Worm Breeder's Gazette 9(3): 14

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.

Genetics of Mutator Strains

B. Saari, J. Collins and P. Anderson

Figure 1

We have learned some things about the mutator strains (especially 
mut-2) that will likely be of interest to others. Most people are 
using the strain TR679 [mut-2(r459); aka. 'high hopper'] to isolate 
spontaneous mutants. The parent of TR679 is strain TR674.              
{Figure 1, part I}
TR679 is a spontaneous WT revertant of TR674. We have been trying to 
map the source of the mutator activity in TR674. Such mapping 
information would be useful for those trying to substitute the mutator 
activity into other genetic backgrounds. 
We have identified an activity on LGII of TR674 that contributes to 
the mutator phenotype. BUT, this activity is not the only source of 
Tc1 instability in TR674. There must be genes located elsewhere that 
contribute to the high levels of Tc1 activity in TR674. Here are the 
data that lead to this conclusion: 
{Figure 1, part II}
the unc-4/unc-4 homozygotes are non-mutator, and the mut-2/mut-2 
homozygotes are mutator. But, the level of Tc1 activity of the mut-
2/mut-2 homozygotes is not as high as in the TR674 parental strain. We 
measure mutator activity by measuring the frequency of reversion of an 
unc-54::Tc1 mutation that is also present in the background: 
{Figure 1, part III}
Each of the above numbers are averages of 12-20 independent strains. 
These data show that linkage group II in TR674 is responsible for 
about a 10-fold increase in reversion frequencies. But there is 
another 10-fold effect (approximately) that must map elsewhere. The 
fact that the unc-4; 23::Tc1) homozygotes are non-
Mut suggests that the additional factors depend upon LGII in TR674 for 
their effect. In these experiments, we do not think that we are simply 
mapping the gene(s) present in Bergerac necessary for Tc1 activity. 
Because of the pedigree of TR674, its linkage group II is primarily 
Bristol material. 
This mapping data makes it difficult to construct systematically 
strains that have the highest levels of Tc1 activity. The best 
strategy is probably to include LGII from TR674 in the constructions, 
plus as much else of TR674 as is possible. Presumably, LGII from TR679 
is equivalent to that from TR674, but we have not independently mapped 
the mutator phenotype of TR679 to linkage group II. 
A cautionary note concerning TR679: We have been propagating our 
stock of TR679 for over a year, and we recently tested the stability 
of its mutator phenotype. Fifteen random clones of TR679 were isolated 
and populations derived from these animals were tested for the 
presence of high-frequency unc-22 twitchers. Eleven of the 15 cultures 
generated lots of twitchers as expected (frequency 10-3 or greater), 
but 4 of the cultures did not. Thus, our TR679 population was mixed 
with non-mutator derivatives. We did not carefully measure the 
frequency of unc-22 mutants in these four strains to know if they were 
as low as Bergerac, but they certainly were not as high as TR679 is 
supposed to be. Since our culture of TR679 had been grown for many 
generations, and since the nonmutator derivatives probably grow better 
than their mutator competitors, it is impossible to know what the 
frequency of mutator loss is. The ability to lose the mutator, however,
means that before undergoing a large mutant hunt it is wise to start 
with single clones of TR679 that are verified to still be active (lots 
of twitchers, Him, etc.). Alternatively, WT revertants of TR674 can be 
used instead of TR679. Such revertants are easy to isolate, and since 
the reversion event will have occurred very recently, they should 
represent the most unstable animals present in the TR674 population. 
Other useful strains: Mike Finney (Bob Horvitz's lab) has generated 
some strains that are possibly more useful than either TR674 or TR679. 
Mike has repeatedly backcrossed TR674 with Bristol males and 
resegregated strains that retain a high frequency of germ-line Tc1 
excision. These strains are discussed by Mike elsewhere in this 
Newsletter. We have measured the frequency of Tc1 transposition in one 
of Mike's strains that had been backcrossed with Bristol four times. 
The frequency of spontaneous unc-22 mutants in this strain is 
approximately 2 x 10-4. This frequency is 5- to 10-fold lower than the 
frequency in TR679, but 4- to 5-fold higher than that in our Bergerac. 
Thus, Mike's strains are intermediate between Bergerac levels and 
mutator levels of Tc1 transposition. Mike's strains have the advantage 
that most of the genome is derived from Bristol, and therefore 
contains fewer Tc1 elements. These strains should be useful to people 
for whom the highest possible level of Tc1 transposition is less 
important than starting in a predominantly Bristol background (ie. 
people having good selections for mutants). For people in need of the 
highest frequency of Tc1 insertion, we still know of nothing better 
than TR679.

Figure 1