Specialized Chromosomes and Their Uses in Caenorhabditis elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 34

A nonreciprocal type of translocation is a chromosome fusion. In a screen for
genes required for germ line immortality, the mrt-2 locus was discovered ( Ahmed
and Hodgkin, 2000 ). mrt-2 encodes an ortholog of a checkpoint gene and in
C. elegans is required to prevent telomere shortening leading to end-to-end chro-
mosome fusions. The meiotic behavior of heterozygous fusion chromosomes has
been studied in the context of crossover control and it has been found that fusions
containing two or even three whole chromosomes behave as a single chromosome in
regard to having mainly only one crossover per meiosis, similar to wild-type ( Hillers
and Villeneuve, 2003 ). X-autosome fusions have been used to investigate dosage
compensation and the relationship between compensations and crossover control
( Gladden et al., 2007; Gladden and Meyer, 2007; Lieb et al., 2000 ).
3. Translocations to Determine Forward Mutation Frequency
The number of lethal events recovered using a translocation heterozygote under
defined conditions can be used to calculate forward mutation frequencies for dif-
ferent doses of specific mutagens, such as ethylmethane sulfonate (EMS) (Raja E.
Rosenbluth et al., 1983 ), ionizing radiation ( Rosenbluth et al., 1985 ), ultraviolet
radiation ( Stewart et al., 1991 ), formaldehyde ( Johnsen and Baillie, 1988 ), trans-
posable element mobilization ( Clark et al., 1990 ), exposure to radiation on the
International Space Station ( Zhao et al., 2006 ), and mutator strains such as the
deletor of G ' s, dog-1( Zhao et al., 2008 ). eT1 has been used extensively for these
purposes, reinforcing its advantages, such as ease of use, size of region balanced, and
stability of genotype.
4. Advantages and Disadvantages
In general, translocations make excellent balancers ( Table III ). They are heritably
stable and relatively easy to use. However, it is important to have some knowledge of
their structure, complexity, and segregational properties in order to use them effec-
tively. Reciprocal translocations balance regions on two different chromosomes, a
feature that may not be as desirable as balancing a single chromosome. In addition,
the large fraction of aneuploid self-progeny segregating from reciprocal transloca-
tion heterozygotes can complicate recognition of a lethal phenotype segregating
with the normal homologs.
B. Inversions
Inversions are chromosomal rearrangements in which a segment of the chromo-
some reinserts in the same location but in the reverse orientation. Although crossing
over can occur in the inverted region, recombination between the breakpoints is
almost completely suppressed in heterozygotes due to topological restraints, absence
of DNA alignment, or disruption of sites required for pairing. When recombination