Worm Breeder's Gazette 7(2): 64

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.

Mutants in Progeny Populations of Hybrids of Two Strains of Caenorhabditis briggsae

A. Fodor, D. Riddle, P. Deak

We are interested to know whether hybrid dysgenesis exists in 
nematodes.  One approach is to study progeny populations of hybrids 
whose mother belongs to a laboratory, while their father to a wild (
recently isolated) strain of the same species.  In the last issue (WBG 
7, (1:)48-49) a newly isolated strain of (then called as 'var.  
Gujarat' but we call it Guj-16/Szeged/) was reported by us and was 
taxonomically determined as C.  briggsae by I.  Andrassy.  Guj-16 
males have been used as fathers and the Zuckerman (Boulder) 
hermaphrodites as mothers of such hybrids needed in our recent studies.
F1 hybrids were selected either as dauer larvae from DRIF (WBG 6, (
1:)25) containing media or simply as NonUnc hermaphrodites.  (The 
Zuckerman's self-progeny are dauer defectives and Uncs,)
We were looking for levamisole resistant (lev) mutants first.  The 
idea behind that was to find mendelian mutations - independent of the 
genetic background - by using the advantages of the selective 
procedure.  We also supposed the target size - 13 non-clustered lev 
gene was found in C.  elegans (Lewis, 1980) large enough to find 
spontaneous or hybrid dysgenesis mutants even if their distribution in 
the genome is not be random.  In preliminary control experiments when 
progeny populations of 200 Guj-16 hermaphrodites were scored on 
levamisole plates there were no resistant animals.  In two run of 
'pilot' experiments (differring from each other in some details but 
based upon Brenner's scheme) we scored progeny populations of 144 F1s 
and found four independent lev mutants.  One of them is a remarkable 
twitcher (hdg-3 or twi-3).  All four mutants are kept in both original 
and outcrossed strains.
Although we do not know whether our mutants confine one or more 
cistrons, we might conclude that the forward mutation rate was not 
lower than 10+E-5 - 10+E-4, which significantly exceeds the 
spontaneous rate.
We were afraid that the wild strain harbored these mutations, but it 
was not the case.  However, the Zuckerman strain harbors several 
mutations like unc, che, daf (defective), so the possibility of 
mutator activity in this strain cannot be ruled out, (Emmons, personal 
communication.) We have to study this strain carefully.  We have not 
observed spontaneous the parental strains so far, but we isolated 
several twitchers from both strains by using EMS.
Mutants affecting size, shape and movement are too difficult to 
analyze.  Considerable time was spent in both Columbia and Szeged 
cleaning up putative dpys, lons, etc.  None of them bred purely and 
some proved sterile.  These phenotypes might be consequences either of 
interacting polygenes or of unstable mutations or both.  The only 
morphological mutants which could be kept, outcrossed and handled were 
the blisters.  Their frequency however, was suspiciously high and we 
cannot be ruled out that this mutation together with its epistatic 
suppressor had not been present in either of the parental strains.  (
In fact we have not found spontaneous bli mutants in any of the 
parental strains so far.)  These bli's can be characterized by low 
penetrance and extremely high expressivity.
The high spontaneous mutation rate (even in a laboratory strain) 
itself might be interesting when looking for transposons in the 
nematode genome.  The question arises whether our mutants were 
generated by hybrid dysgenesis.  To answer this question further 
experiments have to be accomplished (involving determination of the 
spontaneous mutation rates of the parental strains; comparison 
reciprocal hybrid populations as well as F2s and F3s, etc.).  We are 
going to study other phenomena (like different kinds of hybrid 
sterility) which accompany hybrid dysgenesis in other organisms.
The project is continued in Szeged and we want to extend our studies 
to other strains as well.  Therefore, any new nematode strains would 
be welcome and gratefully appreciated.