FERTILITY DIFFERENCES AMONG FLORAL MORPHS FOLLOWING SELFING IN TRISTYLOUS EICHHORNIA-PANICULATA (PONTEDERIACEAE) - INBREEDING DEPRESSION ORPARTIAL INCOMPATIBILITY

Reduction in seed set following self- vs. cross-pollination in floweri ng plants can result from abortion of selfed offspring owing to inbree ding depression and/or partial self-incompatibility. Previous studies on tristylous Eichhornia paniculata (Pontederiaceae) indicate that red uced seed set following self-pollination generally occurs in the short -(S), but not the long-(L) or mid-styled (M) morphs. To determine whet her this pattern results from morph-specific differences in inbreeding depression owing to the sheltering of deleterious alleles at the S lo cus and/or partial self-incompatibility, we conducted controlled hand- pollinations of the floral morphs and measured seed set and levels of seed abortion. There were no significant differences in fertilization success and seed set following self-, illegitimate, and legitimate pol linations in the L and M morphs. In contrast, in the S morph self-, in tramorph and intermorph illegitimate pollinations resulted in signific ant reduction in seed set in comparison with legitimate pollination. T his indicates that the reduced seed set observed in self-pollination i s the result of partial incompatibility rather than inbreeding depress ion. Significantly reduced fertilization success and low levels of ovu le abortion in illegitimate pollinations of S plants also supported th is conclusion. Reduced fertility in the S morph may have implications for the observed loss of this morph from natural populations and the e volutionary breakdown of tristyly.