Editors' ChoiceSteroid Hormones

How to Evolve into a Parasite

Science Signaling  20 Jan 2009:
Vol. 2, Issue 54, pp. ec26
DOI: 10.1126/scisignal.254ec26

Harsh conditions cause the nonparasitic Caenorhabditis elegans larvae to enter the developmentally arrested state of dauer, which requires a decrease in the steroid hormones Δ4-dafachronic acid (Δ4-DA) and Δ7-DA to release the nuclear hormone receptor DAF-12 from its ligand-bound state. Because dauer larvae share morphological similarities with infective larvae of parasitic nematodes, and because exit from dauer in C. elegans appears to use similar signaling mechanisms as activation of infectivity in the parasitic hookworm Ancylostoma caninum, Ogawa et al. reasoned that genes involved in inducing dauer might also be present in parasitic nematodes. They first examined whether Δ4-DA, Δ7-DA, and DAF-12 were involved in inducing dauer in Pristionchus pacificus, a nonparasitic distant relative of C. elegans. Unlike C. elegans, which only enters dauer under extreme conditions, P. pacificus must enter dauer as part of its life cycle and was considered by the authors to be an intermediate between C. elegans and parasitic nematodes. Dauer formation in P. pacificus was prevented by mutations in Ppa-daf-12, a gene encoding a nuclear receptor that resembles C. elegans DAF-12. Furthermore, treatment with C. elegans Δ7-DA suppressed dauer formation in four different P. pacificus mutants that enter dauer constitutively. Strongyloides papillosus is a nematode that has both free-living and parasitic phases during its life cycle. Its eggs can directly develop into infective female larvae and then parasitic adult females or, alternatively, can develop into free-living males and females that produce eggs that give rise to infective larvae. C. elegans Δ7-DA, but not Δ4-DA, inhibited the formation of infective larvae from eggs from parasitic females or from free-living males and females, as demonstrated by the development of morphological features that are only seen in free-living nematodes. According to the authors, their finding that Δ7-DA function appears to be conserved among nematodes supports the idea that infective larvae may have evolved from dauer larvae.

A. Ogawa, A. Streit, A. Antebi, R. J. Sommer, A conserved endocrine mechanism controls the formation of dauer and infective larvae in nematodes. Curr. Biol. 19, 67–71 (2009). [PubMed]