Dual-spindle formation in zygotes keeps parental genomes apart in early mammalian embryos

It takes two to tango Fusion of egg and sperm combines the genetic material of both parents in one cell. In mammals, including humans, each parental genome is initially confined in a separate pronucleus. For the new organism to develop, the two genomes must be spatially coordinated so that the first embryonic division can create two cells that combine both genomes in one nucleus. Reichmann et al. found that at the beginning of the first division, two microtubule spindles organize the maternal and paternal chromosomes and subsequently align to segregate the parental genomes in parallel (see the Perspective by Zielinska and Schuh). Failure of spindle alignment led to two-celled embryos with more than one nucleus per cell. Dual-spindle assembly in the zygote thus offers a potential mechanistic explanation for division errors frequently observed in human embryos in the fertility clinic. Science , this issue p. 189 ; see also p. 128