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The ploidy cycle, which is integral to sexual reproduction, requires meiosis to halve chromosome numbers as well as mechanisms that ensure zygotes are formed by exactly two partners(1-4). During sexual reproduction of the fungal model organism Schizosaccharomyces pombe, haploid P and M cells fuse to form a diploid zygote that immediately enters meiosis(5). Here we reveal that rapid post-fusion reconstitution of a bipartite transcription factor blocks re-fertilization. We first identify mutants that undergo transient cell fusion involving cytosol exchange but not karyogamy, and show that this drives distinct cell fates in the two gametes. The P partner undergoes lethal haploid meiosis, whereas the M cell persists in mating. The zygotic transcription that drives meiosis is rapidly initiated first from the P parental genome, even in wild-type cells. This asymmetric gene expression depends on a bipartite complex formed post-fusion between the cytosolic M-cell-specific peptide Mi and the nuclear P-cell-specific homeobox protein Pi(6,7), which captures Mi in the P nucleus. Zygotic transcription is thus poised to initiate in the P nucleus as fast as Mi reaches it after fusion, a design that we reconstruct using two synthetic interactors localized to the nucleus and the cytosol of two respective partner cells. Notably, delaying zygotic transcription-by postponing Mi expression or deleting its transcriptional target in the P genome-leads to zygotes fusing with additional gametes, thus forming polyploids and eventually aneuploid progeny. The signalling cascade to block re-fertilization shares components with, but bifurcates from, meiotic induction(8-10). Thus, a cytoplasmic connection upon gamete fusion leads to asymmetric reconstitution of a bipartite transcription factor to rapidly block re-fertilization and induce meiosis, ensuring genome maintenance during sexual reproduction.