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transposon (Tc1) insertion (
Rushforth et al., 1993
), and RNAi (
Fire et al., 1998
). The
availability of the complete and well-annotated genome sequence and RNAi librar-
ies currently covering over 94% of the predicted genes in C. elegans (
Ahringer,
2006
) allow investigation of nearly any gene in C. elegans. For example,
Derry et al.
(2001)
used sequence analysis to identify cep-1, the C. elegans ortholog of the
mammalian p53 tumor suppressor gene (
Rubin et al., 2000
). By generating a cep-
1 deletion mutant and using RNAi for functional assays, they identified and char-
acterized the roles of CEP-1 in regulating apoptosis, stress response in somatic cells,
and chromosome segregation in the germ line. This work laid the foundation for
subsequent genetic screens that have added novel insights into how p53 mediates
these conserved processes (
Fuhrman et al., 2009; Gao et al., 2008; Schumacher
et al., 2005; Sendoel et al., 2010
).
As two independent approaches for deciphering gene function, forward and
reverse genetics can often complement each other. A notable example comes from
studies on the C. elegans orthologs of two mammalian polycystin proteins, PKD1
and PKD2, which are defective in human autosomal dominant polycystic kidney
disease (ADPKD), one of the most common monogenic human disorders, affecting 1
in 400-1000 individuals (
Igarashi and Somlo, 2002
). In a forward genetic screen for
male mutants defective in the ability to locate the hermaphrodite vulva,
Barr and
Sternberg (1999)
isolated a mutation in lov-1. Cloning the lov-1 gene revealed it to be
the ortholog of the human disease gene PKD1, which encodes a large transmem-
brane receptor-like protein (
Harris and Torres, 2009; Xiao and Quarles, 2010
). Barr
and Sternberg found this gene to be exclusively expressed in male-specific sensory
neurons. Loss of lov-1 function displayed no phenotype in hermaphrodites, which is
likely the reason this gene was not previously identified, as male-specific pheno-
types, especially behavioral, are not often examined. With this knowledge of lov-1 in
hand, they employed a reverse genetic strategy and generated a deletion mutant of
pkd-2, the worm ortholog of a second PKD disease gene, which encodes a transient
receptor potential channel (
Clapham, 2003
). Strikingly, they found a similar mating
defect in males (
Barr et al., 2001
). Both lov-1 and pkd-2 localize to the ciliated
endings of male-specific sensory neurons, the site of sensory mechanic transduction,
but they are not required for ciliogenesis. This was important, as this study was the
first to indicate that these genes may function in sensory transduction in cilia, a
location where mammalian PKD1 and PKD2 genes were later found to reside and
mediate mechanosensation (
Nauli et al., 2003; Pazour et al., 2002; Yoder et al.,
2002
). Dysfunction of these genes may cause ADPKD due to the inability of renal
epithelial cells to sense fluid flow, which might alter various cell functions, including
gene expression, growth, differentiation, and apoptosis (
Nauli et al., 2003
).
2. Direct Simple Screen
Dissecting genetic pathways involved in a biological process usually starts from
identifying functionally nonredundant components of those pathways. To search for
these key players, direct simple screens are often used. In this type of screen, mutants
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