Non-rodent mammalian zygotes assemble dual spindles despite the presence of paternal centrosomes

Abstract

The first mitosis of the mammalian embryo must partition the parental genomes contained in two pronuclei. In rodent zygotes, sperm centrosomes are degraded and, instead, acentriolar microtubule organizing centers and microtubule self-organization guide the assembly of two separate spindles around the genomes. In non-rodent mammals, including human or bovine, centrosomes are inherited from the sperm and have been widely assumed to be active. Whether non-rodent zygotes assemble a single centrosomal spindle around both genomes, or follow the dual spindle self-assembly pathway is unclear. To address this, we investigated spindle assembly in bovine zygotes by systematic immunofluorescence and real-time light-sheet microscopy. We show that two independent spindles form around the parental genomes despite the presence of centrosomes, which had little effect on spindle structure and were only loosely connected to the two spindles. We conclude that the dual spindle assembly pathway is conserved in non-rodent mammals. This could explain whole parental genome loss frequently observed in blastomeres of human IVF embryos.

Parental genome unification is highly error-prone in mammalian embryos

“…Moreover, the chromosomes in both pronuclei simultaneously attached to microtubules, which in 60% of the cases formed as a single array of microtubules across all chromosomes and in 40% of the cases as two separated asters between the two chromosome masses that later merged in almost 60% of the cases ( Figures S7 H and S7I). As described in a recent study, bovine zygotes sometimes also assembled two separate spindles, although at lower frequency than parthenotes ( Figure S7 H) ( Schneider et al., 2020 ).…”

Fluorescence-Based Ratiometric Analysis of Sperm Centrioles (FRAC) Finds Patient Age and Sperm Morphology Are Associated With Centriole Quality

“…However, the dispensability of sperm centrioles observed in mice does not appear to apply to humans . Recent studies suggest that bovine centrioles play critical and novel roles in the zygote, and their dysfunction leads to aneuploidy (Cavazza et al, 2020;Schneider et al, 2020). Therefore, it is likely that a fraction of human reproductive diseases may be defined as Sperm Centriole Associated Reproductive Disorders (SCARD).…”

A dynamic basal complex modulates mammalian sperm movement

“…However, in mammals, the DC, at the base of the agellum, is structurally atypical 15 . Since, similar to canonical centrioles, the atypical centriole functions postfertilization, the reason for its atypical structure remains unknown [16][17][18] . The PC and DC are embedded in a specialized mass of atypical pericentriolar material: the segmented columns (SCs) and the capitulum.…”