Worm Breeder's Gazette 2(2): 17

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.

Cytological Studies of Non-conditional Sterile Mutants of C. elegans

N. Mounier

Ovotestes of 15 different sterile mutant strains have been examined 
in the hermaphroditic adult, by means of the 'frottis' method 
established in our laboratory.  I classified them in 4 categories 
according to the most apparent altered level, observed in the mutant 
gonad, with regard to wildtype chronological steps of gonadogenesis.  
a) Very early blocking up, which prevents gonial multipIication.  The 
gonad size is greatly decreased (less than 1/4 of wild-type) and the 
30 to 60 cells found in it are completely abnormal : they are very 
large-sized, with various atypical chromatine structures.
b) Blocking-up at the end of spermatogenesis.  The gonad is short, 
1/3 of wild-type, and contains only 50 to 100 cells.  One mutant 
strain has a disturbed spermiogenesis and another one has a 
considerably reduced sperm production.  The last pachytene cells never 
show any cytoplasmic growth, so we may consider that oogenesis cannot 
c) Perturbations in spermatogenesis and oogenesis.  Gonad size is 
shortened of about 1/3 and the cell number varies from 1/10 to 1/2 of 
wild-type, according to the strains.  Sperm production is reduced and 
abnormal in a variable part : from a quarter to the majority of 
atypical spermatozoa.  One to five oocytes are only found and most of 
them have an abnormal or absent nucleus.  In one strain, a cytoplasmic 
breaking is observed, leading to a size-variable anucleated 
cytoplasmic mass.  Aberrant eggs are sometimes found in another strain.

d) Perturbations in oogenesis only, with 2 
- at an early level, after last pachytene cells, there are no or a 
little cytoplasmic growth and no evolution to typical diakinesis;
- oogenesis is slower than in wild-type, and produces some normal 
and abnormal oocytes.
These mutants determine, at least, 4 different levels inducing C.  
elegans sterility : in gonial differentiation, sperm maturation, 
oogenesis and oocyte maturation.  
A cytogenetical and physiological analysis was made on mutants whose 
spermatogenesis is disturbed (2 of the 2nd class, 1 of the 3rd).
Gonia, synapsis and pachytene cells counting shows there are about 
10 times fewer than in wild-type.  The distal arm's cells have a quite 
normal nucleolus, but no rachis has been seen in light microscopy, by 
means of entire worms sections.  Genetical analysis indicates that 
these 3 mutations are single, recessive and autosomal.  The 2 first 
strains do not complement and their genes are not linked.  Work is in 
progress for the third one.
The phenotypic expression of these 3 mutations is the same in the 
hermaphroditic as well as in the male gonad, and does not differ when 
worms are bred at high (24 C) or low (13 C) temperature during 5 
generations.  Although their mating behavior and their copulatory 
bursa seem normal, male mutants could not be proved to be able to 
fertilize hermaphrodites.
Expression of these 3 mutations is therefore independent of animal 
sex and temperature.  Hermaphrodites are male sterile (mas) and female 
sterile (fes).
By means of these mutants, an understanding approach of mechanisms 
directing cell differentiation into oocytes can be made.  Gonad size 
and cells number are all the more reduced as observed perturbations 
stages are earlier.  So, we can suppose that, once a sufficient gonia 
number is produced from primordium cells evolution towards 
spermatogenesis is then possible.  But something else would be 
necessary to a subsequent differentiation.  Rachis would be one of the 
necessary elements to evolution towards oogenesis, and its formation 
would require a large number of gonia, larger than the one observed in 
the 3 mutants whose spermatogenesis is disturbed.