MOLECULAR EVOLUTION OF ANGIOSPERM MITOCHONDRIAL INTRONS AND EXONS

Numbers of substitutions per site for 15 protein-coding genes and six introns of the plant mitochondria were estimated to compare modes and tempos of evolution between exons and introns, and numbers of insertio ns-deletions per site also were investigated in introns, Intra-gene ho mogeneity of numbers of substitutions per site was assessed further am ong different taxa and between mitochondrial and nuclear paralogs tran slocated from the mitochondrial genome, Gene-to-gene differences in nu mbers of substitutions per site were found to be higher for nonsynonym ous than synonymous sites, and this could be due to differential selec tion if mutation rate is assumed constant for the genome. Some mitocho ndrial genes have evolved as fast as chloroplast genes, thus faster th an previously thought, For coxI, relative rate tests showed that woody taxa evolved slower than annuals at synonymous sites, Generation time , population size, and speciation rate are likely factors involved in this rate heterogeneity, Introns were less constrained than their adja cent exons for both overall numbers of substitutions per site and inde ls, but, on average, overall numbers of substitutions per site for int rons were similar to numbers of synonymous substitutions per site for exons, Correlations were generally high between numbers of substitutio ns and numbers of indels per site for the same intron, Mitochondrial g enes transferred to the nucleus had an accelerated rate of substitutio n per site, which was most significant at synonymous sites. These diff erences between paralogs in two different genomes are likely the resul t of different mutation rates.