Abstract
The infraorder Cephalobomorpha is a diverse and ecologically important nematode group found in almost all terrestrial environments. In a recent nematode classification system based on SSU rDNA, Cephalobomorpha was classified within the suborder Tylenchina with Panagrolaimomorpha, Tylenchomorpha and Drilonematomorpha. However, phylogenetic relationships among species within Tylenchina are not always consistent, and the phylogenetic position of Cephalobomorpha is still uncertain. In this study, in order to examine phylogenetic relationships of Cephalobomorpha with other nematode groups, we determined the complete mitochondrial genome sequence of Acrobeloides varius, the first sequenced representative of Cephalobomorpha, and used this sequence for phylogenetic analyses along with 101 other nematode species. Phylogenetic analyses using amino acid and nucleotide sequence data of 12 protein-coding genes strongly support a sister relationship between the two cephalobomorpha species A. varius and Acrobeles complexus (represented by a partial mt genome sequence). In this mitochondrial genome phylogeny, Cephalobomorpha was sister to all chromadorean species (excluding Plectus acuminatus of Plectida) and separated from Panagrolaimomorpha and Tylenchomorpha, rendering Tylenchina non-monophyletic. Mitochondrial gene order among Tylenchina species is not conserved, and gene clusters shared between A. varius and A. complexus are very limited. Results from phylogenetic analysis and gene order comparison confirms Tylenchina is not monophyletic. To better understand phylogenetic relationships among Tylenchina members, additional mitochondrial genome information is needed from underrepresented taxa representing Panagrolaimomorpha and Cephalobomorpha.
Highlights
The suborder Tylenchina sensu De Ley & Blaxter (2002) represents one of the most morphologically and ecologically diverse arrays of nematodes and contains many different trophic types: microbivores, fungivores, and plant- and animal-parasitic forms (Baldwin, Ragsdale & Bumbarger, 2004; Bert et al, 2008; Siddiqi, 1980)
Cephalobomorphs, one of the infraordinal groups in Tylenchina, are the dominant fauna together with other nematodes in nutrient-poor soil such as deserts and dry Antarctic valleys and may play important roles in shaping ecological systems there (Andriuzzi et al, 2018; De Tomasel et al, 2013; Freckman & Mankau, 1986; Freckman & Virginia, 1997; Lee et al, 2019; Sylvain et al, 2014). This group constitutes a microbivorous or saprophagous clade that is found in diverse terrestrial environments including tropical rainforests and hot deserts (Nadler et al, 2006; Timm, 1971; Waceke et al, 2005)
In a further revised nematode classification system (De Ley & Blaxter, 2004) following the SSU phylogeny, cephalobs were treated as the infraorder Cephalobomorpha and placed in the suborder Tylenchina, along with Drilonematomorpha, Panagrolaimomorpha and Tylenchomorpha
Summary
The suborder Tylenchina sensu De Ley & Blaxter (2002) represents one of the most morphologically and ecologically diverse arrays of nematodes and contains many different trophic types: microbivores, fungivores, and plant- and animal-parasitic forms (Baldwin, Ragsdale & Bumbarger, 2004; Bert et al, 2008; Siddiqi, 1980). In a further revised nematode classification system (De Ley & Blaxter, 2004) following the SSU phylogeny, cephalobs were treated as the infraorder Cephalobomorpha and placed in the suborder Tylenchina, along with Drilonematomorpha, Panagrolaimomorpha and Tylenchomorpha. These relationships were supported in subsequent phylogenetic analyses using SSU rDNA data with more extensive taxon sampling (Holterman et al, 2006; Van Megen et al, 2009). We determined the complete mitochondrial genome sequence of Acrobeloides varius, the first full representative of the infraorder Cephalobomorpha, compared its genome structure and organization to other nematodes, and used it to infer phylogenetic relationships among chromadorean nematodes
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