The preeminent model organism Caenorhabditis elegans has earned researchers four “worm Nobel Prizes”. Although lesser nematodes slither in the shadows of such a wonder-worm, C. elegans’ close cousin, Caenorhabditis briggsae, is often studied in comparison with C. elegans. One interspecific difference is that C. briggsae tolerates temperatures that decimate C. elegans populations.

Until now, there has been little understanding of the genetic basis of this phenomenon. Bhagwati Gupta and his colleagues have started filling this knowledge gap with their foundational comparative study on thermotolerance in C. briggsae and C. elegans. Published in the June 2025 issue of GENETICS, this study establishes survival, growth, and reproduction rates of multiple isolates of both species at increasing temperatures and time intervals. It also examines how well C. briggsae tolerates other stressors.

Heat-loving larvae and temperature tradeoffs

All C. briggsae isolates, whether of temperate or tropical origin, demonstrate high tolerance of increased temperatures from two to 12-hour intervals. Therefore, high thermotolerance is likely an innate aspect of this species. The study also provides the first direct evidence that C. briggsae develops thermal tolerance during early larva-hood (L1 stage). However, C. elegans tolerates other stresses, such as oxidative, osmotic, and endoplasmic reticulum stress, better than C. briggsae, which suggests a fitness tradeoff. 

Genetics takeaways

The researchers focused on two conserved heat shock regulators, HSF-1 and HSP-16.2, in investigating the genetic basis of C. briggsae’s high thermotolerance. The transcription factor HSF-1 mediates the heat shock response in C. elegans, which then regulates molecular chaperones, including HSP-16.2. Each species contains one HSF-1 gene. However, there are significant differences in the numbers and arrangements of small heat shock proteins (HSPs) between the two species, with C. briggsae containing 10 members of the HSP16 family. The authors measured transcript levels in seven genes following heat stress. All genes showed significant activation, which implies a role in thermal activation.

After heat exposure, HSF-1 genes started exhibiting modest upregulation at six hours. By 24 hours, these levels remained high in C. briggsae but declined in C. elegans.

However, the star of the transcriptional show was HSP-16.2 (or its closest C. briggsae ortholog, CB19186), which was induced at ~2,500x at one hour post-shock in C. elegans but ~4,600x in C. briggsae. Moreover, HSP-16.2/CBG19186 peaked at 2°C higher in C. briggsae than in C. elegans. This dramatic in vivo evidence of transcriptional response variations reinforces the hypothesis of species-specific differences in the heat shock response.

The expression of another HSP gene, HSP-70, at 35℃ and 37℃ showed analogous differences. These findings further support the hypothesis that high chaperone levels enhance C. briggsae’s thermal response.

Ramifications

In a world where temperatures are becoming more extreme, understanding the genetic basis of thermal response is essential. However, organisms need resilience on many fronts in increasingly toxic habitats. Study author Gupta notes, “[It appears that C. briggsae] diverted all resources and energy to become more heat-tolerant.”

References

  • Heat tolerance and genetic adaptations in Caenorhabditis briggsae: insights from comparative studies with Caenorhabditis elegans
    Nikita Jhaveri, Harvir Bhullar, Paul W Sternberg, Bhagwati P Gupta
    Genetics, Volume 230, Issue 2, June 2025, iyaf061, https://doi.org/10.1093/genetics/iyaf061

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