What ecological factors favor the shift from distyly to homostyly? A study from the perspective of reproductive assurance

Abstract

Aims Distyly is one of the most widespread floral polymorphisms promoting cross-fertilization. Evolutionary transition from obligate cross-fertilized distyly to predominantly self-fertilized homostyly is frequently documented in various groups. However, empirical studies concerning the ecological factors connected with this transition are still lacking. Primula chungensis, suggested to be evolving from distyly to homostyly, provides an ideal model for the study of the ecological factors concerned with this transition. We study P. chungensis to understand if autonomous self-fertilization would provide reproductive assurance for the self-fertilized homo-styled morph in the field. Methods The incompatibility features of P. chungensis were tested with handpollination experiments. We compared the capacity of autonomous self-fertilization between the distylous and homo-styled morph of P. chungensis in the field by excluding the pollinators with bags. In addition, the degrees of herkogamy of some P. chungensis plants were between the short-styled and homo-styled morphs. These plants were studied to understand whether they were able to obtain greater reproductive assurance when the herkogamy in the flowers was reduced. Important Findings All three morphs of P. chungensis were highly self- and intra-morph compatible. The degree of herkogamy positively correlated with the capacity for autonomous self-fertilization. A negative correlation between the degree of herkogamy and the magnitude of pollen limitation was found, but no significant correlation was found between the degree of herkogamy and the contribution of cross-fertilization to overall fertilization. This study suggests that reducing the degree of herkogamy can significantly increase the reproductive assurance for a self-compatible plant. Our results provided evidence that the homo-styled morph of P. chungensis had the highest capacity for autonomous self-fertilization and the highest seed production in the field, because autonomous self-fertilization provided reproductive assurance for the homo-styled morph. This may cause selection towards the transition from distyly to homostyly.