Distyly and cryptic heteromorphic self‐incompatibility inMussaenda macrophylla(Rubiaceae)

Abstract

AbstractTypical 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 inMussaendamacrophyllaWall. 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 inM. 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 thatM. macrophyllais morphologically distylous with a cryptic heteromorphic self‐incompatibility breeding system, which functions differently in S‐morph with self‐pollination compared with other illegitimate pollination.