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type of balancer provides a wild-type allele by means of an extrachromosomal or
integrated segment of DNA. This group includes duplications and transgenic arrays.
It is useful to have marked balancers expressing a recognizable phenotype
( Tables I-III ). In some cases, the rearranged chromosome is lethal and the hetero-
zygote the only viable phenotype, which can be especially useful for growing large
numbers of animals for experimental purposes ( Jones et al., 2007 ). Equally valuable
for strain maintenance are dominant markers. An excellent example of a dominant
marker is an insertion of a transgene that express GFP, such as myo-2::GFP, which
expresses GFP in the pharynx. Popular examples of balancers that have insertions of
myo-2::GFP are the inversion, mIn1 and the translocations, nT1 and hT2 ( Table I ).
A. Nomenclature for Chromosomal Rearrangements
C. elegans is a self-fertilizing hermaphrodite that produces both sperm and
oocytes. Males, which are XO (no Y chromosome), occur spontaneously as a result
of X-chromosome nondisjunction or loss ( Brenner, 1974 ). The diploid genome
consists of six chromosomes, five autosomes (I-V), and a sex chromosome (X).
The genome is approximately 100 Mb in size and contains approximately 20,000
genes. To facilitate communication, C. elegans researchers have adopted a consis-
tent nomenclature ( Horvitz et al., 1979 ). Genotypes are written as three or four
italicized (or underlined) lower case letters, for example, dpy, refers to the wild-type
DNA sequence of the gene. Protein products encoded by the genes are written in
upper case, for example, DPY. Phenotypes are written nonitalicized with an upper
case letter, for example, Dpy or written out in full, for example, dumpy, refers to the
phenotypic consequence of a mutation. The category of Dpy may be used for any
animal that has a short fat body and includes mutants in at least 30 dpy genes
(different DNA coding sequences). Each gene is distinguished by an Arabic number
separated from the general name by a hyphen, for example, dpy-5 and dpy-11 are
different genes. Gene names may also include a Roman numeral to indicate the
chromosome in which that gene maps (I, II, III, IV, V, X, as well as f for ''free'' or
unattached to a chromosome), for example, dpy-5 (I) indicates that dpy-5 is on
chromosome I. Specific mutations in the gene (alleles) are referred to by a lowercase
letter and Arabic number, for example, dpy-5(e61). The letter identifies the labora-
tory in which the mutation originated, for example, ''e'' is the laboratory in which
Sydney Brenner did his initial research on C. elegans. Each individual strain carry-
ing one or more mutations is identified by uppercase letters and Arabic numbers, for
example, CB61 is the original strain carrying dpy-5(e61). The laboratory in which a
strain was generated is identified by an uppercase letter combination, for example,
CB for Cambridge. However, whether or not a particular allele is dominant or
recessive, temperature sensitive, etc. is not indicated in the allele name and requires
further notation. When a strain is moved into a different genetic background (either
by outcrossing or mutagenesis), the new strain gets a new strain name. A full list of
laboratory designations is available in WormBase ( www.wormbase.org ) .
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