Abstract
The nucleotide composition of the light (L-) and heavy (H-) strands of animal mitochondrial genomes is known to exhibit strand-biased compositional asymmetry (SCA). One of the possibilities is the existence of a replication-associated mutational pressure (RMP) that may introduce characteristic nucleotide changes among mitochondrial genomes of different animal lineages. Here, we discuss the influence of RMP on nucleotide and amino acid compositions as well as gene organization. Among animal mitochondrial genomes, RMP may represent the major force that compels the evolution of mitochondrial protein-coding genes, coupled with other process-based selective pressures, such as on components of translation machinery- tRNAs and their anticodons. Through comparative analyses of sequenced mitochondrial genomes among diverse animal lineages and literature reviews, we suggest a strong RMP effect, observed among invertebrate mitochondrial genes as compared to those of vertebrates, that is either a result of positive selection on the invertebrate or a relaxed selective pressure on the vertebrate mitochondrial genes.