Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes

Abstract

Stilbonematinae are a subfamily of conspicuous marine nematodes, distinguished by a coat of sulphur-oxidizing bacterial ectosymbionts on their cuticle. As most nematodes, the worm hosts have a relatively simple anatomy and few taxonomically informative characters, and this has resulted in numerous taxonomic reassignments and synonymizations. Recent studies using a combination of morphological and molecular traits have helped to improve the taxonomy of Stilbonematinae but also raised questions on the validity of several genera. Here, we describe a new circumtropically distributed genus Paralaxus (Stilbonematinae) with three species: Paralaxus cocos sp. nov., P. bermudensis sp. nov. and P. columbae sp. nov. We used single worm metagenomes to generate host 18S rRNA and cytochrome c oxidase I (COI) as well as symbiont 16S rRNA gene sequences. Intriguingly, COI alignments and primer matching analyses suggest that the COI is not suitable for PCR-based barcoding approaches in Stilbonematinae as the genera have a highly diverse base composition and no conserved primer sites. The phylogenetic analyses of all three gene sets, however, confirm the morphological assignments and support the erection of the new genus Paralaxus as well as corroborate the status of the other stilbonematine genera.Paralaxus most closely resembles the stilbonematine genus Laxus in overlapping sets of diagnostic features but can be distinguished from Laxus by the morphology of the genus-specific symbiont coat. Our re-analyses of key parameters of the symbiont coat morphology as character for all Stilbonematinae genera show that with amended descriptions, including the coat, highly reliable genus assignments can be obtained. K E Y W O R D S16S rRNA, 18S rRNA, cytochrome c oxidase subunit I, ectosymbionts, molecular phylogeny, Paralaxus, systematics, thiotrophic symbiosis 380 |

Taxonomic and functional nematode diversity in Maldivian coral degradation zones: patterns across reef typologies and depths

“…This taxon occurs globally in sheltered intertidal and subtidal habitats and is frequently observed in tropical carbonate sediments ( Semprucci et al, 2010 ; Armenteros, Ruiz-Abierno & Decraemer, 2014 ). Species of this subfamily show an ectosymbiosis with sulphur oxidising bacteria, which cover the cuticle of the nematodes and are often genus- and species-specific consortia ( Bayer et al, 2009 ; Scharhauser et al, 2020 ). As sediment-dwellers, the Stilbonematinae migrate up and down through the redox cline and, thus, provide the bacteria with electron donors and electron acceptors, such as nitrate and oxygen ( Hentschel et al, 1999 ; Bayer et al, 2009 ).…”

FtsZ-mediated fission of a cuboid bacterial symbiont

“…Catanema sp. nematodes were identified based on morphological characteristics ( Scharhauser et al., 2020 ).…”

“…For the 16S phylogenetic analysis, 48 Ca . Thiosymbion 16S sequences from Scharhauser et al., (2020) were retrieved from GenBank together with 16S sequences from 3 outgroup species (see Table S1 for accession numbers). The alignment was performed with mafft v7.427 (L-INS-I mode, Katoh and Standley, 2013 ), trimmed in TrimAl v1.4.rev15 (-gt 0.7, Capella-Gutiérrez et al., 2009 ) and the maximum likelihood phylogeny reconstructed using IQ-TREE v2.1.2 ( Minh et al., 2020 ) with the best-fit model automatically selected by ModelFinder Plus ( Kalyaanamoorthy et al., 2017 ) and 1,000 ultrafast bootstraps ( Hoang et al., 2018 ).…”

Eubostrichopsis johnpearsei n. gen., n. sp., the first stilbonematid nematode (Nematoda, Desmodoridae) from the US West Coast

“…The shape and geometry of the arrangement of the symbiotic bacteria are genus-specific in Stilbonematinae and should be included into the definition of the respective genera (Scharhauser et al 2020). The new genus is remarkable for a new type of symbiont coat consisting of up to 10 μm long slightly curved rods that are pointed at both poles.…”