Biology Reference
In-Depth Information
manual manipulation ( Fig. 5 E-G ). We will only briefly mention genetic balancers
here, referring the reader to Chapter 7 in the previous edition of this text and the
chapter by Jones et al. in this edition for a more thorough discussion ( Edgley et al.,
1995 ).
Many types of balancers can be used to maintain sterile mutations, including
chromosomal rearrangements (translocations, duplications, and inversions) and
transgenes specifically constructed for this purpose. Regardless of type, all good
balancers used for existing strain maintenance share common characteristics; het-
erozygotes (or transgene-containing animals) must be viable and fertile as well as
have a unique, easy-to-score phenotype or mutations that effectively eliminate
homozygote balancers
from the population (see chapter by Jones
et al.)
( Edgley et al., 1995 ).
When working with fundamental processes such as fertility, one must take care to
ensure that balanced strains are not lost. Fertility is highly selected, and any rever-
sions or mutations conferring a fertility advantage will quickly take over a stock.
Further, we have observed weakly fertile stocks become more fertile over many
generations (I. Chatterjee and A. Singson, unpublished). Conversely, spontaneous
mutations that further negatively affect fertility may arise in the balancer strains,
confounding analyses. In addition, balancers themselves can occasionally stop func-
tioning to DNA rearrangements, resulting in the loss of balancing. Therefore, it is
very important to carefully monitor balanced strains and to keep frozen stocks soon
after their construction.
DNA injected into the worm gonad can form heritable extrachromosomal arrays
( Mello and Fire, 1995 ) that can be used to balance mutations. Because these bal-
ancing transgenes can often be lost, mosaic animals can be studied to determine
whether a given sterile gene is required in the germ line for its action. For example,
spe-19 is maintained via an extrachromosomal balancing array that also contains a
GFP marker (myo3::gfp, which expresses in body wall muscle) ( Geldziler et al.,
2005 ). Studies of mosaic (glowing sterile and nonglowing fertile) animals revealed
that glowing Spe animals failed to carry the transgene in the germ line while rare
nonglowing fertile animals sired glowing progeny (and therefore contained the
transgene in the germ line). These results strongly suggest that spe-19 is required
in the germ line for function, but do not rule out an additional requirement in somatic
tissue. Note that transgene expression is often repressed in the germ line (so-called
''germ line silencing''), somewhat limiting the usefulness of this type of analysis for
fertility study ( Putiri et al., 2004 ).
Maintenance of spe-8 mutations provides another balancer example. Using the
free duplication sDp2 which contains wild-type copies of both dpy-5 and spe-8, the
strain is maintained as a homozygous spe-8 dpy-5 double mutant and affords an easy
visual assay for the spe-8 mutation. Animals complemented by the duplication are
phenotypically wild-type; animals that do not contain the duplication are Spe and
Dpy ( Edgley et al., 2006; Singson et al., 1999 ).
Of course, many sterile mutations are conditional and do not require balancers.
Temperature-sensitive (ts) alleles of many sterility genes (both sperm and egg) exist
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