A quantitative evaluation of the distylous syndrome in Sebaea grandis (Gentianaceae)

Distyly is a rare polymorphism characterized by short-styled [S] and long-styled [L] floral morphs. It has been hypothesized to reduce the gender conflict inherent in hermaphrodite flowers. In particular, the reciprocal arrangement of male and female parts promotes precise pollen transfer between morphs, and the heteromorphic incompatibility system that is associated with distyly can eliminate uniparental inbreeding depression. We studied the syndrome of floral traits associated with distyly in Sebaea grandis, a perennial herb common in South African grasslands. Flowers have the reciprocal reproductive organ arrangement characteristic of the distylous syndrome, and L- and S-morphs occurred at equal frequencies in the two populations that were studied. S- and L-morph flowers did not differ significantly in size, although the S-morph produced larger pollen grains and pollen exine sculpturing differed between the two morphs. Approximately 80% of the pollen deposited on the stigmas of either morph was legitimate (intermorph), suggesting that distyly in this species promotes cross-pollination. A controlled pollination experiment suggests that S.grandis possesses a heteromophic incompatibility system: no fruits were produced following intramorph pollinations, whereas intermorph pollinations resulted in 70% of flowers setting fruit. Fluorescence microscopy revealed clear differences among morphs in the site of incompatibility. Intramorph pollen grains did not grow beyond the stigmatic surface of the L-morph, and growth was arrested in the stylar canal of the S-morph, while intermorph pollen grains germinated on the stigmatic surface, grew through the stylar canal and entered the ovary. This study demonstrates that S.grandis displays the morphological traits and genetically controlled incompatibility system associated with heterostylous plants.