Abstract

The introns of chloroplast trnKUUU belong to Group II introns and contain an open reading frame denoted as matK. The trnK5’-matK-trnK3’ structure is consistent in almost all examined higher land plants and in Characeae, but not in other green algae examined. The putative gene product MatK is the only maturase in chloroplasts. Functional chloroplast matK genes are retained even in the nonphotosynthetic parasite, Epifagus virginiana and the fern, Adiantum capillus-veneris, in which chloroplast genome rearrangement has left matK free-standing, apart from trnK exons. Among lower land plants, the chloroplasts of Psilotum, mosses and liverworts all have trnK5’-matK-trnK3’ structure, but matK is a pseudogene in hornwort Anthoceros formosae. In this study we found a clear trnK5’-matK-trnK3’ structure in Ophioglossum petiolatum, Lycopodiella cernua and Selaginella doederleinii, but PCR with degenerate primers failed to amplify any trnK or matK fragments from other ferns and fern allies. However, dot blot hybridization showed distinct signals in these plants that failed to amplify matK fragments by PCR, indicating that the matK sequences in those taxa may be too divergent to amplify by an ordinary PCR approach. RT-PCR results showed matK genes are expressed in Ophioglossum petiolatum and Lycopodiella cernua, but no signal was detected in Selaginella doederleinii. Overall, the expression patterns of matK are not consistent in lower land plants. Phylogenetic analysis of matK sequence showed that Pinus, Ginkgo, and Cycas form a monophyletic group, which is sister to angiosperms. Together, they form a clade that is sister to Gnetales. This ad hoc reconstruction is likely due to the high evolutionary rate in matK.

中文摘要

高等植物葉綠體trnKUUU 基因的內插子屬於第二類內插子，其中包含一段開讀框， 因其序列及結構和酵母菌粒線體的maturase 相似，故稱之為matK，matK 是葉綠體中唯一具有去除內插子功能的基因，只有較高等的輪藻和陸生植物的葉綠體trnK 基因才有內插子和trnK5’-matK-trnK3’的結構，其他綠藻葉綠體的trnK 並沒有matK 開讀框、也沒有第二類內插子。葉綠體trnK5’-matK-trnK3’的結構在高等植物相當一致，但先前研究發現兩個matK 脫離trnK 而存在的例子，像是葉綠體嚴重退化的寄生植物Epifagus virginiana 及因葉綠體基因重組而導致trnK 外顯子丟失的鐵線蕨，它們的matK 仍然具有完整的開讀框，顯示了matK 的確十分重要而不可缺失，蘚苔類和松葉蕨也被證實具有trnK5’-matK-trnK3’的結構，但是在角蘚中卻發現其matK 是偽基因。本論文鑑定原始陸生植物其他未被檢視的重要類群中葉綠體matK 基因的存在，結果發現瓶爾小草、過山龍、生根卷柏都有trnK5’-matK-trnK3’的結構，以RT-PCR 偵測matK 表現時，瓶爾小草和過山龍都有偵測到訊號，但生根卷柏卻沒有，顯示matK 的表現在原始陸生植物似乎不是那麼一致。利用PCR 的偵測並未能在水韭、木賊、觀音座蓮、紫萁、海金沙、芒萁等物種中放大出trnK 或matK 的片段。但是利用點式雜合反應則顯示這些物種可能有matK 序列的存在，顯示這些序列可能因變異太大而無法以PCR 方式放大。故有關蕨類葉綠體matK 的存在與否仍有待進一步研究。以葉綠體matK 核甘酸序列進行譜系分析，結果顯示部分裸子植物(松柏、銀杏、蘇鐵)和被子植物形成姊妹群，兩者再和麻黃類植物形成姊妹群。作者認為這個結果主要是由於matK 基因的演化速率太快，而造成譜系分析的不可靠性。

Keyword: Chloroplast matK, Lycopodiella cernua, Selaginella doederleinii, Chloroplast matK, Lycopodiella cernua, Selaginella doederleinii, Ophioglossum petiolatum, Evolutionphioglossum petiolatum, Evolution