The POU Geneceh-18Promotes Gonadal Sheath Cell Differentiation and Function Required for Meiotic Maturation and Ovulation inCaenorhabditis elegans

Abstract

InCaenorhabditis elegans,specialized contractile myoepithelial cells of the somatic gonad, the gonadal sheath cells, are closely apposed to oocytes and are required for normal meiotic maturation and ovulation. Previously we found that mutations in theceh-18gene, 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, ovulation, and early embryogenesis inceh-18mutants. Inceh-18mutants sheath cell contractions are weaker, less frequent, and uncoordinated throughout the sequence of ovulation events, and ovulation is defective. Defective ovulation can result in the formation of endomitotic oocytes in the gonad, the formation of haploid embryos, and reversals in embryonic polarity.ceh-18mutant oocytes exhibit defects prior to nuclear envelope breakdown, suggesting that they are physiologically different from the wild type. We observed delays in meiotic maturation, as well as maturation out of the normal spatial and temporal sequence, suggesting that proximal sheath cells directly or indirectly promote and spatially restrict meiotic maturation. Analysis of sheath cell differentiation inceh-18mutants using antibodies to proteins of the contractile apparatus reveals that although contractile proteins are expressed, the sheath cells appear disorganized. Transmission electron microscopy reveals thatceh-18mutant sheath cells are morphologically irregular and only loosely cover oocytes. Taken together, these observations indicate thatceh-18is a crucial determinant of sheath cell differentiation, a function required for normal meiotic maturation and ovulation.