In the wild-type C. elegans germ line, the partially cellularized nuclei ("cells") near the distal tip are a mitotic stem population. These cells exit mitosis at about 20 cell diameters from the distal tip, transit into meiotic pachytene in the remainder of the distal arm and around the bend, and undergo gametogenesis in the proximal arm. How are these different cell fates specified within the germ line? A gene on chromosome I,
gld-2, is involved in a cell fate decision between meiotic and mitotic fates in the germ line. We initially identified one
gld-2 allele in which germline cells enter meiosis but fail in pachytene; cells in the proximal arm are mitotic in 100% of gonad arms. We now report that this "allele" is actually a compound mutation in two closely linked genes,
gld-2 (
q497) and an enhancer, Enh (
q525). In the absence of Enh (
q525), the proximal arm of
q497 homozygotes is filled with enlarged oocyte-like cells that have failed to exit pachytene; no proximal mitosis is observed. However, a second allele of
gld-2,
h292[1], does not appear to have a linked enhancer and does have proximal mitosis in 56% of gonad arms, just proximal to a region of enlarged cells such as those seen in
q497. Analysis of trans-heterozygotes suggests that
q497 is loss-of-function, whereas
h292 may have recessive gain-of-function character. In order to obtain possibly null alleles of
gld-2, we have been screening for mutations that fail to complement
q497. The characterization of these alleles will be presented at the meeting. Null alleles of another locus on chromosome I,
gld-1, have proximal germline mitosis similar to that in
gld-2[2]. However,
gld-2 is different from
gld-1 in that both male and hermaphrodite
gld-2 mutants are unable to undergo gametogenesis, whereas
gld-1 males are normal. Hence, while
gld-1 is required for oogenesis but not for male spermatogenesis,
gld-2 is required for both types of gametogenesis. We hypothesize that
gld-2 functions at an early stage of meiosis common to both spermatogenesis and oogenesis. To determine the nature of ectopic proximal mitosis in the germ line, we have used electron microscopy to examine three mutants with this phenotype (
gld-1(0),
gld-2 (
q497) Enh (
q525), and
lin-12 (0). These results as well as progress in cloning of
gld-2 will be presented. [1]Thanks to Jennifer McDowall and Ann Rose for sharing this allele! [2] Francis et al., (1995) Genetics 139: 579-606