Classification of Non-Animals and Invertebrates Based on Amino Acid Composition of Complete Mitochondrial Genomes

Amino acid compositions were predicted from data sets of 47 non-animal and 58 invertebrate animal complete mitochondrial genomes, which were chosen alphabetically based on scientific names without considering biological characteristics. Using Ward’s clustering method with amino acid composition or nucleotide content as traits, non-animals were classified into Plantae, Chromalveolata, and Fungi, and invertebrates were classified into Animalia and primitive groups, Amoebozoa, Excavata, Protista, and Choanozoa. A combined sample set of primitive eukaryotes was also examined by cluster analysis using amino acid composition and nucleotide content. Some Amoebozoa comprised a single cluster, whereas other Amoebozoa were grouped with other organisms (Excavata, Prosista, Chromaleolata, Fungi and Plantae), indicating their close relationships. Choanozoa (choanoflagellates; Monosiga brevicollis), considered the closest living relatives of animals, were found to be instead closely related to Fungi (Smittium culisetae, Pleurotus ostreatus, and Epidermophyton floccosum) and Excavata (Malawimonas jakobiformis). Our results demonstrate that amino acid composition and nucleotide content are useful indices for characterizing non-animal and invertebrate complete mitochondrial genomes.