Worm Breeder's Gazette 2(2): 17
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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 succeed. 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 modalities - 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.