真雙子葉植物花部B群同源基因之重複與演化

雖然花部器官決定的ABC 模組解釋了大部份被子植物花部特徵的形成，但在被子植物基群以及真雙子葉基群的植物，其花部器官決定的基因表現仍存在許多變異。在真雙子葉植物基群及核心真雙子葉植物的基群的花部大小以及花被片的輪數都與典型核心真雙子葉植物有很明顯的差異，比如昆欄樹的花被片缺失，而檀香目植物的花被片通常只有一輪。顯然在此之中必定在花部發育的調控中發生了一些未知但重要的改變，因此這些植物在探討被子植物花被片的演化上扮演著非常重要的角色。近年來的研究顯示，在ABC 三群基因在演化至核心真雙子葉植物的過程中各發生了一次主要的基因重複事件。然而，二個 B 群同源基因的系群只有其中之一在此主要重複事件中固定下來，而變成euAP3 或TM6 次系群。因此，這樣的重複事件可能與真雙子葉植物基群花部特徵的多樣性有關。本論文分別自演化上屬於關鍵類群的黃楊科(臺灣黃楊)、昆欄樹科(昆欄樹)、第倫桃科(第倫桃)以及檀香目植物(百蕊草、椆樹桑寄生以及高氏桑寄生) 中選殖花部B 群同源基因，希望能釐清B 群同源基因重複事件是否在真雙子葉植物基群中發生。在六種植物中，一共得到17 個與B 群同源基因相似的cDNAs 序列。譜系分析結果顯示，前兩種植物具有paleoAP3 同源基因，而後四種植物則具有euAP3 或?? TM6 同源基因。由譜系結果推論，因此本論文將B 群同源基因的重複事件的發生時間推定於昆欄樹科出現之後，核心真雙子葉植物分歧前的這段演化期間。此外，選汰壓力偵測的結果亦顯示PI-like 基因系群或是AP3- like 基因系群在各演化階段均受到強烈的純化選汰(purifying selection)壓力影響，尤其在被子植物基群與真雙子葉植物基群之間，選汰壓力的變動相當地顯著，但是基因重複與選汰壓力在不同植物類群間的關連性，仍有待進一步的釐清。有趣的是，在檀香目植物的基因選殖中，並未發現有euAP3-like 基因的存在，如果euAP3-like 基因真的在花部器官不表現或甚至已自檀香目中丟失，則表示其TM6-like 基因勢必要負責起所有必備的功能。

關鍵字

重複；演化；選汰限制；花部器官； B群基因；真雙子葉

並列摘要

Although the floral ABC model is generally accepted as ubiquitous in flowering plants, variation in gene expression has been found in basal angiosperms and basal eudicots. Accordingly, the basal eudicots and basal core eudicots show highly variable floral features in terms of flower size and floral organ numbers, for example, the perianth is absent in Trochodendron aralioides and flowers is generally reduced in Santalales. These plants are indeed critical to understand the evolutionary developmental program of perianth evolution in flowering plants. Previous studies have shown that there is a major duplication event of all ABC class genes along the branch leading to core eudicot lineages. However, only one of the two B class homologues (AP3-like) is found duplicated and fixed in this major event, i.e. to euAP3 and TM6 sublineages, but not the PI-like homologues. Such duplication may be responsible to the diversification of floral features among basal eudicots. Here we chose taxa from Buxaceae, Trochodendraceae, Dilleniaceae and Santalales to explore the B class gene duplications among basal core eudicots. Seventeen cDNA clones encoding B class gene homologues were determined from Buxus microphylla ssp. sinica, Trochodendron aralioides, Dillenia indica, Thesium chinense, Loranthus kaoi and Loranthus delavayi. All of these taxa have one to three PI homologues, and various AP3 homologues. Results from phylogenetic analysis suggest that the first two taxa have paleoAP3 homologues whereas the later four are most likely having either TM6 or euAP3-like homologues instead. Based on the phylogenetic analysis, we proposed the euAP3/TM6 duplication occurred after the separation of the Trochodendraceae but before the divergence of the core eudicots. Besides, the analysis for detecting selective constraints also suggests that both PI and AP3 lineages have been subjected to strong purifying selection, and the switch of selective constraint is apparently present between basal eudicots and core eudicots. A thorough survey is necessary to elucidate the correlations between gene duplications and the selective constraints among different clades of basal and core eudicots. Interestingly, euAP3 homologue was not found in the screening of B class genes from Santalales species, suggesting the function would be complemented by PI or TM6 homologues if they are truly absent in Santalales.

並列關鍵字

evolution ； B class genes ； eudicots ； floral organ ； selective constraint ； duplication ； MADS box gene

參考文獻

systematics, molecular developmental genetics and the angiosperm flower.

Page 349-374 in Molecular Systematics of Plants II (Soltis, P., Soltis, D., and

evolution beyond flowers: expression in pollen, endosperm, guard cells, roots and

2000b. An ancestral MADS-box gene duplication occurred before the divergence

BAUM, D. A. 1998. The evolution of plant development. Current Opinion in Plant

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真雙子葉植物花部B群同源基因之重複與演化

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