Abstract
BACKGROUND: Nematodes belonging to the genus Steinernema are insect parasites and are used as effective biological agents against soil-dwelling insect pests. Although the full nuclear genomes of multiple Steinernema species have become available recently, mitochondrial genome information for the genus is limited. In this study, we sequenced the complete mitochondrial genomes of four species of Steinernema and analysed their structure, codon usage and phylogenetic relationships. RESULTS: Mitochondrial genomes of Steinernema carpocapsae, S. glaseri, S. kushidai and S. litorale comprised 13,924, 13,851, 15,182 and 21,403 bp, respectively, with highly AT-rich nucleotide contents (AT ratio of 71.05-76.76 %). All the expected genes, including 12 protein-coding genes (encoding ATP6, CYTB, COX1-3, ND1-6 and ND4L), two rRNA genes and 22 tRNA genes were identified in the four genomes. Phylogenetic analyses based on the amino acid sequences of the 12 protein-coding genes identified the Steinernema species as monophyletic, representing a sister clade of Rhabditina and Ascaridida. In addition, they were more closely positioned to other Clade 10 nematodes, including Bursaphelenchus xylophilus, Aphelenchoides besseyi and Panagrellus redivivus, than to Strongyloides species. Gene arrangements and codon usage analyses supported this relationship. Mitochondrial genome comparison of two distinct strains of S. carpocapsae detected high intra-specific diversity. CONCLUSIONS: The mitochondrial genomes of four species of Steinernema determined in this study revealed inter- and intra-species divergences/diversities of mitochondrial genomes in this genus. This information provides useful insights into the phylogenetic position of the genus Steinernema within the Nematoda and represents a useful resource for selecting molecular markers for diagnosis and population studies. These data will increase our understanding of the interesting biology of insect parasites.