Abstract: To augment the databases of Mermithidae mitochondrial genome, to study the mitochondrial genome characteristics and phylogenies of the nematodes, and to provide the theoretical information for the biological control, we carried out the research on the entomopathogenic nematode Ovomermis sinensis. The characterization of mitochondrial genomes from individual O. sinensis nematodes showed that numerous mtDNA haplotypes, ranging in size from 16.5 to 24.5 kb. To understand the molecular basis of this hypervariation, complete nucleotide sequences of two O. sinensis mtDNA haplotypes were determined by two-step long-PCR, with the length of 18 864 and 16 777 bp, respectively. Our data revealed that O. sinensis mtDNA can be divided into a constant region and a hypervariable region. The hypervariable segment, residing between ND2 and ND4, contributes exclusively to O. sinensis mtDNA size variation. Within this region, mtDNA coding genes and putative nonfunctional sequences are duplicated and rearranged to varying extents. Based on the above analysis and the comparison of the mitochondrial genomes of O. sinensis and other six species of Mermithidae, the following characteristics of the mitochondrial genomes of Mermithidae were summarized: firstly, the gene orders of mitochondrial genomes in Mermithidae are different; secondly, the frequency of gene duplication is variable among some species; and thirdly, the sizes of complete mitochondrial genomes are different.

Key words: Ovomermis sinensis, entomogenous nematodes, mitochondrial genome, polymorphism