The mitochondrial DNAs of flowering plants are the largest and most complex of any eukaryotes. One approach to better understanding the complex behavior of this genome exhibited by higher plants is to identify changes in genetic structure and composition of this genome across the landscape of land plant evolution. A comparison of the genomes of Arabidopsis and Maize with Marchantia reveals that very few gene clusters have been conserved during land plant evolution. Presently we have examined two gene clusters that are conserved among most eukarytotic mitochondrial DNA genomes but which have been disrupted in the flowering plant genomes. These gene clusters include the ribosomal DNA cistron and rps12-rps7 genes . The ribosomal DNA cistron of Marchantia consists of the following genes in the order: SSU, two tRNAs, 5S, two tRNAs, and LSU. In flowering plants only the SSU and 5S, without the transfer RNAs, remain linked. A survey of land plants using PCR primers designed to amplify both the SSU-5S and 5S-LSU gene regions reveals that the entire ribosomal cistron is present in representatives of all the main lineages of land plants except gymnosperms and angiosperms. Further, based on nucleotide sequences of selected samples and sizes of PCR products it is likely that the two tRNAs found between the SSU and 5S of Marchantia were lost in the common ancestor of the gymnosperms. The rps12-rps7 gene cluster is conserved among the same organisms as the entire rDNA cistron but is also found among the gymnosperms surveyed and thus appears to have been lost in the ancestor of the angiosperms. The distribution of the linkage of nad3 and rps12 previously reported among gymnosperms and angiosperms will be discussed in the context of the rps12-rps7 data.

Key words: land plants, mitochondria, rDNA cistrons, rps7-rps12