The Ethology of Stress in Nematodes

Citation

Shih, Pei-Yin (2019) The Ethology of Stress in Nematodes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1E7G-K373. https://resolver.caltech.edu/CaltechTHESIS:01062019-215120039

Abstract

Animals can respond to stress in two ways: one is through innate, reflexive behaviors and physiological responses. For example, bees sting invaders when they feel threatened, and heat shock proteins in our body ensure the proper folding of proteins under stressful conditions. The other strategy is through the more active and dynamic phenotypic plasticity responses, for example the transformation of spadefoot tadpoles into cannibals in crowded environments.

When Caenorhabditis elegans roundworms face harsh environmental conditions they can develop into the dauer larvae stage instead of reproductive adult. Dauers are long-lived, stress-resistant, and specialized for dispersal. Dauer biology has much to reveal about stress resistance, neural state, and tissue coordination.

Using RNA-seq we compared dauers vs non-dauers and found 8,042 genes that are differentially expressed. By bioinformatically clustering these genes, we discovered the significant up-regulation of neuropeptide genes during dauer development. In particular, the FMRFamide neuropeptides are coordinatelly up-regulated as a family. Peptidergic signaling downstream of sbt-1 promotes dauer entry decision and nication coordination, and it is necessary for CO₂ chemoattraction. We further identified that flp-10 and flp-17 together have the same effect as sbt-1 on nictation and CO₂ attraction. Finally, we showed that the upregulation of flp might be a shared strategy in the host-seeking parasitic infective juvenile (IJ) stage.

From the RNA-seq data we also identified four good marker genes for labeling the dauer entry decision and driving gene expression, specifically during dauer commitment. By overexpressing daf-9 in the hypodermis during dauer-commitment, we can manipulate the decision and promote reproductive development. Combining the markers with partial dauer mutants allowed me to confirm their subtle phenotypes in tissue-coordination breakdown. Furthermore, this approach allowed me to uncover the novel neuronal partial dauer phenotype for daf-18 mutants.

In work done outside of the lab, I investigated the innate stress response of extremophiles to Mono Lake. I isolated nine new nematode species that were diversely related in phylogeny, morphology, and feeding lifestyles. We were able to culture one of the species, Auanema tufa, in the laboratory, and demonstrated a high level of arsenic stress-resistance in the species. These data suggest that Mono Lake—particularly its more buffered tide zone—has been invaded independently and multiple times by nematodes. We also speculate that pre-adaptation to arsenic in the tide zones on Mono Lake could lead to the genomic evolution necessary to adapt to the high pH and salinity of inner Mono Lake.

Altogether, I have investigated innate and plastic stress responses in and outside of the lab through my work on dauer development and arsenic resistance in Mono Lake. This has allowed me to survey the strategies nematodes use to maximize the use of their simple body plans. In particular, dauers up-regulate 64 neuropeptide genes that encode for 215 peptides to massively rewire their neural state. This likely allows them to overcome the physical limitations of their un-compartmentalized nervous system, and I speculate that such a strategy would be useful in other organisms lacking compartmentalized brains, as well as in local regions of a brain that are low complexity.

Thesis Files