Both‐strand gene coding in a plastome‐like mitogenome of an enoplid nematode

The phylum Nematoda remains very poorly sampled for mtDNA, with a strong bias toward parasitic, economically important or model species of the Chromadoria lineage. Most chromadorian mitogenomes share a specific order of genes encoded on one mtDNA strand. However, the few sequenced representatives of the Dorylaimia lineage exhibit a variable order of mtDNA genes encoded on both strands. While the ancestral arrangement of nematode mitogenome remains undefined, no evidence has been reported for Enoplia, the phylum's third early divergent major lineage. We describe the first mitogenome of an enoplian nematode, Campydora demonstrans, and contend that the complete 37‐gene repertoire and both‐strand gene encoding are ancestral states preserved in Enoplia and Dorylaimia versus the derived mitogenome arrangement in some Chromadoria. The C. demonstrans mitogenome is 17,018 bp in size and contains a noncoding perfect inverted repeat with 2013 bp‐long arms, subdividing the mitogenome into two coding regions. This mtDNA arrangement is very rare among animals and instead resembles that of chloroplast genomes in land plants. Our report broadens mtDNA taxonomic sampling of the phylum Nematoda and adds support to the applicability of cox1 gene as a phylogenetic marker for establishing nematode relationships within higher taxa. The phylum Nematoda remains poorly sampled for mtDNA, with a strong bias toward parasitic, economically important or model species of the Chromadoria lineage. Most chromadorian mitogenomes share a specific order of genes encoded on one mtDNA strand. We describe the mitogenome of Campydora demonstrans, the first representative of the Enoplia major lineage, and contend that the complete gene repertoire and both‐strand gene encoding are the phylum's ancestral states. It contains a noncoding perfect inverted repeat, an arrangement very rare among animals and instead resembling that of chloroplast genomes in land plants. Research Highlights Genes are encoded on both strands in the mitochondrial genome of enoplid nematode Campydora demonstrans. The coding portion of C. demonstrans mitochondrial genome is subdivided in two regions by two arms of a long noncoding inverted repeat. cox1 gene can serve as a suitable phylogenetic marker for resolving within‐higher‐taxon relationships among nematodes.