Latent endogenous giant viruses drive active infection and inheritance in a multicellular algal host.

Endogenous viral elements inserted in host genomes are often regarded as inert relics of past infections. Whether they can retain infective potential and contribute to active viral cycles has remained largely unresolved. Here we demonstrate that giant viral elements in the multicellular alga Ectocarpus can reactivate and drive productive viral infections. Using long-read sequencing and transcriptomics, we identify full-length, transcriptionally active phaeoviruses integrated within the host genome, and we use classical genetics and CRISPR-Cas to demonstrate that these elements are stably inherited through the germline, while their reactivation is precisely regulated by developmental and environmental cues including temperature. We resolve the genomic integration sites and propose a mechanism for phaeovirus integration and replication. Our work provides direct evidence and uncovers the mechanisms by which giant viral elements can reactivate, replicate and transmit both horizontally and vertically in a multicellular eukaryote, establishing a new model of latency, inheritance and evolutionary impact of giant dsDNA viruses.