A weekly digest. Found something missing? Leave a comment below.
New genomic resources, speciation, natural variation, and meeting/conference.
Genome resources
A gap-free, telomere-to-telomere (T2T) reference genome for AF16. O’Connor et al. have released a T2T assembly for an isogenic AF16 culture, named CGC2, no gaps, no unplaced sequence, and chromosome-level contiguity throughout. This fills a long-standing gap and supports C. briggsae experimental work. The previous CB4 assembly was not complete (Ross et al. 2011). The authors used deep RNA-seq across all developmental stages to predict protein-coding annotations of comparable quality to the existing AF16 gene models, and lifted over 108 validated indel variants to the new coordinate system so existing mapping reagents remain usable.
O’Connor et al. (2026) G3 16(8):jkag158
T2T assemblies for C. nigoni and C. briggsae put a number on satellite DNA loss. A companion paper (preprint) reports a 139 Mb T2T genome for C. nigoni strain JU1422, resolving 57 gaps and 149 unassigned scaffolds from the previous assembly, alongside a 107 Mb T2T C. briggsae genome mentioned above. Comparing the two, the dominant driver of the genome size difference between the outcrossing and the selfing sister species is not gene content but deletion of satellite DNA arrays, totalling roughly 9.6 Mb. Given that the two species split only ~3.5 Mya, this is a strikingly fast turnover, and it raises the question of what selfing actually does to repeat landscapes.
Pellow et al., bioRxiv (Feb 2026)
Speciation and hybrid incompatibility
Haldane’s rule in Caenorhabditis works through X:autosome incompatibility. Harbin et al. paper from the Ellis lab tackled a century-old question using two routes that are hard to take in most systems: crosses involving sex-determination mutants of both hybridising species, and crosses involving tetraploids. Their conclusion is that the critical ingredient is incompatibility between a sex chromosome contributed by one species alone and autosome pairs contributed by both, not the hemizygosity of the X chromosome per se. They also show that the machinery for reading the X:autosome ratio has itself diverged over recent nematode evolution.
Harbin et al. (2026) Nature Communications 17:1679
Pangenome analysis points at F-box/ubiquitin-ligase adaptors as divergence drivers. A pangenome comparison across C. briggsae and C. nigoni, from Zhao lab, finds that C. nigoni consistently carries larger genomes and higher gene counts, driven mainly by large interspecific unaligned regions and by expansion of species-specific dispensable gene families. Both the unaligned regions and the expanded dispensable genes are significantly enriched for rapidly evolving Cullin-E3 ubiquitin-ligase adaptors, in particular F-box proteins, a family long suspected of mediating immune and stress responses in nematodes, and now implicated in hybrid incompatibility.
Xie et al., Genome Research (2026)
A toxin-antidote pair where the antidote came first. A new PNAS paper, also from the Zhao lab (and, the same first author as Genome Research paper), reports a novel toxin-antidote element in C. nigoni in which the antidote, Cni-shls-2, is a C. nigoni-specific F-box gene present in three identical tandem copies. Its absence causes embryonic lethality in C. nigoni and in hybrids with C. briggsae. The toxin, Cni-hlix-1, is a maternally deposited chimera fusing duplicated host sequence with novel sequence of possible bacterial or archaeal origin. Evolutionary reconstruction suggests the antidote predates the toxin, which is a reverse of the usual assumption about how these elements assemble.
Natural variation
Ancestral hyper-diversity versus globally distributed haplotypes. A new study from the Andersen lab (Moya et al., currently in bioRxiv) examining C. briggsae population structure reports ancestral genomic hyper-diversity set against genome-wide haplotypes that are distributed globally. This is relevant to anyone choosing wild isolates for mapping or for adaptation studies, and a useful companion to the tropical/temperate clade structure the field has worked with for two decades.
Moya et al., bioRxiv, Dec 2025
And, in the neighbourhood. Yet another paper from Andersen lab (also in bioRxiv) reports global survey of genomic diversity in the selfing nematode C. tropicalis. The study finds diversity correlating with geography, which is a useful third data point for comparisons across the three selfing Caenorhabditis lineages.
Wang et al., bioRxiv, Apr 2026
Meetings
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- X Spanish Worm Meeting: 14–15 September 2026, Príncipe Felipe Research Center, Valencia, Spain. The meeting marks 20 years of the Iberian worm community. Plenary by Victor Ambros; keynotes from Christian Frøkjær-Jensen, Ana X. de Carvalho, David Vílchez, Arantza Barrios and Enrique Martínez-Pérez. Registration and abstracts closed 15 June. Programme
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- 26th International Worm Meeting (#Worm27): 18–22 July 2027, Kobe Convention Center, Kobe, Japan. It is the first International Worm Meeting being held in Asia. Abstract submission and registration open 13 January 2027. Details
Compiled from PubMed, bioRxiv, and community sources. Corrections and additions are welcome.