Fertility differences among floral morphs following selfing in tristylous Eichhornia paniculata (Pontederiaceae): inbreeding depression or partial incompatibility?

Reduction in seed set following self‐ vs. cross‐pollination in flowering plants can result from abortion of selfed offspring owing to inbreeding depression and/or partial self‐incompatibility. Previous studies on tristylous Eichhornia paniculata (Pontederiaceae) indicate that reduced seed set following self‐pollination generally occurs in the short‐ (S), but not the long‐(L) or mid‐styled (M) morphs. To determine whether this pattern results from morph‐specific differences in inbreeding depression owing to the sheltering of deleterious alleles at the S locus 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 pollinations in the L and M morphs. In contrast, in the S morph self‐, intramorph and intermorph illegitimate pollinations resulted in significant reduction in seed set in comparison with legitimate pollination. This indicates that the reduced seed set observed in self‐pollination is the result of partial incompatibility rather than inbreeding depression. Significantly reduced fertilization success and low levels of ovule abortion in illegitimate pollinations of S plants also supported this conclusion. Reduced fertility in the S morph may have implications for the observed loss of this morph from natural populations and the evolutionary breakdown of tristyly.