Double trouble at the beginning of life

Every human life begins with the fertilization of an egg (1). Once the egg and the sperm have fused, the parental chromosomes need to be united. To this end, the egg and sperm chromosomes are first packaged into two separate membrane-enclosed nuclei. These nuclei then slowly move toward each other and break down in the center of the fertilized egg, called the zygote. Only then are the maternal and paternal chromosomes united—but not quite. Surprisingly, the parental chromosomes do not mix immediately but instead occupy distinct territories in the zygote throughout the first cellular division (2, 3). How the autonomy of parental genomes is retained after fertilization has remained unclear. On page 189 of this issue, Reichmann et al. (4) used elegant microscopy methods to illuminate this special moment, when the parental chromosomes first meet in live mouse zygotes, and follow how the chromosomes become distributed as the zygote divides. Their findings reveal an unexpected mechanism that keeps the parental genomes apart during the first division of the embryo: The male and female chromosomes each assemble their own chromosome separation machineries. This increases the probability that chromosomes are separated into multiple, unequal groups, which may compromise em