The switch from mitosis to meiosis is a critical decision in the life of all metazoan germ cells. Once this commitment to meiosis is made, the sperm and oocyte must coordinate differentiation with the meiotic cell cycle. While examining spermatogenesis defective (spe) mutants in the nematode C. elegans, we discovered two dominant mutants that show major defects in sperm proliferation. Light and electron microscopic examination reveal that both mutants show similar aspects of cellular differentiation in the absence of either cell division or successful meiosis. Both dominant mutants affect sperm, but do not affect oocytes or any somatic cells. Using these dominant alleles, one can select for a second mutation within
spe-37 (gf) that allows production of functional sperm by
spe-37 (gf) / wild type heterozygotes. Several of these "second site"
spe-37 mutants have been selected and all have recessive phenotypes. Genetic analysis of these recessive mutants uncovers a role for this gene in early embryogenesis and germline proliferation, in addition to its role in sperm as revealed by the dominant mutants). These recessive mutants allow utilization of positional cloning techniques, which reveal that
spe-37 encodes the metazoan specific
wee-1 homolog Myt1. Many higher eukaryotes, including humans, have a soluble Wee-1p as well as Myt1, which is a single pass integral membrane protein that localizes to the endoplasmic reticulum and Golgi. The kinase region of Myt1 is presumed to face the cytoplasm while the C terminal tail lies within these organelles. The C terminal region is known to be essential for proper Myt1 function, but prior work has not revealed its role. Two independently derived dominant
spe-37/Myt1 mutants have an identical missense mutation in this C-terminal region, so they should provide crucial insight into the function of this domain. We have initiate a large scale mutant hunt to recover new dominant and recessive
spe-37/Myt1 mutants and to identify other genes in the pathway in which Myt1 functions. This is the first time that it has been possible to combine cell biological and genetic techniques to study this important metazoan kinase. It should be possible to determine the genetic pathway that controls exit from mitosis and entry into meiosis through study of C. elegans Myt1.