Mitochondrial DNA in early tracheophytes: Towards complete chondriome sequences of Isoetes, Lygodium and Welwitschia

与叶绿体DNA相比，植物线粒体DNA表现出明显不同的进化模式和大量特殊的分子现象，如RNA编辑、反式剪接、从细胞核和叶绿体获取序列等。到目前为止，对线粒体进化的见解主要基于几个单基因研究，以及七种开花植物和地钱草的完整线粒体基因组序列。我们建议获得维管植物(维管植物)中三个古老的(和孤立的)谱系的完整的线粒体DNA序列：昆虫属、蕨类和裸子植物。对石蒜等位体的初步研究已经形成了一个有序的Fosid文库，通过杂交鉴定了一些含有线粒体DNA的Fosid克隆，这可以通过亚克隆和初步测序来验证。第一个序列数据在以下几个方面证实了等丝虫确实是一个具有特殊mtDNA的物种：(I)RNA编辑极其丰富，(Ii)它以迄今未见的频率影响tRNA，(Iii)新的内含子伴随着先前在被子植物和苔类中观察到的那些内含子，以及(Iv)频繁的基因组重组导致共存的mtDNA排列。我们相信，对蕨类植物和裸子植物线粒体的分析将有力地加深我们对苔藓植物向早期管状植物系统发育过渡带中分子细胞器基因组进化的理解。

Compared to their chloroplast counterparts the mitochondrial DNAs of plants show strikingly different modes of evolution and a plethora of peculiar molecular phenomena such as RNA editing, trans-splicing, and the acquisition of sequences from the nucleus and the chloroplast. The insights into chondriome evolution gathered so far are largely based on several single gene studies, and the completed mitochondrial genome sequences of seven flowering plants and the liverwort Marchantia. We here suggest to obtain complete mitochondrial DNA sequences for three representatives of ancient (and isolated) lineages among vascular plants (tracheophytes): the quillwort Isoetes, the fern Lygodium and the gymnosperm Welwitschia. Preliminary work on the lycophyte Isoetes has already resulted in a ordered fosmid library, the identification of a number of fosmid clones containing mitochondral DNA by hybridization which could be verified by subcloning and initial sequencing. The first sequence data confirm that Isoetes indeed is a species with a special mtDNA in several ways: (i) RNA editing is extremely abundant and (ii) it affects tRNAs in a hitherto unseen frequency, (iii) novel introns accompany those that have previously been observed in angiosperms and the liverwort and (iv) frequent genomic recombination results in co-existing mtDNA arrangements. We believe that the analysis of pteridophyte and gymnosperm chondriomes will strongly increase our understanding of molecular organelle genome evolution in the important phylogenetic transition zone from bryophytes to early tracheophytes.