|其他摘要||Self-incompatibility, the physiological inability of a fertile hermaphrodite plant to produce viable seeds upon self-pollination, is the principal and most effective mechanism preventing self-fertilization in flowering plants. However, in extremely harsh environments where pollinators are scarce and unstable, self-incompatibility might break down, followed by self-pollination to assure sexual reproduction. Since the evolution of selfing races or species from primarily outcrossing ancestors is one of the most frequent evolutionary transitions in the plant kingdom, empirical studies comparing mating systems and pollination mechanisms in closely related species within one genus will help us to better understand the evolution of plant mating system. In this dissertation, I performed field experiments on Anisodus luridus Link and A. carniolicoides (C. Y. Wu and C. Chen) D'Arcy and Z. Y. Zhang, which belongs to the Hyoscyameae in Solanaceae, attempting to explore the evolution of self-compatibility and pollinator shift in the genus Anisodus. Additionally, I also explored the adaptive significance of pendulous flowers during anthesis in these two Anisodus species, and upward fruits with persistent calyx in A. luridus. The major findings of my investigations are summarized as follows:
1. Both fruit set and seed number from manual selfing and outcrossing did not differ significantly in the two species, showing that both A. luridus and A. carniolicoides were fully self-compatible. The low fruit set and seed production after pollinator isolation without emasculation indicated a potential ability of autonomous selfing in both species. Seed production with supplemental outcross pollen was greatly enhanced compared with natural flowers, suggesting pollen limitation in these two species. Vespula rufarufa L. and Dolichovespula saxonica Fabr. were the most frequent pollinators of A. luridus and A. carniolicoides, respectively. Based on the total number of pollen grains deposited on virgin stigmas within one day from 10:00 to 17:00 (7 h in total) to evaluate total pollination efficiency, I found that pollination efficiency of intact flowers was significantly higher than that of emasculated flowers in A. carniolicoides, but not in A. luridus. Therefore, visitations of D. saxonica to A. carniolicoides were speculated to result in selfing within flower, i.e. facilitated selfing, whereas V. rufarufa could facilitate outcrossing among flowers in A. luridus. Additionally, A. tanguticus (Maxim.) Pascher, the putatively primitive species in Anisodus, was reported to be self-incompatible/partial self-compatible and pollinated by bumblebees, ants and flies. Compared with the pollination mechanism of A. tanguticus, it was obvious that self-compatibility occurred in the derived species and the evolutionary transition from self-incompatibility to self-compatibility together with pollinator shift did happen during the speciation in this genus. Moreover, the unpredictable and frequent rain in alpine environment might depress pollinator activity, which could be the principal reason for pollen limitation in A. luridus and A. carniolicoides.
2. Flower stalks of A. luridus and A. carniolicoides bend during flowering, but erect when fruiting only in A. luridus, together with enlarged and persistent calyx. My results showed that (1) in the two Anisodus species, there was no significant difference in pollinator visitation frequency and pollination efficiency between pendulous and erected flowers in the field, so pendulous flowers were unlikely to attract more pollinators or enhance pollen deposition. (2) In artificially erected flowers, pollen grains in anthers greatly decreased after rain-wash in A. luridus and A. carniolicoides. Additionally, for A. luridus, pollen germination rate reduced significantly due to water immersion and direct exposure to sunlight, indicating that nodding flowers could maintain viability of pollen grains in front of rain immersion and intense solar radiation. (3) After supplemental pollination in A. luridus, the number of mature seeds, seed set and weight of 100 seeds from fruits without any water in persistent calyx were much lower compared with intact control fruits, but no significant difference existed in seed germination rate. I conclude that the reflexed flower stalk made the campanulate corolla in two Anisodus species act as an umbrella to avoid rain damage to pollen quantity and quality, protecting pollen grains from direct exposure to intense solar radiation and improving male fitness. The upright fruit pedicel in A. luridus could support the elongated persistent calyx to hold water inside when rainfall was frequent, providing a stable microenvironment for the fertilized ovules and then promote their development, thus enhance female fitness. Nodding flowers in the two species and upright fruits in A. luridus seem to be an effective strategy to adapt the typical alpine environment of the QTP, characterized by frequent and unpredictable rains, intense solar radiation in the daytime and chilling at night.|