Previous research has identified two genes required specifically to determine that germ cells develop as sperm:
fog-1 I (Barton and Kimble 1990) and
fog-2 V (Schedl and Kimble 1988). Mutations in either of these genes feminize the germline (the Fog phenotype) by causing germ cells that would have differentiated as sperm to become oocytes instead; these mutations do not affect the soma. However, the
fog-1 and
fog-2 genes play distinct roles in germline development: (1) Mutations in
fog-1 affect the germ cells of both sexes, whereas mutations in
fog-2 -onlyaffect hermaphrodites. (2) Mutations in
fog-1 prevent spermatogenesis in all genetic background tested, whereas mutations in
fog-2 can be suppressed by mutations in
tra-1 ,
tra-2 ,and
tra-3 ,as well as by
fem-3 (gf)mutations. These results suggest that
fog-2 acts early in the process of germline sex-determination in hermaphrodites, whereas
fog-1 acts late in both sexes to specify spermatogenesis. We have characterized four new Fog mutations (
q441 ,
q443 ,
q469 ,and
q470 )that were isolated by Phil Balandyk in screens for sterile mutants. These recessive mutations all fail to complement each other, but do complement
fog-1 mutations. Furthermore, they map just to the left of
lin-11 on LGI, far from
fog-1 .Thus these mutations define a new gene,
fog-3 .Because
fog-3 mutations complement the deficiencies nDf23 ,nDf24 ,and nDf25 ,
fog-3 is probably located to the right of these deficiencies, and thus lies between the genes
mel-26 and
lin-11 .This region is spanned by the duplication sDp1 .As expected from the recessive behavior of the
fog-3 mutations,
unc-13 fog-3 ;sDp1 animals (which are fog/fog/+) produce sperm. In both tests discussed above, the
fog-3 gene behaves like
fog-1 and differs from
fog-2 .In both
fog-1 and
fog-3 XO animals, all germ cells differentiate as oocytes. Furthermore, in
fog-3 ;
tra-2 (
e1425am)mutants, in
fog-3 ;
tra-1 (
e1099lf)mutants, and in
fog-3 ;
fem-3 (
q95gf)mutants all germ cells differentiate as oocytes. These results suggest that
fog-3 acts at the same place in the process of sex-determination as does
fog-1 ,and that both genes are absolutely required for spermatogenesis to occur. Although the germ cells of
fog-1 and
fog-3 mutants develop identically, two genetic tests discriminate between these genes. First,
fog-1 mutations are semi-dominant in males:
fog-1 /+XO animals produce both sperm and oocytes. Second,
fog-1 mutations are semi-dominant suppressors of
fem-3 (gf)mutations (Barton and Kimble 1990). By contrast,
fog-3 mutations show no semi-dominance in either test. These results suggest that the amount of
fog-1 gene product might be limiting in some situations in which the amount of
fog-3 gene product is adequate. To learn whether the
fog-3 mutations reduce or eliminate gene function, we are now screening for new
fog-3 mutations and deletions by non-complementation. Furthermore, to better understand the interactions between
fog-1 and
fog-3 we are continuing our efforts to clone these genes for molecular analysis.