher than that of the M morph under all mo rph structures, and the frequency of self-fertilization by both morphs increased as morph diversity of experimental populations declined. In trimorphic populations, total transmission by the M and M' morphs did not differ. The small, nonsignificant increase in selfing by the M' r elative to the M morph was balanced by decreased outcross siring succe ss, particularly on the S morph. In populations lacking the S morph, m ale gamete transmission by the M' morph was approximately 1.5 times gr eater than that by the M morph because of both increased selfing and i ncreased success through outcross pollen donation. Therefore, gamete t ransmission strongly favored the M' morph only in the absence of the S morph, a result consistent with the distribution of the M' morph in n ature. This study indicates that floral traits that alter the selfing rate can have large and context-dependent influences on outcross polle n donation.