Asymmetry is defined during meiosis in the oocyte of the parthenogenetic nematode Diploscapter pachys.

Asymmetric cell division is an essential feature of normal development and certain pathologies. The process and its regulation have been studied extensively in the Caenorhabditis elegans embryo, particularly how symmetry of the actomyosin cortical cytoskeleton is broken by a sperm-derived signal at fertilization, upstream of polarity establishment. Diploscapter pachys is the closest parthenogenetic relative to C. elegans , and D. pachys one-cell embryos also divide asymmetrically. However how polarity is triggered in the absence of sperm remains unknown. In post-meiotic embryos, we find that the nucleus inhabits principally one embryo hemisphere, the future posterior pole. When forced to one pole by centrifugation, the nucleus returns to its preferred pole, although poles appear identical as concerns cortical ruffling and actin cytoskeleton. The location of the meiotic spindle also correlates with the future posterior pole and slight actin enrichment is observed at that pole in some early embryos along with microtubule structures emanating from the meiotic spindle. Polarized location of the nucleus is not observed in pre-meiotic D. pachys oocytes. All together our results are consistent with the idea that polarity of the D. pachys embryo is attained during meiosis, seemingly based on the location of the meiotic spindle, by a mechanism that may be present but suppressed in C. elegans. [Display omitted] • In parthenogenetic D. pachys , the first embryonic cell division is asymmetric. • In D. pachys embryos, the nucleus inhabits principally the future posterior pole. • Embryo centrifugation revealed that the posterior pole is defined during meiosis. • Pre-meiotic D. pachys oocytes do not show signs of asymmetry. • The meiotic spindle appears to trigger the location of the future posterior pole. [ABSTRACT FROM AUTHOR]