Natural variation and antiviral resistance
The gtnt-1 preprint from the Félix lab, covered earlier ( see August 10 updates), has appeared in the journal Molecular Biology and Evolution. The C. elegans gene has a single P182L substitution and it accounts for the resistance of the MY10 strain to Orsay virus. The change was mapped in recombinant inbred lines and confirmed by genome editing.
The published version names the C. briggsae alleles in the gene Cbr-gtnt-1 (CBG09519), and among the 40 wild C. briggsae strains previously assayed for Santeuil and Le Blanc virus infection. The isolate JU516 carries a 45 bp deletion predicted to remove amino acids 286-300, while JU1564 carries a missense E194K variant. Edited derivatives swapping these alleles into each other’s backgrounds confirmed the effect on susceptibility, and worms carrying an intact Cbr-gtnt-1 were the more strongly affected by SANTV infection. The loss-of-function allele raises relative fitness under infection, not just infection rate.
The six C. briggsae strains carrying the deletion alleles were all isolated in Taiwan. In both species the resistant alleles are recurrent and rare, which the authors read as transient adaptation in a shifting eco-evolutionary context rather than a sweep.
Richaud et al. (2026), Molecular Biology and Evolution (pre-print)
Phylogeny and the origins of parasitism
A recalibrated nematode timetree puts the ancestor of the phylum in the early Palaeozoic and the parasites much later.
Lü et al. combined 184 representatives including 156 nematode species with fossil constraints. They date the last common ancestor of nematodes to 508.80-477.64 Ma, which is late Cambrian to Middle Ordovician, and infer that the ancestor was free-living. Their analysis place the first shift to parasitism at 382.85–303.37 Ma, at the common ancestor of Trichinellida and Dioctophymatida within Dorylaimia.
The headline result is about rates rather than origins. For most of their history parasitic lineages did not diversify faster than free-living ones. That changes at roughly 90 Ma, when three obligate zooparasitic groups – Ascarididae, Onchocercidae and Strongyloidea – radiate in step with terrestrial mammals and modern birds. Strongyloidea itself is dated to 100.88–79.46 Ma.
The most relevant part here is the number for the age of C. briggsae‘s branch: Rhabditina (Clade V) is dated to 330.96–291.66 Ma and inferred to have originated free-living, with parasitic groups derived from within it later. The taxon sample includes C. briggsae, C. elegans and C. remanei alongside Pristionchus, Oscheius and Diploscapter.
Lü et al. (2026) Nature Ecology & Evolution 10:1547–1558
Gene models, small RNAs and a caution
A short-read reference annotation hid the causal polymorphism. Okahata and colleagues examined natural variation in temperature acclimation that separates N2 and AB1 strains from CB4856. The responsible locus is smrn-1. Because the original whole-genome sequences were built from 110 bp Illumina reads, they did not align across the gene’s repeat region. The PacBio long reads recovered a 269 bp sequence, containing multiple repeats, absent from the WormBase model entirely.
The corrected sequence identified CB4856 smrn-1 as 1,813 bp long mRNA that carries four exons and 11 polymorphisms relative to the identical N2 and AB1 transcripts. smrn-1 turns out not to encode protein at all! It is transcribed only in embryos, transiently between 210 and 330 minutes after fertilization. The transcript is a major source of 22G small RNAs that load preferentially onto the Argonaute HRDE-1 and act through crml-1 to control axon branching in the O2-sensing BAG neurons.
There is no C. briggsae work here, but the methodology is relevant. A repeat-containing region invisible to short reads sat inside a reference gene model in the best-annotated nematode genome, and it was the causal region. With the AF16 T2T-assembly published recently and C. briggsae gene models still poorly curated, mapping experiments that get stuck on a locus with no obvious candidate are worth re-examining against long-read sequence before the candidate list is trusted.
Okahata et al. (2026) PNAS 123(21):e2538076123
Compiled from PubMed, bioRxiv, online, and community sources. Corrections and additions are welcome.