Worm Breeder's Gazette 7(1): 48

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.

Discovery of a Wild-Type Caenorhabditis briggsae (var. Gujarat)

A. Fodor, D. Riddle, K. Nelson, J. Golden

A hermaphroditic nematode strain isolated from the soil (by A. F.) 
in the State of Gujarat, India, has been identified as Caenorhabditis 
briggsae by genetic and morphological criteria.  The Dougherty strain 
of C.  briggsae, which has been cultured axenically in various 
laboratories since 1954, was obtained by the CGC from Dr.  Bert 
Zuckerman.  No other sub-lines of C.  briggsae are known to exist (see 
Friedman, Platzer and Eby, 1977, J.  Nematol.  9:198).  A male stock 
of the Gujarat strain was established and used for mating tests, 
genetic analysis and comparisons of male copulatory bursa morphology 
together with N2 males and heat-shock-generated Zuckerman males.  (
Historically, the number and arrangement of bursal rays has been the 
basis for species classification in this genus.) The Gujarat males 
mate very well with the Zuckerman strain and not at all with C.  
elegans N2.  Conversely, N2 males fail to mate with either Gujarat or 
Zuckerman hermaphrodites.  Examination of bursal ray patterns confirms 
the conclusion that the Gujarat strain is C.  briggsae.  In general, 
the behavior, appearance, and growth properties of the Gujarat strain 
are very similar to C.  elegans strain N2.  However, both of the C.  
briggsae strains grow and reproduce well at 27.5 C while N2 does not.  
The Gujarat generation time from egg to egg is about 37 hrs. at 27.5 C,
and about 56 hrs. at 20 C.
The Zuckerman C.  briggsae strain differs markedly from both Gujarat 
and N2 in many respects.  The Zuckerman strain is variably 
uncoordinated, it exhibits chemotaxis defects, it is dauer-defective 
the hermaphrodites produce relatively small broods (140 + 18 is the 
best at 22.5 C in comparison with 238 + 25 of Gujaratat the same 
temperature), spontaneous males will not mate but instead crawl off 
the plate, and the male bursa is abnormally small with stunted rays.  
Bursa size and ray length of Gujarat males are the same as N2, but 25% 
greater than Zuckerman males.  Gujarat/Zuckerman hybrid males possess 
a bursa indistinguishable from Gujarat itself, so we conclude that 
this Zuckerman trait is autosomal and recessive.  However, the hybrid 
males are uncoordinated, non-chemotactic, and dauer-defective.  The 
dauer-defective phenotype can be scored as a failure to form dauer 
larvae in response to the Caenorhabditis 'crowding' pheromone (Golden 
and Riddle, 1981, C.  elegans Meeting Abstracts).  These traits are 
all X-linked, recessive, and may represent pleiotropic effects of a 
single mutation.  An attempt was made to induce revertants of the 
Zuckerman dauer-defective trait with EMS, but no revertants were found 
in a screen of almost 10+E5 mutagenized chromosomes.  This leaves open 
the possibility that the X-linked defect may be a non-revertible, 
multi-site mutation of some sort.
What does all this mean? Perhaps not much, but we believe this may 
be a particularly grandiose example of evolution (or de-evolution) in 
the laboratory.  The Dougherty strain of C.  briggsae was reported in 
1969 to form dauer larvae (Yarwood and Hansen, 1969, J.  Nematol.  
1:184), but the strain we have today forms dauers at very low 
frequency.  We are tempted to speculate that prolonged liquid axenic 
cultivation may have actually favored the type of genetic variation in 
sensory behavior, movement and dauer formation that we have observed 
in the Zuckerman strain.  Interestingly, the Zuckerman strain does not 
exhibit uncoordinated movement in liquid, but it seems to thrash about 
more slowly than N2 or Gujarat.  Slower movement also might be an 
advantage in liquid suspension culture.
Does cultivation on agar plates spread with E.  coli also select 
particular types of variation from 'true wild-type'? Well, our N2 
stock is not noticeably different in growth or behavior from the 
recently-isolated Gujarat C.  briggsae strain, with one interesting 
exception.  The exception involves sensitivity to the dauer larva 
pheromone.  All three strains produce the Caenorhabditis specific 
'crowding' pheromone.  While the Zuckerman strain does not respond to 
the pheromone (it is dauer-defective), the Gujarat strain responds to 
even lower concentrations of pheromone than does N2.  Presumably as a 
consequence of its pheromone sensitivity, the Gujarat strain actually 
forms some dauers prior to starvation on NGM plates.  If N2 had such a 
characteristic when it was first cultured on E.  coli, it almost 
certainly would have been at a selective disadvantage to spontaneous 
genetic variants less sensitive to the pheromone.