Cell fate choices are often executed by precise control of gene expression. In the C. elegans germline, levels of GLD-1 protein are tightly correlated with germ cell fate; GLD-1 levels are low in the germline stem cell pool and high in germ cells in meiotic prophase. Ectopically high GLD-1 levels drive germline stem cells to prematurely enter meiosis, while failure to elevate GLD-1 contributes to defects in meiotic entry.
gld-1 is controlled post-transcriptionally, with the
gld-1 mRNA being translationally repressed in the pool of germline stem cells and switching to a translationally active state upon germ cell entry into meiosis. While regulators have been identified that contribute to
gld-1 repression/activation, the quantitative influence these regulators impart on GLD-1 as germ cells transition from a stem cell to meiotic fate is unknown. We established a sensitive quantitative approach to measure GLD-1 levels in the distal germline. In wild type, base GLD-1 levels, which we define as the levels in the distal-most germ cells, are approximately 5% of peak GLD-1 levels, which occurs when all germ cells have entered meiotic prophase ~26 cell diameters from the distal tip. We described the sigmoidal rise in GLD-1 from base to peak with an equation where the slope value defines the GLD-1 accumulation rate. Known
gld-1 repressors, such as
fbf-1, specifically keep GLD-1 base levels low, while known
gld-1 activators, such as
gld-2, specifically keep GLD-1 peak levels high. Interestingly,
gld-3 mutants had elevated GLD-1 base levels but low peak levels, suggesting that
gld-3 contributes to both
gld-1 repression and activation. Change in GLD-1 levels was specific to regulators of
gld-1 as loss of genes whose activity indirectly promotes a normally large germline progenitor pool, such as
eat-5 and
daf-1, did not affect GLD-1 base or peak levels, but instead had elevated GLD-1 accumulation rate. Based on these data, we interpret quantifiable changes in GLD-1 base or peak levels to specifically define genes that act to regulate
gld-1, and thus conclude that the miR-35 family of microRNAs - previously reported repressors of
gld-1 - do not control GLD-1 levels in germ cells. Lastly, we used a temperature sensitive allele of
glp-1 to manipulate GLD-1 levels and found that germ cells with 40% of peak GLD-1 levels are irreversibly committed to entering meiosis, thereby describing a quantifiable feature of meiotic commitment that prefaces germ cells executing the early events of meiotic prophase.