Abstract: Typical distylous species display both reciprocal herkogamy and heteromorphic incompatibility, which exclude self- and intramorph pollination and accordingly promote intermorph pollination. Here we explore an unusual distylous flower associated with self- and intramorph compatibility in Mussaenda macrophylla Wall. The species did not exhibit a precise reciprocal herkogamy, and the populations studied had a slight bias in favor of the short-styled morph (S-morph). In addition, pollen tube growth in intermorph pollination was faster than either in intramorph or self-pollination, indicating that cryptic heteromorphic self-incompatibility occurred in M. macrophylla. Specifically, pollen tube growth rates after legitimate and illegitimate pollination were much more highly differentiated in long-styled morph (L-morph) than in S-morph, which might be attributed to the increased de-esterified pectin in the tip of L-morph pollen tube wall in comparison with S-morph. Transmission electron microscopic observation revealed illegitimate self-pollinated pollen tubes of two morphs were seriously degraded with the large autolysosome-like vacuoles in cytoplasm of L-morph pollen tube but accumulating small vacuoles in that of S-morph pollen tube, suggesting that incompatible pollen tubes might undergo autophagic programmed cell death except self-incompatible S-morph pollen tubes. Our results indicate that M. macrophylla is morphologically distylous with a cryptic heteromorphic self-incompatibility breeding system, which functions differently in S-morph with self-pollination compared with other illegitimate pollination.

Key words: cryptic self-incompatibility, distyly, Mussaenda macrophylla, pollen tube growth, Rubiaceae