Biology Reference
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phenotype. Most likely this will be either an exaggeration of the Prim phenotype
(enhancement), or a reduction/elimination of the Prim phenotype (suppression).
It is also possible to identify modifier mutations that cause a qualitative change in
the Prim phenotype (neomorphism), particularly in the case of a GFP expression
reporter, where loss-of-function mutations that affect the specification or differ-
entiation of cells that express the reporter can lead to an altered expression
pattern (e.g., if the labeled cell is eliminated, duplicated, or mispositioned). In
some cases, modifier screens are performed that involve primary mutations in
more than one locus, for example, a loss-of-function allele, prim-1(*), in com-
bination with an integrated GFP reporter, prim-2(*) ( Grote and Conradt, 2006;
Hammarlund et al., 2009; Hatzold and Conradt, 2008; Nehme et al.,2010 ). When
performing mapping and characterization, the same fundamental principles apply,
but attention must be given to maintaining both prim-1(*) and prim-2(*) through
the mapping procedures.
B. Initial Characterization
The first experimental step in characterization of a Mod stock is to determine
whether the prim-1 locus itself has been affected. For example, if prim-1(*) is a weak
loss-of-function allele, then second-site mutations within prim-1 could potentially
result in either enhancement or suppression of the Prim phenotype, depending on the
nature of the modifier allele. Or, if prim-1(*) is an integrated GFP reporter, mutation
of the reporter (or recombination, in the case of integrated extrachromosomal arrays)
could result in attenuation of GFP expression.
The simplest method for determining whether the prim-1 locus has been altered in
a given Mod stock is to perform an outcross to a non-Mod prim-1(*) stock. I will
illustrate this procedure by using an example of a modifier screen that was per-
formed in my laboratory:
C. An Example: Mapping Suppressors of gon-2(q388)
We began with a strain of genotype gon-2(q388) unc-29(e1072). gon-2(q388) is a
recessive, temperature-sensitive, loss-of-function mutation that causes a highly
penetrant gonadless/sterile phenotype when animals are raised at restrictive temper-
ature ( Sun and Lambie, 1997 ). unc-29 is tightly linked to gon-2, and the e1072 is a
recessive allele that causes a mild, but easily scorable Unc phenotype. The unc-29
(e1072) mutation serves two purposes: it enables cross progeny to be distinguished
from self-progeny during outcrossing, and it allows identification of revertant muta-
tions within gon-2 based on linkage. We mutagenized gon-2(q388) unc-29(e1072)
hermaphrodites, and then raised progeny at the restrictive temperature to select for
fertile Mod derivatives ( Church and Lambie, 2003 ).
For initial characterization, hermaphrodites from each Mod stock were crossed
with males of genotype
gon-2(q388); him-8(e1489)
that had been raised at
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