Dynamic evolution of plant mitochondrial genomes: Mobile genes and intronsand highly variable mutation rates

We summarize our recent studies showing that angiosperm mitochondrial (mt) genomes have experienced remarkably high rates of gene loss and concomitant transfer to the nucleus and of intron acquisition by horizontal transfer. Moreover, we find substantial lineage-specific variation in rates of these structural mutations and also point mutations. These findings mostly arise from a Southern blot survey of gene and intron distribution in 281 diverse angiosperms. These blots reveal numerous losses of mt ribosomal protein gen es but, with one exception, only rare loss of respiratory genes. Some linea ges of angiosperms have kept all of their mt ribosomal protein genes wherea s others have lost most of them. These many losses appear to reflect remark ably high land variable) rates of functional transfer of mt ribosomal prote in genes to the nucleus in angiosperms. The recent transfer of cox2 to the nucleus in legumes provides both an example of interorganellar gene transfe r in action and a starting point for discussion of the roles of mechanistic and selective forces in determining the distribution of genetic labor betw een organellar and nuclear genomes, Plant mt genomes also acquire sequences by horizontal transfer. A striking example of this is a homing group I int ron in the mt cox1 gene. This extraordinarily invasive mobile element has p robably been acquired over 1,000 times separately during angiosperm evoluti on via a recent wave of cross-species horizontal transfers. Finally, wherea s all previously examined angiosperm mtDNAs have low rates of synonymous su bstitutions, mtDNAs of two distantly related angiosperms have highly accele rated substitution rates.