ABSTRACT
When the fi rst complete mitochondrial DNA (mtDNA) sequence-that of humans-was determined (Anderson et al. 1981), it was fi ttingly described by the phrase “small is beautiful” (Borst and Grivell 1981). Mitochondrial genomes of bilaterian animals are indeed small (~16 kpb), not only because of their limited coding capacity (typically 37 genes), but also due to the remarkable economy of their genomic organization (Fig. 1). Genes encoded in bilaterian mtDNA are compactly arrayed, separated by no, or only a few, nucleotides and typically contain neither introns (but see Valles et al. 2008) nor regulatory sequences. Protein and transfer RNA genes are even often truncated and completed by either posttranscriptional polyadenylation (Yokobori and Pääbo 1997) or, in some cases, editing (Lavrov et al. 2000). Furthermore, changes in the genetic code allowed animals to reduce the set of mitochondrial tRNA genes to 22, several tRNAs fewer than the minimum number required for translation under the standard genetic code (Marck and Grosjean 2002). Encoded ribosomal RNAs are also reduced in size, lacking many secondary structures present in homologous molecules in other groups. In addition to small size, bilaterian mtDNA displays several unusual genetic and genomic
253 Bessey Hall, Department of Ecology, Evolution and Organismal Biology, Iowa State University, Ames, Iowa 50011. E-mail: dlavrov@iastate.edu
features such as unorthodox translation initiation codons, highly modifi ed structures of encoded transfer RNAs, a high rate of sequence evolution, a relatively low rate of gene rearrangements, and the presence of a single large non-coding “control” region (Wolstenholme 1992b).
