Worm Breeder's Gazette 9(3): 18

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Mutators Necessary for Germ-Line Transposition and Excision of Tc1 can Transpose

I. Mori, D. Moerman and B. Waterston

Figure 1

In a previous newsletter (WBG 9, #1;25) we reported experiments on a 
Bergerac/Bristol mixed strain, called RW7037 in which a single locus 
located near unc-37 I was responsible for most germ-line transposition 
and excision of Tc1. In further experiments with derivatives of this 
strain we have shown that most likely this mutator activity can itself 
transpose, and in the process have obtained a low Tc1 copy number 
strain with high mutator activity.
After a further backcross of RW7037 [ genotype: mut-4(st700) I; unc-
22(st136::Tc1) IV ] with Bristol males we recovered a strain RW7080 
that behaved as if it contained a second mutator, i.e., replacement of 
chromosome I with the Bristol homolog did not result in loss of 
mutator activity as it had with the parental RW7037 line. An active 
strain with a Bristol LGI was established and used to map this second 
mutator activity. In crosses with dpy-10 / + males, replacement of 
LGII with the Bristol homolog resulted in loss of activity: 0/26 dpy-
10 ;nc-22(st136::Tc1) lines yielded revertants in 
approximately 104 animals. In the reciprocal experiment, retention of 
the RW7080 derived LGII by elimination of dpy-10 led to retention of 
the mutator activity : 10/11 unc-22(st136::Tc1) Iines reverted in 104 
animals. This new mutator linked to dpy-10 was designated mut-5(st701) 
II. A three factor cross was carried out to confirm this result : 10 + 
rol-1 chromosomes were recovered from dpy-10 + rol-1 / mut 
136::Tc1) parents. Seventeen of the lines 
homozygous for the + rol-1 chromosomes retained the mutator activity; 
2 did not. We conclude that during the cross to obtain RW7080 either a 
cryptic factor on LGII activated or the mutator activity itself 
These results led us to look more directly for mutator transposition,
possibly into the unc-22 locus itself. We began with an RW7037 
derived strain of the genotype dpy-5(e61) 00) in 
which the unc- 22(st136::Tc1) allele had been replaced by a Bristol 
LGIV. We isolated 26 spontaneous twitchers in this background and then 
replaced the dpy-5 me in each of the 26 
established lines by the wild type Bristol homolog. In 24 cases this 
produced a stable strain. However, in 2 cases the unc-22 allele 
remained unstable after elimination of mut-4 from the background, 
demonstrating that the mutator activity was no longer linked to 
chromosome I. More detailed examination of the strain RW7096 harboring 
the new unc-22(st192::Tc1) allele shows it has a new mutator, 
designated mut-6(st702), which maps on LGIV, near or left of dpy-13 ( 
Of 17 dpy-13 tablished from dpy-13 / mut-6 
192::Tc1 ) parents, 16 failed to revert in 10 
animals; the other gave an Intermediate reversion frequency ).
Table I lists some of the properties of the strains carrying these 3 
mutators. In each case the principal activity can be mapped to a 
single region of the genome, and yet the activity varies from strain 
to strain, or mutator to mutator. In one case we have a strain RW7096 
which has activity comparable to our original BO strain in which we 
had obtained evidence for a polygenic origin of mutator activity (
Genetics 108:859). If the new mutators do represent transposition of 
mut-4, the difference in activity of the derived and original strains 
is intriguing. It may be a 
effect, perhaps reflecting a negative correlation between the 
mutator activity and the overall Tc1 copy number. Or it could be the 
result of changes in the mutator activity itself, perhaps reflecting a 
position effect on expression. In any case the strain RW7096 and its 
unc-22(+) derivative RW7097 are proving useful in the isolation of 
spontaneous mutations in a healthy low Tc1 copy number background.

Figure 1