THE POU GENE CEH-18 PROMOTES GONADAL SHEATH CELL-DIFFERENTIATION AND FUNCTION REQUIRED FOR MEIOTIC MATURATION AND OVULATION IN CAENORHABDITIS-ELEGANS

In Caenorhabditis elegans, specialized contractile myoepithelial cells of the somatic gonad, the gonadal sheath cells, are closely apposed t o oocytes and are required for normal meiotic maturation and ovulation . Previously we found that mutations in the ceh-18 gene, which encodes a POU-class homeoprotein expressed in sheath cells, result in oocyte defects. To determine the basis for these oocyte defects, we have used time-lapse video Nomarski microscopy to observe meiotic maturation, o vulation, and early embryogenesis in ceh-18 mutants. In ceh-18 mutants sheath cell contractions are weaker, less frequent, and uncoordinated throughout the sequence of ovulation events, and ovulation is defecti ve. Defective ovulation can result in the formation of endomitotic ooc ytes in the gonad, the formation of haploid embryos, and reversals in embryonic polarity. ceh-18 mutant oocytes exhibit defects prior to nuc lear envelope breakdown, suggesting that they are physiologically diff erent from the wild type. We observed delays in meiotic maturation, as well as maturation out of the normal spatial and temporal sequence, s uggesting that proximal sheath cells directly or indirectly promote an d spatially restrict meiotic maturation. Analysis of sheath cell diffe rentiation in ceh-18 mutants using antibodies to proteins of the contr actile apparatus reveals that although contractile proteins are expres sed, the sheath cells appear disorganized. Transmission electron micro scopy reveals that ceh-18 mutant sheath cells are morphologically irre gular and only loosely cover oocytes. Taken together, these observatio ns indicate that ceh-18 is a crucial determinant of sheath cell differ entiation, a function required for normal meiotic maturation and ovula tion. (C) 1997 Academic Press.