Getting Creative with an Extreme Animal Model

Tardigrades are eight-legged, translucent, microscopic animals. They crawl within dirt, moss, and lichen, and also live in the world’s harshest regions such as mountaintops, the deep sea, and the Antarctic. Also known as water bears, tardigrades survive extreme conditions, including desiccation, extreme heat and cold, and radiation exposure, thanks to their unique biology. Bob Goldstein, a biologist at the University of North Carolina at Chapel Hill, uses tardigrades as a model system to understand body plan evolution and their extraordinary survival skills.

What inspired you to work on tardigrades?

Evolution alters animal shapes by tinkering with genes that control development and cellular functions. When I first trained as a developmental biologist, I was mostly using Caenorhabditis elegans as a model system, and I wondered how it evolved. In the late 1990s, researchers found that C. elegans and Drosophila belong to a clade called Ecdysozoa.¹ That is when my obsession began. I wanted to find something in this group that was outside of the arthropods and nematodes, so that I could have a good comparative organism for studying C. elegans and Drosophila evolution and development.

Tardigrades are members of Ecdysozoa, so I decided to collect different species from the outdoors and from biological supply companies. I identified ones that had small cells and rapid cell cycles—those things might roughly correlate with small genome size. I also wanted ones that were optically clear like C. elegans, so that I could view their development without injecting individual embryonic cells to trace cell lineage. That narrowed it down to the species that we typically use, which is called Hypsibius exemplaris.

What aspects of tardigrade biology do you research?

Tardigrades have a different body form than other models such as Drosophila, which have many segments. We found that the tardigrade had only one head segment and four body segments.² They are missing the parts that would correspond in Drosophila to the entire thorax and almost the entire abdomen. We think that the origin of tardigrades came with losing a big part of their ancestor’s body.

We also study how tardigrades survive crazy extremes that animal life should not be compatible with. They are loaded with protein protectants and exceptional repair mechanisms. We have published papers on desiccation and radiation tolerance mechanisms, and for each of those, I am sure we just scratched the surface.^3,4 Animal life should not survive desiccation, and the level of radiation we can give them is 1,000 times more than what humans can survive. We are fascinated to understand these mechanisms, and the protein protectants that they make could be useful. For example, medicines come with expiration dates—tardigrades do not. We think they make protectants that could protect biomedical materials.

What techniques do you employ to study tardigrades?

Tardigrades are easy to work with. We keep them in spring water, feed them unicellular algae, and they live indefinitely in the laboratory. However, compared to using C. elegans, there are constant challenges because fewer methods are worked out. Because of this, tardigrade research demands creativity.

The genome of H. exemplaris is sequenced and we developed an RNA interference (RNAi) method, which we can use to study protectants. Since then, other scientists have developed CRISPR and transgenic methods.

We started soaking the tardigrades in fluorescently tagged molecules simply to know what can get into them. We exposed the tardigrades to some crazy conditions, such as electroporation, to see if that helped the dyes get in. That project provided us with a collection of markers that can label sub-cellular compartments in an organism that was taken from the wild. This is also important for CRISPR and RNAi to avoid injecting individual animals, which is laborious.

We have also come up with some unusual ways to identify protectants. Jonathan Hibshman, a former lab member who studied desiccation, made a cDNA library so that each bacterium was expressing a tardigrade gene. Then he would torture tubes of bacteria like they were tardigrades, killing almost all the cells, and sequence the survivors. This was a clever way to identify protectants that did not depend on doing forward genetic screens within the animal. So, our research is like a hard-fought battle and we are proud when we win it.