Two mechanisms drive pronuclear migration in mouse zygotes

A new life begins with the unification of the maternal and paternal chromosomes upon fertilization. The parental chromosomes first become enclosed in two separate pronuclei near the surface of the fertilized egg. The mechanisms that then move the pronuclei inwards for their unification are only poorly understood in mammals. Here, we report two mechanisms that act in concert to unite the parental genomes in fertilized mouse eggs. The male pronucleus assembles within the fertilization cone and is rapidly moved inwards by the flattening cone. Rab11a recruits the actin nucleation factors Spire and Formin-2 into the fertilization cone, where they locally nucleate actin and further accelerate the pronucleus inwards. In parallel, a dynamic network of microtubules assembles that slowly moves the male and female pronuclei towards the cell centre in a dynein-dependent manner. Both mechanisms are partially redundant and act in concert to unite the parental pronuclei in the zygote's centre. In a newly fertilized egg, maternal and paternal chromosomes are enclosed in two separate pronuclei but the mechanisms in mammals for pronuclear movement are unclear. Here, the authors report that both F-actin and microtubule polymerization act in concern to drive inward movement of pronuclei towards the cell centre.

Automated summary

In fertilized mouse eggs, the unification of parental genomes is driven by the concerted action of F-actin and microtubule polymerization.