大花細辛授粉生物學及種子傳播機制

Pollination Biology and Dispersal Mechanism of Asarum macranthum Hook. f.

莊貴竣(Kuei-Chun Chuang)

指導教授 ： 王震哲

國立臺灣師範大學/理學院/生命科學研究所/碩士(2011年)

摘要

在植物的繁殖過程當中，授粉及種子傳播為相當關鍵的兩項步驟。植物長期和生物媒介共演化的結果也使得這兩項步驟衍伸出許多精巧且多樣的設計，但由於缺乏研究也使得這些機制未明且充滿不確定性。因此，對於授粉系統與傳播模式詳盡且深入的研究有助於了解與記錄植物的族群動態以及繁殖策略的成功與否，更進一步地了解植物與媒介共演化後的巧妙機制。
細辛屬植物因為花果皆位於地表，特殊的習性使得其生殖生物學充滿臆測，再加上族群大多呈現小族群群集分布的模式，使得授粉及種子傳播這兩項步驟倍受矚目。故我們針對授粉及種子傳播來進行研究以釐清細辛屬植物的繁殖機制。
植物常利用擬態的方式來吸引授粉媒介前往拜訪，特化的花部構造模擬食物資源、配偶以及產卵場所。Vogel於1978年對於細辛屬植物的授粉提出一項假說，認為細辛萼筒內壁具有許多網格狀的稜脊，其形態與擔子菌綱的蕈褶相仿，故可吸引原來在蕈褶內產卵的蕈蠅轉移產卵場所，藉模擬蕈褶吸引蕈蠅將卵產在細辛萼筒內部，在進出產卵的過程中協助授粉。但有關此授粉機制的假說迄今仍缺乏有統計效力的數據予以驗證，故在本研究中，觀察並記錄台灣特有種大花細辛的生殖生態以驗證Vogel’s hypothesis，並進一步以統計方式呈現蕈蠅產卵現象對成功授粉的影響，且於台灣北部低海拔山區設立一樣區進行長期觀察。根據自然及人工授粉結實率、花型變化、雌雄蕊活性檢測的研究結果顯示大花細辛為一雌先熟(protogynous)且自交親合(self-compatible)的物種，但雄蕊不會自行運動，因此不具自力授粉機制，必須藉由媒介來完成授粉。本研究觀察到蕈蠅會於細辛萼筒內部的網格中產卵，支持Vogel於1978年所提出的授粉假說，顯示蕈蠅是大花細辛的重要傳粉媒介。Fisher’s exact test也證實產卵行為與結實率之間具有顯著相關。氣味分析的結果也佐證了大花細辛與蕈類的氣味相仿，皆具有類似油酸的氣味釋出，更加支持大花細辛模擬蕈蠅生育環境的授粉假說。
此外，本研究也首次觀察到鼠婦傳粉的現象，由於細辛花開在接近地表處，其陰暗且潮試的萼筒內部形成了良好的庇護所，鼠婦躲藏及進出的行為也完成授粉。兩個授粉系統並存之下使得大花細辛的野外族群具有約五成的結實率。
在種子傳播機制方面，本研究確認了大花細辛的種子是藉由螞蟻來進行傳播，根據長期野外觀察及定點攝影，九種螞蟻被認定具有傳播大花細辛種子的能力，相較於脊椎動物的傳播距離，螞蟻傳播的距離顯得較短，在本研究中的平均傳播距離為77.5公分。本研究也證實了種子上的油質體在螞蟻傳播上扮演極為重要的角色。此外，藉由分布位置的分析，顯示螞蟻蟻巢的分布與細辛幼苗的分布具有高度的相關性。因此，大花細辛的分布模式深受螞蟻傳播模式的影響。
綜合上述的研究結果，大花細辛在繁殖的策略上顯現了兩項重要的機制，一為兩種共存的授粉系統；二為以螞蟻為媒介的傳播機制，此兩項關鍵性機制的作用之下，造就了大花細辛成功的繁殖。

關鍵字

大花細辛 ； 蕈蠅 ； 鼠婦 ； 授粉 ； 種子傳播 ； 螞蟻傳播

並列摘要

Pollination and dispersal are two critical steps among plant reproduction. Delicate and diverse mechanisms on these two steps are derived from the results of coevolution. But these mechanisms are still unclear and full of uncertainty due to the lack of researches. Therefore, detailed and extensive studies on pollination system and dispersal pattern are helpful to document the population dynamic, delicate reproductive strategies and mechanisms.
Plants in genus Asarum bloom and set fruits near the ground. Its pollination biology is full of speculations due to the extraordinary habit. Besides, distribution of small and isolated population may be hugely influenced by seed-dispersal pattern. In brief, we take Asarum macranthum as our material in this study to investigate on these two reproductive steps: ‘pollination’ and ‘seed dispersal’.
Plants used to attract their pollinators by mimicry. The strategies are involved not only with food and sexual deception, but also with brood site imitation. Vogel raised a hypothesis of Asarum pollination. The ridges inside the calyx tube of Asarum have similar morphology with gills of basidiomycetes. They deceive fungus gnats to translocate the brood site and to help pollen transfer. No study with further statistical data can prove this hypothesis. Therefore, this study attempts to realize the pollination biology of Asarum macranthum and to prove Vogel’s hypothesis. Moreover, we use the statistical analysis to testify the relationship between oviposition state and fruit ratio. One study area is set in the low altitude mountain of Northern Taiwan for long-term observation. By means of reproductive system, floral morphology and receptivity test, I conclude that Asarum macranthum is a protogynous and self-compatible species without any stamen movement. It most relies on pollinators to achieve pollination. Fungus gnats are discovered to oviposit on the ridges inside the calyx tube, fully supporting Vogel’s hypothesis. Fungus gnat is proved to be the important agent of Asarum macranthum. This study also proved the oviposition state is positively correlated with the fruiting phenomenon by Fisher’s exact test. Volatile analyses also prove that similar chemical substances are emitted between A. macranthum and fungus, which is another evidence to support the pollination hypothesis of brood site imitation.
Besides, this study also observed the isopods-pollination phenomenon for the first time. Dark and dampness environment inside the calyx tube provide a shelter for isopods. The intrafloral behavior of isopods is helpful for pollination. The combination of two pollination systems enhances the fruiting ratio of A. macranthum in the field.
A. macranthum is proved to be a myrmecochorous plant. By field observation and videotaping, nine ant species are recognized as the seed dispersers. Average dispersal distance is 77.5 cm, which is relatively shorter than the distance that dispersed by vertebrate animals. Seed removal rate decreased significantly when elaiosome is removed. Distribution analysis indicated that the distribution of Asarum seedlings is strongly correlated with the distribution of ant nests. Population dynamic and distribution of A. macranthum are strongly influenced by myrmecochorous phenomenon.
Combining results from the above, A. macranthum presents dual-pollination system and myrmecochory on the reproductive strategy. Two critical mechanisms contribute to the successful reproduction.

並列關鍵字

Asarum macranthum ； fungus gnat ； isopods ； Vogel's hypothesis ； seed dispersal ； myrmecochory

參考文獻

21. Wilson, E. O., Durlach, N.I. and Roth, L. M. 1958. Chemical releasers of necrophoric behavior in ants. Psyche. 65: 108-114.

15. Ollerton, J., R. Alarco´n, N. M. Waser, M. V. Price, S. Watts, L. Cranmer, A. Hingston, C. I. Peter, and J. Rotenberry. 2009. A global test of the pollination syndrome hypothesis. Ann Bot. 103: 1471-1800.

8. Huang, S. F. 1996. Asarum. In Huang T. C. et al. (eds.), Fl. Taiwan 2nd ed. Vol. 2. pp. 642- 651.

1. Ayasse, M., F. P. Schiestl, H. F. Paulus, C. Löfstedt, B. Hansson, F. Ibarra, and W. Francke. 2000. Evolution of reproductive strategies in the sexually deceptive orchid Ophrys sphegodes: how does flower-specific variation of odor signals influence reproductive success? Evolution 54: 1995–2006.

2. Azuma, H., J. I. Nagasawa, and H. Setoguchi. 2010. Floral scent emissions from Asarum yaeyamense and related species. Biochem. Syst. Ecol. 38 (4): 548-553.