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.
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.