Sequencing and phylogenomics of the complete mitochondrial genome of Allodiplogaster sp. (Rhabditida: Diplogasteridae): A new gene order and its phylogenetic implications.

Gene order has been utilized as a phylogenetic signal for many taxa. However, its phylogenetic performance has not been evaluated in Nematoda. As there is only one nematode mitogenome available to date, in the Diplogasteridae family, we sequenced the mitogenome of Allodiplogaster sp. and constructed a phylogeny for Nematoda using this updated mitogenome dataset. We then compared this phylogeny to one constructed using gene order. The complete mitochondrial genome of Allodiplogaster sp. was 13,953 bp in size and included 22 tRNAs, two rRNAs, and 12 protein-coding genes. To assess how Allodiplogaster sp. is related to other nematode species, we used Bayesian inference and maximum likelihood algorithms to construct phylogenetic trees of the Nematoda. We found that: 1) The target species Allodiplogaster sp. is closely related to Allodiplogaster sudhausi. The topology of the mitogenome based phylogeny was nearly identical to previous phylogenies created using 18S rRNA data, except for the placement of the Strongyloididae family. 2) The maximum likelihood tree constructed using gene order was roughly consistent with the mitogenome-based tree at the family level, but not at the species level. 3) Protein-coding genes were ordered differently in Allodiplogaster sp. versus Allodiplogaster sudhausi; this represents the first report of such a reordering in the class Chromadorea in our study. Our study confirms that gene order represents useful phylogenetic information for the Nematoda: the maximum likelihood tree based on gene order provided additional support for the nematode phylogeny constructed using molecular data.