A unique life cycle transition in the red seaweed Pyropia yezoensis depends on apospory

摘要

Plant life cycles consist of two temporally separated stages: a haploid gametophyte and a diploid sporophyte. In plants employing a haploid–diploid sexual life cycle, the transition from sporophyte to gametophyte generally depends on meiosis. However, previous work has shown that in the red seaweed Pyropia yezoensis, this transition is independent of meiosis, though how and when it occurs is unknown. Here, we explored this question using transcriptomic profiling of P. yezoensis gametophytes, sporophytes, and conchosporangia parasitically produced on sporophytes. We identify a knotted-like homeobox gene that is predominately expressed in the conchosporangium and may determine its identity. We also find that spore-like single cells isolated from the conchosporangium develop directly into gametophytes, indicating that the gametophyte identity is established before the release of conchospores and prior to the onset of meiosis. Based on our findings, we propose a triphasic life cycle for P. yezoensis involving production of gametophytes by apospory. Koji Mikami et al. report transcriptome analysis of the haploid and diploid life stages of the red seaweed Pyropia yezoensis. They identify a homeobox gene that may be responsible for determining the identity of the conchosporangium and propose that this parasitic structure may represent a third life stage of the alga.