Fast zygotic genome activation in brown algae

A defining event of early development is the maternal-to-zygotic transition (MZT), during which control shifts from the maternally deposited transcripts to the zygotic genome. In animals, early embryogenesis depends on maternal provisioning, with zygotic genome activation (ZGA) typically delayed until later developmental stages. In plants, however, the timing of this transition remains debated, in part because embryogenesis occurs within maternal tissues and is difficult to resolve experimentally. Whether maternal provisioning universally delays ZGA, as observed in animals, or whether this relationship differs across distinct multicellular lineages remains unknown. Brown algae, which have evolved complex multicellularity independently, and span a gradient of maternal provisioning from isogamy to oogamy, provide a powerful system to test the relationship between maternal provisioning and ZGA timing. Here, using allele-specific expression across multiple genetic crosses, we resolve parental contributions during early development. We show that this relationship is reversed: species with extensive maternal provisioning activate their zygotic genomes immediately after fertilisation, whereas limited provisioning is accompanied by a prolonged ZGA. Rather than delaying activation, maternal provisioning enables rapid and large-scale genome activation, demonstrating that the timing of the MZT is evolutionarily flexible across multicellular eukaryotes.