The sporophyte generation of the brown alga Ectocarpus sp. exhibits an unusual pattern of development compared to the majority of brown algae. The first cell division is symmetrical and the apical/basal axis is established late in development. In the IMMEDIATE UPRIGHT mutant the initial cell undergoes an asymmetric division to immediately establish the apical/basal axis. We provide evidence which suggests that this phenotype corresponds to the ancestral state of the sporophyte. The IMM gene encodes a protein of unknown function, containing a repeated motif also found in the EsV-1-7 gene of the Ectocarpus virus. Brown algae possess large families of EsV-1-7 domain genes but these genes are rare in other stramenopiles suggesting that the expansion of this family may have been linked with the emergence of multicellular complexity. EsV-1-7 domain genes have a patchy distribution across eukaryotic supergroups and occur in several viral genomes, suggesting possible horizontal transfer during eukaryote evolution.