The evolution of the land plant life cycle
New Phytologist (2009) Summary 1I. Introduction 2II. Developmental constraint or a phyletic legacy? 3III. Green plant phylogeny 3IV. The ancestral green plant life cycle 5V. Haplobiontic or diplobiontic life cycles? 6VI. Pseudo-archegonia, plasmodesmata, and parenchyma 7VII. Genomic re-deployment and embryophyte reproduction 9VIII. Developmental homologies? 9IX. Isomorphic or dimorphic?10X. Conclusions12Acknowledgements13References13 The extant land plants are unique among the monophyletic clade of photosynthetic eukaryotes, which consists of the green algae (chlorophytes), the charophycean algae (charophytes), numerous groups of unicellular algae (prasinophytes) and the embryophytes, by possessing, firstly, a sexual life cycle characterized by an alternation between a haploid, gametophytic and a diploid, sporophytic multicellular generation; secondly, the formation of egg cells within multicellular structures called archegonia; and, thirdly, the retention of the zygote and diploid sporophyte embryo within the archegonium. We review the developmental, paleobotanical and molecular evidence indicating that: the embryophytes descended from a charophyte-like ancestor; this common ancestor had a life cycle with only a haploid multicellular generation; and the most ancient (c. 410 Myr old) land plants (e.g. Cooksonia, Rhynia and Zosterophyllum) had a dimorphic life cycle (i.e. their haploid and diploid generations were morphologically different). On the basis of these findings, we suggest that the multicellular reproductive structures of extant charophytes and embryophytes are developmentally homologous, and that those of the embryophytes evolved by virtue of the co-option and re-deployment of ancient algal homeodomain gene networks.