Evidence for Natural Selection in Nucleotide Content Relationships Based on Complete Mitochondrial Genomes: Strong Effect of Guanine Content on Separation between Terrestrial and Aquatic Vertebrates

Kenji Sorimachi1, 2, *, Teiji Okayasu3
1 Educational Support Center, Dokkyo Medical University, Mibu, Tochigi 321-0293, Japan
2 Life Science Research Center, Higashi-Kaizawa, Takasaki, Gunma 370-0041, Japan
3 Center for Medical Informatics, Dokkyo Medical University, Tochigi 321-0293, Japan

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© Sorimachi and Okayasu; Licensee Bentham Open.

open-access license: This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

* Address correspondence to this author at the Life Science Research Center, Higashi-Kaizawa, Takasaki, Gunma 370-0041, Japan; Tel: +81-27-352-2955; E-mail:


The complete vertebrate mitochondrial genome consists of 13 coding genes. We used this genome to investigate the existence of natural selection in vertebrate evolution. From the complete mitochondrial genomes, we predicted nucleotide contents and then separated these values into coding and non-coding regions. When nucleotide contents of a coding or non-coding region were plotted against the nucleotide content of the complete mitochondrial genomes, we obtained linear regression lines only between homonucleotides and their analogs. On every plot using G or A content purine, G content in aquatic vertebrates was higher than that in terrestrial vertebrates, while A content in aquatic vertebrates was lower than that in terrestrial vertebrates. Based on these relationships, vertebrates were separated into two groups, terrestrial and aquatic. However, using C or T content pyrimidine, clear separation between these two groups was not obtained. The hagfish (Eptatretus burgeri) was further separated from both terrestrial and aquatic vertebrates. Based on these results, nucleotide content relationships predicted from the complete vertebrate mitochondrial genomes reveal the existence of natural selection based on evolutionary separation between terrestrial and aquatic vertebrate groups. In addition, we propose that separation of the two groups might be linked to ammonia detoxification based on high G and low A contents, which encode Glu rich and Lys poor proteins.

Keywords: : Ammonia detoxification, aquatic and terrestrial, evolution, mitochondrial genome, natural selection, normalization, nucleotide content, vertebrate..