The germline of a wild-type C. elegans hermaphrodite is a reflexed tube-like syncytium with a distal to proximal axis relative to the vulva. The fate of the nuclei within the syncytium depends on position; nuclei in the distal arm are a mitotic stem population, those around the bend of the tube are in meiotic pachytene, and those in the proximal arm undergo gametogenesis. How is this pattern of fates established and maintained? We are studying a gene required for this pattern: in loss-of-function mutants, meiotic nuclei are found not only in the distal region but also in the most proximal region, and meiotic nuclei are found around the bend. Due to the lack of differentiation and failure of normal regulation of mitosis, we call this a "tumorous" phenotype. Two recessive, EMS-induced alleles of this locus,
q497 and
h292 (1), map just to the right of
bli-4 on chromosome I. The meiotic nuclei in
q497 homozygotes appear to be in pachytene, although the morphology by DAPI staining seems slightly abnormal. The phenotype of the
h292 allele appears weaker: the meiotic nuclei become much more enlarged than those in
q497 ,and occasionally a nucleus is observed that appears to have gone beyond pachytene and into diakinesis, as would an oocyte. We have recently isolated a deficiency, qDf16 ,that fails to complement either
q4970r
h292 .The phenotype of qDf16 /q497 issimilar to that of
q497 homozygotes and the phenotype of qDf16 /h292 is similar to that of
h292 homozygotes. Because
q497 /h292 also resembles
h292 homozygotes,
q497 may be a null allele. Null mutations in another gene,
gld-1 ,also have a tumorous phenotype.(2) However, there are two key differences between mutations in these two genes: 1 ) the
q497 and
h292 mutations cause tumorous germlines in both males and hermaphrodites, whereas
gld-1 males have normal differentiation of gametes unless forced into oogenesis by a mutation such as
fog-1 or
fog-3 (Tim Schedl, Ron Ellis, personal communication). Hence, while
gld-1 appears to be primarily required for oogenic meiosis, this new gene appears to be required for progression through both spermatogenic and oogenic meiosis. 2) In
q497 ;
glp-1 (0)double mutants, nuclei fail to proliferate mitotically, but, unlike the
glp-1 (0)single mutant, they also fail to differentiate into sperm. In contrast,
gld-1 (Tum);
glp-1 (0)double mutants undergo an extra round of proliferation, then differentiate into sperm2 .Hence, the new gene represented by
q497a
ndh292 appears to be required for a step of meiosis common to spermatogenesis and oogenesis, and may work at an earlier stage of meiosis than does
gld-1 . Another question we are addressing is whether the mitotic proliferation observed in the mutant is signal-dependent. Two cells of the somatic gonad are capable of stimulating mitotic proliferation in the germline: the distal tip cell is normally required for distal proliferation(3), and the anchor cell can, in some circumstances, stimulate abnormal proximal proliferation, similar to that observed in
q497a
ndh292 .(4)We have ablated either the two distal tip cell precursors or the two anchor cell precursors in
q497 mutants in early L1 ,when there are only four somatic gonadal and four germline cells. In either case, proliferation is still observed. However, preliminary results show that ablation of all four of the somatic gonadal precursor cells prevents any germline proliferation. Thus, some signal from the somatic gonad is required for the mitotic proliferation in these mutants. In first attempts to clone this gene, we have seen partial rescue with a single cosmid mapping to the right of the cosmid known to rescue
bli-4 . (1) Thanks very much to Jennifer McDowall and Ann Rose for sharing the
h292 allele with us! (2) Schedl, et al., (1991) Worm Meeting Abstracts, p. 362 (3) Kimble and White (1981) Devel. Biol. 81: 208-219 (4) Seydoux, et al. (1990) Cell 61: 939-951