DUFF, R. JOEL*, MARK DAVIS, AND ANGELA BOYLE. Department of Biology, University of Akron, Akron, OH 44325-3908. - The fate of conserved ribosomal DNA and protein coding gene clusters during the evolution of land plant mitochondrial genomes.
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