Mitochondrial genome sequence evolution in Chlamydomonas
Cristina E. Popescu 1 and Robert W. Lee 1*
1 Dalhousie University
* To whom correspondence should be addressed. E-mail: robert.lee{at}dal.ca.
Submitted on July 8, 2006
Revised on September 13, 2006
Accepted on 15 November 2006
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ABSTRACT
The mitochondrial genomes of the Chlorophyta exhibit significant diversity with respect to gene content and genome compactness, however, quantitative data on the rates of nucleotide substitution in mitochondrial DNA, which might help explain the origin of this diversity, are lacking. In order to gain insight on the evolutionary forces responsible for mitochondrial genome diversification we sequenced to near completion the mitochondrial genome of the chlorophyte Chlamydomonas incerta, estimated the evolutionary divergence between Chlamydomonas reinhardtii and C. incerta mitochondrial protein-coding genes and rRNA-coding regions, and compared the relative evolutionary rates in mitochondrial and nuclear genes. Synonymous and nonsynonymous substitution rates do not differ significantly between the mitochondrial and nuclear protein-coding genes. The mitochondrial rRNA-coding regions, however, are evolving much faster than their nuclear counterparts, and this difference might be explained by relaxed functional constraints on the mitochondrial translational apparatus due to the small number of proteins synthesized in Chlamydomonas mitochondria. Substitution rates at synonymous sites in a non-standard mitochondrial gene (rtl) and at intronic and synonymous sites in nuclear genes expressed at low levels suggest that the mutation rate is similar in these two genetic compartments. Potential evolutionary forces shaping mitochondrial genome evolution in Chlamydomonas are discussed.
Key Words:
Chlamydomonas, mitochondrial genes, nuclear genes, substitution rates
