Specialized Chromosomes and Their Uses in Caenorhabditis elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 33

1. Translocations for the Maintenance of Lethal Mutations
As heterozygotes, crossing over is eliminated within the regions no longer cis-
lined to an HRR, making translocations valuable tools for the isolation and main-
tenance of lethal mutations ( Clark et al., 1990; Clark et al., 1988; Ferguson and
Horvitz, 1985; Johnsen and Baillie, 1991; Rogalski and Riddle, 1988; Rosenbluth
et al., 1990; Zhao et al., 2006 ). In the case of eT1, the translocated regions cover a
large fraction (one-sixth) of the genome and thus balance a very large number of
genes. Fortunately, in the case of eT1, the breakpoint on chromosome III provided a
viable visible marker, unc-36. In order to easily distinguish the heterozygotes,
morphological mutations in the recombination-suppressed region of the normal
homologs were used. A frequently used strain of eT1[unc-36] (III:V) has dpy-18
(e364) (III) and unc-46(e177) (V) mutations on the normal homologs. The hetero-
zygote has a wild-type phenotype and can be easily distinguished from either
homozygote. eT1 heterozygotes segregate: (1) eT1 homozygotes (which have an
Unc-36 phenotype because the translocation breakpoint lies in the unc-36 gene on
chromosome III); (2) Dpy-18 Unc-46 homozygotes; and (3) a large fraction of
aneuploid progeny (10/16) that arrest development as embryos or larvae as a con-
sequence of abnormal chromosome numbers ( Adames et al., 1998; Rosenbluth and
Baillie, 1981 )( Fig. 2 ).

2. Translocations for the Study of Meiotic Processes
It is not immediately obvious why translocations in C. elegans suppress crossing
over. If they paired as described in textbooks, forming a cruxiform structure, only
limited crossover disruption would be expected. Thus, based on the genetic evi-
dence, it was proposed that in animals heterozygous for a reciprocal translocation,
the ''translocated'' arms did not pair, recombine, or disjoin ( Rosenbluth and Baillie,
1981 ). Subsequent molecular analysis confirmed the predictions ( Adames et al.,
1998 ), which are shown in Fig. 2 . An examination of all available translocations
leads to the proposal that one end of each chromosome contained a region that
facilitated pairing between homologs and initiated the meiotic cascade of pairing,
recombination, and disjunction. This region was designated as the HRR.
Subsequently, the regions were molecularly identified ( Phillips et al., 2009 ).

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bar, with a line indicating the position of the dpy-18 mutation. The normal chromosome V is shown as a
light gray bar, with a vertical line indicating the position of the unc-46 mutation. The half-translocation eT1
(III) is shown as a half-black, half-shaded bar with a vertical line indicating the position of the unc-36
mutation caused by the translocation breakpoint on chromosome III. The half-translocation eT1 (V) is
shown as a half-black, half-shaded bar with no vertical line. All wild-type progeny are heterozygous for the
translocation chromosomes and the normal chromosomes. Unc-36 progeny are eT1 homozygotes, and
Dpy-18 Unc-46 progeny are homozygous for the normal chromosomes. Aneuploid progeny account for
10/16ths of the total progeny.
(Adapted from Edgley and Riddle (2001) ).