| Summary: | 碩士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 105 === Angiosperms exhibit great diversity of sexual systems, as the majority of extant species are hermaphrodite, the evolution of unisexuality has drawn attention since Darwin’s age. The reproductive biology of Daphne arisanensis Hayata (Thymelaeaceae), an endemic shrub in Taiwan, had rarely been studied before. Field observations implied that populations of D. arisanensis comprise morphologically bisexual and male-sterile individuals, hence this species may be gynodioecious. However, no unequivocal evidence had been presented yet. To clarify the true sexual system of D. arisanensis, I investigated the spatial and temporal expression of sexuality, and tested the reproductive potential of each floral morph.
In this study, phenology surveys were conducted in Meifeng, Yuanfeng, Hehuanshan, Dayuling and Guanyuan during 2014-2017. Pollination experiments were operated in Guanyuan and the Hehuanshan-nearby alpine populations respectively. No individual altered the flowering type in this three-year observation. Male-sterile flowers had the ability to set fruits either with pollination or without fertilization (i.e. apomixis). The gynoecia of morphologically bisexual flowers displayed receptivity to pollens from either itself or other plants, but they hardly set fruits among all pollination treatments. In addition, unusual flowers with different degrees of male-sterility were occasionally found on morphologically bisexual plants. The extremes were quite similar to the typical “female” flowers in male-sterile plants and indeed set fruit under open pollination. The seeds from all kinds of treatments showed germination potential. Great variations in morph ratio, floral traits and floral phenology were found among populations. Compared to the Guanyuan population, individuals in alpine areas showed larger flower size, longer floral lifespan and lower fruit-set rates under both open pollination and apomixis. Various nonspecific nocturnal moths were observed pollinating in both of the populations.
To provide materials for inferring the possible evolutionary scenario of the gender divergence in D. arisanensis, I also conducted bagging experiments to another congeneric hermaphroditic species, D. kiusiana Miq. var. atrocaulis (Rehder) Maekawa, in Yangmingshan. The results show that 49% of the bagged flowers had set fruits via autonomous selfing, suggesting that self-pollination may be common in genus Daphne.
These findings indicate that the sexual system of D. arisanensis is cryptic and leaky dioecy. The flowering phenology and floral traits varied among populations, and the evolution toward dioecism would probably be correlated with the self-pollination pressure in genus Daphne.
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