The nonreciprocal heterostyly and heterotypic self-incompatibility of Ceratostigma willmottianum

Ceratostigma willmottianum (Plumbaginaceae) is a perennial herb native to China. Many species of Plumbaginaceae have been reported to exhibit heterostyly. Determining the functional breeding system of C. willmottianum can improve our understanding of the reproductive ecology of heterostylous plants. We investigated the floral traits and pollen and stigma characteristics in a natural population, and artificial pollination was carried out in an artificial population. It was found that C. willmottianum was distylous with short (S)- and long (L)-styled morphs, did not exhibit precise reciprocal herkogamy and was partially self-compatible but primarily outcrossing. In the artificial pollination experiments, the pollen tubes reached the base of the style under intermorph pollination, whereas they rarely penetrated the style under intramorph pollination and self-pollination. Both the L and S morphs exhibited a high seed set after intermorph pollination, whereas the seed set under intramorph pollination was lower. Therefore, C. willmottianum may spread the chance of receiving pollen between the two morphs by nonreciprocal heterostyly, which may be a unique mode of ecological adaptation in Plumbaginaceae. We believe that our discovery could provide new ideas regarding the origin and evolution of heterostyly.