Mapping Mutations in C. elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 5

phenotypes have been identified, but for the practical purpose of mapping, these
can be divided into two categories: homozygous viable and homozygous inviable.
Since different strategies are required for each of these, they are considered sepa-
rately below.
Many genetic screens involve searching for mutations that have no detectable
phenotype in an otherwise wild-type background. These include suppressor,
enhancer, and synthetic lethal screens, as well as screens that depend on specialized
markers (e.g., GFP reporters) for detection of the mutant phenotype. The underlying
strategy for mapping such mutations is similar to that used for mutations that can be
scored independently, but also includes consideration of events that affect the pri-
mary locus. Therefore, these are also considered in a separate section.
Although the procedures described herein will be applicable to most newly
identified mutations, it is not possible to anticipate all potential idiosyncrasies that
might be encountered for a given mutant stock. Therefore, it may be necessary to
make appropriate modifications for temperature effects, growth rate, pleiotropy, sex-
specificity, and other factors.
III. Mutations That Can Be Propagated as Homozygotes
These include visible mutations such as the standard dpy and unc loci that were
used to define the initial genetic map of C. elegans. In this chapter, I will use the term
''New'' to denote the phenotype that has been sought when isolating new mutations,
and the terms new-1(+) and new-1(*) to indicate, respectively, the wild-type and
mutant alleles of the affected locus. Although unlikely, it is possible that New is a
synthetic phenotype, that is, it results from mutations in more than one gene. In the
case of two unlinked genes, the genotype would be written as new-1(*); new-2(*).If
multiple mutations are responsible for the mutant phenotype, this is usually obvious
from the results of the initial mapping cross ( Fig. 1 ). However, if the mutations are
linked to each other (written new-1(*) new-2(*)) this might not be clear until after
further tests of gene identity have been performed.
A. Preliminary Mutant Characterization
Before proceeding with mapping crosses, address the following two issues.
1. Mutant Similarity
Use available information resources (e.g., WormBase, PubMed) to determine
whether the mutant phenotype fits the profile of any previously identified genes,
or if it is similar to that of other mutations that were identified in the same mutant
screen. If either of these conditions holds, then the most efficient course of action is
to perform a complementation test to assess allelism and/or proceed directly with