Mitochondrial genome of the eyeworm, Thelazia callipaeda (Nematoda: Spirurida), as the first representative from the family Thelaziidae

Abstract

Human thelaziosis is an underestimated parasitic disease caused by Thelazia species (Spirurida: Thelaziidae). The oriental eyeworm, Thelazia callipaeda, infects a range of mammalian definitive hosts, including canids, felids and humans. Although this zoonotic parasite is of socio-economic significance in Asian countries, its genetics, epidemiology and biology are poorly understood. Mitochondrial (mt) DNA is known to provide useful genetic markers to underpin fundamental investigations, but no mt genome had been characterized for any members of the family Thelaziidae. In the present study, we sequenced and characterized the mt genome of T. callipaeda. This AT-rich (74.6%) mt genome (13,668 bp) is circular and contains 12 protein-coding genes, 22 transfer RNA genes and two ribosomal RNA genes, but lacks an atp8 gene. All protein-coding genes are transcribed in the same direction; the gene order is the same as those of Dirofilaria immitis and Setaria digitata (Onchocercidae), but distinct from Dracunculus medinensis (Dracunculidae) and Heliconema longissimum (Physalopteridae). Phylogenetic analyses of the concatenated amino acid sequence data for all 12 protein-coding genes by Bayesian inference (BI) showed that T. callipaeda (Thelaziidae) is related to the family Onchocercidae. This is the first mt genome of any member of the family Thelaziidae and should represent a new source of genetic markers for studying the epidemiology, ecology, population genetics and systematics of this parasite of humans and other mammals.

Figure 1. Organization of the mitochondrial genome of Thelazia callipaeda.

Scale is approximate. All genes have standard nomenclature except for the 22 tRNA genes, which are designated by the one-letter code for the corresponding amino acid, with numerals differentiating each of the two leucine- and serine-specifying tRNAs (L₁ and L₂ for codon families CUN and UUR, respectively; S₁ and S₂ for codon families UCN and AGN, respectively). All genes are transcribed in the clockwise direction. ‘AT’ indicates the non-coding region.

Figure 2. Substitution ratios in the mitochondrial genomes of spirurid nematodes.

The rate of non-synonymous (Ka), the rate of synonymous (Ks) substitutions, and the respective ratios (Ka/Ks) for individual protein-coding genes are shown.

Figure 3. Relationship of Thelazia callipaeda with other selected spirurid nematodes based on mitochondrial sequence data.

The concatenated amino acid sequences of 12 protein-coding genes were subjected to analysis by Bayesian inference (BI) using Ascaris suum as an outgroup. Posterior probability (pp) values are indicated.