Abstract
Nematodes are presumably the most abundant Metazoa on Earth, and can even be found in some of the most hostile environments of our planet. Various types of hypobiosis evolved to adapt their life cycles to such harsh environmental conditions. The five most distal major clades of the phylum Nematoda (Clades 8–12), formerly referred to as the Secernentea, contain many economically relevant parasitic nematodes. In this group, a special type of hypobiosis, dauer, has evolved. The dauer signalling pathway, which culminates in the biosynthesis of dafachronic acid (DA), is intensively studied in the free-living nematode Caenorhabditis elegans, and it has been hypothesized that the dauer stage may have been a prerequisite for the evolution of a wide range of parasitic lifestyles among other nematode species. Biosynthesis of DA is not specific for hypobiosis, but if it results in exit of the hypobiotic state, it is one of the main criteria to define certain behaviour as dauer. Within Clades 9 and 10, the involvement of DA has been validated experimentally, and dauer is therefore generally accepted to occur in those clades. However, for other clades, such as Clade 12, this has hardly been explored. In this review, we provide clarity on the nomenclature associated with hypobiosis and dauer across different nematological subfields. We discuss evidence for dauer-like stages in Clades 8 to 12 and support this with a meta-analysis of available genomic data. Furthermore, we discuss indications for a simplified dauer signalling pathway in parasitic nematodes. Finally, we zoom in on the host cues that induce exit from the hypobiotic stage and introduce two hypotheses on how these signals might feed into the dauer signalling pathway for plant-parasitic nematodes. With this work, we contribute to the deeper understanding of the molecular mechanisms underlying hypobiosis in parasitic nematodes. Based on this, novel strategies for the control of parasitic nematodes can be developed.
Highlights
Nematodes, referred to as roundworms, play an important role in many ecosystems, from ocean floors to arable soils
In the insulin/insulin-like growth factor 1 (IGF-1) pathway, aap-1, age-1, pdk-1 and daf-16 sequences are conserved (Fig. 5) and there is a global overlap in transcriptional patterns [121, 122], in B. xylophilus, dauer gene transcription is divided over its two dispersal stages (D3 and D4) [114, 122]
Transcriptional patterns in hatched J2 of H. glycines did not match those of C. elegans dauers [159], and only a few homologues of dauer pathway genes were found in the G. pallida genome, and treatment with cyclic guanosine monophosphate (cGMP) unexpectedly inhibited hatching in quiescent nematodes under the influence of potato root diffusate (PRD) [158]
Summary
Referred to as roundworms, play an important role in many ecosystems, from ocean floors to arable soils. The dauer hypothesis Dauer-like features are observed in both free-living and parasitic nematodes throughout Clades 8 to 12, even though their life cycles differ substantially They do show some conserved genes in the TGF-β and insulin/IGF1 pathways, these are not specific to nematodes and not indicative of a dauer stage.
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