Unique generation transition in the life cycle of Pyropia yezoensis (Rhodophyta, Bangiales)

Abstract

The sexual life cycle in plants generally consists of a haploid gametophyte and a diploid sporophyte, both of which appear reciprocally at temporally different stages via fertilization of gametes to produce sporophytes and meiosis to produce gametophytes. The red seaweeds Bangiales display a heteromorphic sexual life cycle containing a leafy gametophyte (thallus) and a filamentous sporophyte (conchocelis). Unlike other multicellular eukaryotes, the production of gametophytic spores, termed conchospores, in a conchosporangium is not associated with meiosis in Bangiales. To explore this question, we performed transcriptomic profiling of Pyropia yezoensis gametophytes, sporophytes and conchosporangia and identified a knotted-like homeobox gene that is predominately expressed in the conchosporangium and may determine its identity. Further, spore-like single cells isolated from conchosporangia developed directly into thalli. We therefore concluded that gametophytic identity is established in the conchosporangium before release of conchospores without meiosis and proposed a triphasic life cycle in P. yezoensis where apospory is responsible for production of gametophyte generation.