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Abstract

The molecular processes that regulate life cycle progression provide an essential foundation for the correct initiation of diverse biological processes, including multicellular development and sexual reproduction1. The life cycle of the model brown alga Ectocarpus2–4 involves an alternation between two independent multicellular organisms, the sporophyte and the gametophyte. We have shown that the identities of the two generations are not determined by ploidy, but rather are determined genetically. Mutations affecting either the OUROBOROS (ORO) or the SAMASARA (SAM) locus result in complete conversion of the sporophyte generation into a gametophyte5. Both ORO and SAM encode three amino acid loop extension homeodomain transcription factors (TALE HD TFs). Similarities between ORO/SAM and HD-TF-based life cycle regulation systems in other eukaryotic supergroups indicate not only that these systems have an extremely ancient origin but also that TALE HD TFs have been independently recruited to regulate sporophyte developmental programs in at least two different complex multicellular eukaryotic supergroups, Archaeplastida and Chromalveolata. The presentation will discuss genetic regulation of life cycle progression and will also look at some of the genetic and epigenetic events that occur downstream of the switch to either the sporophyte or gametophyte developmental programs.