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.
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.