Devil Worms & Extreme Heat

A “devil worm” manages to live in hellish conditions. First described in 2011, H. mephisto is one of the deepest-living land animals found to date. The only live one ever caught in the wild was filtered out of water from an aquifer 1.3 kilometers underground in a South African gold mine At that depth, devil worms must cope with low oxygen, high methane levels and temperatures around 37° Celsius.. The captured worm laid eight eggs. Now, thanks to that one worm’s descendants, scientists have some genetic clues to how the nematodes tolerate these conditions.

The nematodes have duplications of two genes involved in heat shock and cell survival decisions, Bracht and his team report November 21 in Nature Communications. Picking up those extra copies over time likely helped the devil worms cope with extreme conditions and move deeper underground.The researchers found that H. mephisto has about 112 copies of the gene that makes Hsp70 proteins, which refold damaged proteins that have unraveled due to heat stress. That’s a big leap from the devil worm’s closest relative that has had its genetic instruction book, or genome, analyzed already — a nematode that has 35 copies of the Hsp70 gene. Heat stress tests in the lab exposing the devil worms to temperatures from 38° to 40° Celsius show that these genes ramp up to make more Hsp70 proteins when the heat is on. That suggests that these proteins somehow help the devil worms take the heat. The Hsp70 protein likely is one avenue to prevent damage or clean damage up.

But further research is needed to directly link an expansion of Hsp70 genes to an adaptation that helps the worms live underground. So far, it’s not proven that these changes help survival. The devil worms also have extra copies of AIG1, a gene that controls whether a cell lives or dies. A fungus that associates with plant roots might have transferred this gene to devil worms far back in the nematode’s ancestry, the researchers say. Now, devil worms have about 63 copies of AIG1. (The archetype of nematodes, Caenorhabditis elegans, has only one gene that looks somewhat similar.) In heat stress tests with the devil worms, the productivity of these genes didn’t change with temperature. Instead, extra copies of the AIG1 gene might help the worms deal with some other stress in their environment. Another creature, the Pacific oyster (Crassostrea gigas), also has extra Hsp70 and AIG1 genes, Bracht and colleagues report in the December Journal of Molecular Evolution. Oysters are exposed to extreme fluctuations in temperature as the tide ebbs and flows. Because the same genetic pattern is present in two animals far apart on the tree of life, it’s likely that duplication of both the Hsp70 and AIG1 genes is a general strategy for animals to adapt to extreme environments.

Citations:

D. Weinstein et al. The genome of a subterrestrial nematode reveals adaptations to heat. Nature Communications. Published November 21, 2019. doi: 10.1038/s41467-019-13245-8.

G. Borgonie et al. Nematoda from the terrestrial deep subsurface of South Africa. Nature. Vol. 474, June 1, 2011, p. 79. doi:10.1038/nature09974.

M. Guerin, D. Weinstein and J. Bracht. Stress adapted mollusca and nematoda exhibit convergently expanded Hsp70 and AIG1 gene families. Journal of Molecular Evolution. Vol. 87, December 2019, p. 289. doi:10.1007/s00239-019-09900-9.