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

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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.