STAR: Deconstructing Theta: Quantifying the rate and spectrum of mutation in non-model Caenorhabditis nematodes

Understanding the forces responsible for variation in and among populations is important to both basic and applied biology. Mutations are a major cause of human disease, and are the ultimate cause of heritable differences between individuals and species. The rate and type of mutations passed from parent to offspring varies at all biological levels, from Kingdoms (animals vs. bacteria) to individual humans.

The causes of that variation are both environmental and genetic. The environment cannot be controlled in human studies, so model organisms provide the best opportunity to understand the interplay between genes and environment as a cause of biological variation. The nematode Caenorhabditis elegans is a model organism that has informed many areas of biology (four Nobel prizes so far).

This project will quantify the rate and types of mutations in two species related to C. elegans which are extremely different from C. elegans in certain respects. Populations of C. elegans are about as genetically variable as humans ("not too variable"), whereas C. brenneri is among the most genetically variable animals yet discovered. Physically, C. briggsae is virtually indistinguishable from C. elegans, but many of its indistinguishable features develop by very different genetic mechanisms.

This work will extend the utility of one of the most important model systems in biology, and will shed light on the question of why organisms that appear so similar can be so different under the surface. A critical element of the project is the collection of a large sample of wild-caught nematodes, done by local middle-school students under the supervision of teachers trained in the lead investigator's lab.

Previously, the lead investigator constructed and cryopreserved several hundred "mutation accumulation" (MA) lines of these species, maintained under minimal selection. The goal is to sequence the genomes of those lines to quantify the rate and molecular spectrum of mutation in two genotypes each of C. briggsae and C. brenneri, along with a C. elegans control.

If mutation rates are similar to C. elegans, it is expected to identify approximately 1500 new mutations in C. brenneri and 5000 in C. briggsae. The data will inform two unresolved questions: first, does the well-documented discrepancy between the spectrum of spontaneous mutations and that of segregating nucleotide variants in C. elegans extend beyond that species; and second, is the astronomical nucleotide variation present in C. brenneri due to a huge population size or an extremely high mutation rate (or both)?

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.