The eggshell is required for meiotic fidelity, polar-body extrusion and polarization of the C. elegansembryo

Abstract

Background: Fertilization restores the diploid state and begins the process by which the singlecell oocyte is converted into a polarized, multicellular organism. In the nematode, Caenorhabditis elegans, two of the earliest events following fertilization are secretion of the chitinous eggshell and completion of meiosis, and in this report we demonstrate that the eggshell is essential for multiple developmental events at the one-cell stage.

The C. elegans eggshell

“…This layer is larger at the anterior end where the first extruded polar body resides. It is thought that this layer is fluid-filled and contains a number of proteins (Olson et al, 2012; Johnston et al, 2006). The fifth layer, which is separated from the trilaminar outer eggshell by the EEM, is now thought to be the lipid-rich layer, which makes up the osmotic/permeability barrier for the developing embryo.…”

“…The presence of chitin in the nematode eggshell has been directly demonstrated using either a fluorescently-conjugated or immunogold-labeled chitin-binding domain (CBD) probe (New England Biolabs) in C. elegans (Olson et al, 2012; Parry et al, 2009; Maruyama et al, 2007; Johnston et al, 2006) (Figure 1 and Figure 2). Chitin is evident in the eggshell of a newly fertilized zygote at metaphase I (Maruyama et al, 2007).…”

“…Most embryos accumulate in the uterus as an indistinct massthose that make it out of the spermatheca and uterus without being crushed are multinucleate, polyploid one-cell embryos that fail to undergo cytokinesis. It should be no surprise that such embryos also do not establish embryonic polarity (Benenati et al, 2009; Johnston et al, 2006). The polarity phenotypes described for many eggshell synthesis genes could be due to the observation that polarity cues depend on microtubules.…”

“…The enzymatic pathway that generates UDP-GlcNAc, the substrate for chitin synthase, is the hexosamine pathway. This pathway consists of at least five enzymes that are essential for the synthesis of chitin from glucose-6-phosphate, and individual depletion of four of these enzymes by RNAi has been reported to generate an OID phenotype (Johnston and Dennis, 2012; Johnston et al, 2006; Sonnichsen et al, 2005). The best-studied of these genes is gna-2 , which codes for glucosamine 6-phosphate N-acetyltransferase.…”

“…The best-studied of these genes is gna-2 , which codes for glucosamine 6-phosphate N-acetyltransferase. Hermaphrodites homozygous for the deletion allele gna-2(qa705) are 100% Maternal Effect Lethal (Mel) and lay round, osmotically-sensitive one-cell embryos (Johnston et al, 2006). In addition, the sperm pronucleus centrosome complex (SPCC) never associates with the cortex and pseudocleavage furrows are not observed (Johnston et al, 2006).…”