Specialized Chromosomes and Their Uses in Caenorhabditis elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 31

pairing, resulting in a corresponding suppression of recombination in those regions.
A heterozygous mutation in the recombination suppressed regions can subsequently
be retained for an indefinite number of generations. Translocations are often marked
with a mutation, which results in a phenotype that makes the homozygote readily
observable (''Visible''), thus facilitating selection of the heterozygote. In this way,
either lethal or visible mutants can be balanced and the strain can be maintained in a
heritably stable manner.
The reciprocal translocation, eT1, illustrates the properties and uses of transloca-
tions in C. elegans. eT1, originally thought to be a mutation in an unc gene, was
shown to have exchanged the right portion of chromosome III for the left portion of
chromosome V without the loss of any essential material, and thus is homozygous
viable ( Rosenbluth and Baillie, 1981 ). The physical positions of the breakpoints in
eT1 have been characterized. On chromosome III, the break occurs within the unc-36
gene, and on chromosome V is in cosmid H14N18 between rol-3 and

unc-42

( Zhao et al., 2006 )( Fig. 1 ).

Fig. 1 Structure of the reciprocal translocation eT1. eT1 is a reciprocal translocation between the right
end of chromosome III and the left half of chromosome V. The breakpoint on chromosome III falls within
the gene unc-36, such that homozygous eT1 animals display an Unc phenotype. eT1 (III) pairs with theWT
chromosome III while eT1 (V) pairs with WT chromosome V. The recombination suppressed regions
available for balancing mutations are shown by dashed lines in (a).