台灣產蕘花葉灰木與厚皮香之隱性雌雄異株之研究

Abstract

「雄花/兩性異株花」(androdioecy)在被子植物中是極端少見的生殖系統 (breeding system)，理論模式預測這樣的演化條件非常嚴苛，且大部分外觀為雄花/兩性異株花的植物最後都被證實為隱性雌雄異株 (cryptically dioecious)。在台灣，只有蕘花葉灰木 (Symplocos wikstroemiifolia Hayata)與厚皮香 (Ternstroemia gymnanthera (Wight & Arn.) Beddome)這兩種植物曾被記錄為雄花/兩性異株花。過去文獻指出蕘花葉灰木可能是隱性雌雄異株，但是對於厚皮香的生殖系統為何仍然不清楚。本論文的目的為釐清台灣究竟有沒有植物屬於雄花/兩性異株花，以及增進有關隱性雌雄異株的知識。為此我研究台灣產蕘花葉灰木與厚皮香的生殖生物學 (reproductive biology)。 在蕘花葉灰木的研究中，為了釐清其生殖系統的狀態，我分別調查了在北插天山與坪林的植株之性別表現；另外，為了知道蕘花葉灰木與其近緣的雌雄同株物種相比時，是否具有較高的結實率，我對野外的天然結實率加以觀察記錄，並對同樣分布在北插天山的雌雄同株植物尾葉灰木 (S. caudata Wall. ex G. Don)的結實率進行比較；為了檢驗在外觀兩性花上的雄蕊或假雄蕊潛在的授粉者吸引功能，我也一併進行了人工授粉實驗與去雄處理。生殖系統的調查結果顯示，來自外觀兩性花上的雄蕊並不具花粉，因此這種型態的花朵應被視為雌花。幾乎所有的開花植株在調查期間只開雄花或雌花，且上述這些植株中沒有記錄到性別轉換的例子。族群中雖然記錄到較多的雄株，但是性別比例並沒有明顯偏離1:1。這些結果符合預想中隱性雌雄異株的特徵。然而，有兩株個體產生了生殖能力較弱的兩性花。這顯示蕘花葉灰木的性別表現幾乎完全分化為雄性與雌性兩種型態 (gender dimorphism)，但是還是有一些小變異存在於族群中。花部外形測量顯示雄花平均具有約12枚雄蕊；一般而言，每朵雌花其子房具有兩室，而每室中具有4枚胚珠，且胚珠依照大小可以分成兩型。雄株會產生類總狀的聚繖花序 (racemose cymes)與密錐花序 (thyrses)，而雌株只會產生類總狀的聚繖花序。以上這些外形特徵與過往文獻中的敘述並不一致。蕘花葉灰木與尾葉灰木的天然結實率沒有顯著不同，顯示雖然蕘花葉灰木的族群中大約只有一半的植株可以產生果實與種子，這個雌雄異株的物種或許並不依靠提高結實率做為補償策略。另一方面，人工授粉實驗產生的各項結實率並無顯著不同，這可能暗示在蕘花葉灰木中，雌花上的假雄蕊並不具有吸引傳粉者的功能。 在厚皮香的研究中， 由於外觀兩性花的雄蕊具有花粉， 因此在生殖系統的調查上，主要聚焦在探討來自不同型態花朵的花粉可孕性。雄花相較於外觀兩性花，具有較大的花瓣、較長的雄蕊與較多的花粉。來自雄花的花粉具三溝孔 (tricolporate)，形狀為縱橢圓形 (prolate)至球狀縱橢圓型 (prolate-spheroidal)，而來自外觀兩性花的花粉則沒有萌發孔 (inaperturate)且形狀為球形 (spheroidal)。無孔花粉的平均體積幾乎是三溝孔花粉的九倍。花粉萌發實驗顯示無孔花粉無法在蔗糖溶液中萌發。人工授粉實驗顯示，使用來自外觀兩性花的花粉授粉的花朵只有少數發育為果實，而且果實內都只有一枚種子；反之，使用來自雄花的花粉授粉的花朵幾乎全都發育為果實，果實內的種子數範圍從二到六不等。這些結果指出來自外觀兩性花的花粉很有可能不孕，外觀兩性花應該視為雌花。因此厚皮香應是一種隱性雌雄異株的植物，且具有花部與花粉形態二型性 (floral and pollen dimorphism)。然而，有一株雄株在調查期間產生了少量具有生殖能力的兩性花，這或許暗示了在某些條件下厚皮香的性別表現具有可塑性。 依照本文的研究結果，目前在台灣還沒有發現任何功能上雄花/兩性異株花的植物物種。

Androdioecy is an extremely rare breeding system in angiosperms. Theoretical models predicted that the condition for the evolution of androdioecy is stringent and most morphologically androdioecious species were proved to be cryptically dioecious. In Taiwan, Symplocos wikstroemiifolia Hayata and Ternstroemia gymnanthera (Wight & Arn.) Beddome were the only two species documented as morphologically androdioecious. Symplocos wikstroemiifolia had been proposed to be cryptically dioecious, whereas in T. gymnanthera the sexual expression of its breeding system is unclear. This thesis is aimed to validate the existence of androdioecious plants in Taiwan and to increase the knowledge on cryptic dioecy. Thus I laid out experiments of reproductive biology for S. wikstroemiifolia and T. gymnanthera in Taiwan. In S. wikstroemiifolia, to clarify the functional status of breeding system, patterns of sexual expression were examined in Beichatian Mountain and Pinglin; furthermore, to understand whether S. wikstroemiifolia exhibits a higher level of fruit-set than its hermaphroditic relatives, the natural fruit-set was recorded and compared to that in S. caudata Wall. ex G. Don, a hermaphroditic species co-distributed in Beichatian Mountain. Artificial pollination experiments with emasculation treatments were conducted to evaluate the putative pollinator attraction roles of the stamens or staminodes on morphologically hermaphroditic flowers. Breeding system investigations showed anthers on morphologically hermaphroditic flowers did not produce pollen grains, thus this flower type should be considered as female flowers. Nearly all flowering individuals produced only either male or female flowers, and none of them showed changes of sexes between the two-year field investigations. The sex ratios in the population were slightly toward male-biased, but did not significantly deviate from 1:1. These findings were congruent with the proposed cryptic dioecy status. However, two individuals produced functionally hermaphroditic flowers with limited reproductive abilities. It suggested S. wikstroemiifolia exhibited a nearly complete gender dimorphism but with certain flexibility in sexual expression. In general, there were about 12 stamens per male flowers; and two locules within the ovary of the female flowers, where four ovules (two large, two small) in each locule were found. Male individuals produced racemose cymes and thyrses, whereas female individuals produced racemose cymes only. These morphological characteristics were different from previous discriptions. Patterns of natural fruit-set were not significantly different between S. wikstroemiifolia and S. caudata, which suggested that although only about half of the individuals in the population of S. wikstroemiifolia can produce fruits and seeds, this dioecious species may not maintain a high level of fruit-set as a compensatory strategy. On the other hand, the fruit-set among different artificial pollination treatments also did not differ significantly, which may imply in S. wikstroemiifolia, the staminodes on female flowers might not serve as pollinator attraction. In T. gymnanthera, because morphologically hermaphroditic flowers produced pollen grains, breeding system investigations largely focused on the pollen fertility from different flower forms. Male flowers showed larger perianth, longer stamens and more pollen grains than morphologically hermaphroditic flowers. Pollen grains from male flowers were tricolporate and prolate to prolate-spheroidal, whereas pollen grains from morphologically hermaphroditic flowers were inaperturate and spheroidal. The mean volume of inaperturate pollen grains was almost nine times as large as that of tricolporate pollen grains. In vitro pollen germination experiments revealed that the inaperturate pollen grains could not germinate in sucrose solution. Artificial pollination experiments showed that in flowers pollinated with pollen grains from morphologically hermaphroditic flowers, only a few of them developed into fruits, and only one seed per fruit were found. In contrast, nearly all flowers pollinated with pollen grains from male flowers developed into fruits containing two to six seeds. These results demonstrated that pollen grains from morphologically hermaphroditic flowers had very high sterility. Morphologically hermaphroditic flowers could be regarded as functionally female flowers. Therefore T. gymnanthera was cryptically dioecious, which exhibited a floral and pollen dimorphism. However, I also observed one male plants producing few functionally hermaphroditic flowers, which may imply an occasional plasticity of sexual expression occur in T. gymnanthera. In conclusion, there is still no plant species in Taiwan can be proved as functionally androdioecious so far.