The brown alga Ectocarpus has alternating haploid (gametophyte) and diploid (sporophyte) generations. Morphologically, these are distinguished by a more complex system of basal filaments in the sporophyte, initiated via symmetric divisions before the apical-basal axis is defined and the upright filaments form. This mode of development is unusual – in most brown algae, and in the Ectocarpus gametophyte generation, the first division is asymmetric to establish the apical-basal axis. The immediate upright (imm) mutation of Ectocarpus displays an asymmetric first division, and was initially thought to represent a partial switch from the sporophyte to the gametophyte developmental programme. Here (p. 409), Mark Cock and colleagues identify the gene responsible for this phenotype and provide a detailed analysis of its evolutionary history. The IMM gene is a member of the large, rapidly evolving, EsV-1-7 domain family, which exhibits an unusual distribution across eukaryotic lineages – potentially as a result of horizontal gene transfer. Transcriptional profiling suggests that, rather than imm causing a switch from the sporophyte to the gametophyte programme, the mutation blocks the extensive development of the basal filament system, such that the mutant displays a more canonical mode of sporophyte development. While the molecular and cellular function of IMM has yet to be determined, this gene appears to represent an evolutionary innovation in the Ectocarpus lineage that altered early sporophyte development.
