The mechanism for maintenance and evolution of the distylous syndrome has attracted wide attention among ecological and evolutionary researchers. The reproductive characteristics, such as the viability of pollens and of stigmas, the ratio of pollen grains and ovules per flower, the fecundity of pistils and their ability of receiving pollens under natural conditions, and self-compatibility levels, of pin and thrum flowers in Primula merrilliana Schltr. were comparatively investigated by experiments in greenhouse and surveys in wild populations.
The studies of 13 wild populations indicated that the diameter of corolla and the width of corolla lobe were similar between pin and thrum morphs (P>0.05). The corolla tube length of thrum (short-styled) flower (7.33 mm) was significantly longer than that of pin (long-styled) flower (6.35 mm). In thrum morph, anther (7.30 mm high) was at the mouth of the corolla tube, and the stigma (3.98mm high) was situated near the middle of the tubular corolla. In pin morph, however, the stigma (6.50 mm high) was near the mouth of the corolla tube, and the anther (4.09mm high) was at the middle of the tube. The mean number of the pin pollen grains per flower was significantly higher than that of thrum, but the diameter of the former was distinctly smaller than that of the latter. No significant difference was observed in the number of ovule between the two morphs, so the ratio of pollen grains to ovule was obviously higher in pin morph than in thrum morph. Under natural conditions, total pollen deposition at long-styled stigma was significantly higher than at short-styled stigma, but the legitimate pollen grains number (from the opposite morph flower) and filled seeds per fruit were not different significantly between the two morphs. In the cultivated condition, the longevity of pin and thrum flower was similar (about 20-21days). The pollen viability and stigma receptivity of pin flower were also similar to those of thrum. Pollen and stigma of two types flower kept fairly high viability during both prophase and metaphase of flowering (about 16 days) and only reduced distinctly at the end of flowering. And this phenomenon was more obvious in the pollen viability of long-styled flower. The results of artificial pollination indicated that self-compatibility in both morphs were similar, and that inter-morph, intra-morph and self pollinated all could bring filled seeds, but the mean filled seeds per fruit of inter-morph pollinated was higher than that of self- and intra-morph pollinated. The tubes of legitimate pollens (from the opposite morph flower) grew faster than those of illegitimate pollens (from the same morph or self flower) in pin style, but similarly in thrum style. The germination percentage of legitimate pollens was higher than that of illegitimate pollens at thrum stigma, but no such pattern was found in plants with pin stigma. Based on the results above, the breeding system and the mechanism of maintenance of the distylous syndrome in P. merrilliana were discussed.