The evolution of the land plant life cycle

Abstract

SummaryThe 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.

Characterization of the Heterotrimeric G-Protein Complex and Its Regulator from the Green Alga Chara braunii Expands the Evolutionary Breadth of Plant G-Protein Signaling

“…The charophyte green algae are especially interesting from an evolutionary perspective, since they are the closest living relatives of the ancestors of the embryophytes (Becker and Marin, 2009; Niklas and Kutschera, 2010;Wodniok et al, 2011;Laurin-Lemay et al, 2012;Timme et al, 2012;Raven, 2013;Zhong et al, 2013). Superficially, these algae (e.g.…”

Green Algae and the Origins of Multicellularity in the Plant Kingdom

“…Nonetheless, the Ulvophytes (a predominantly multicellular order of chlorophyte algae) contain genera with a significant diploid sporophytic stage. This derived state of sporophytic growth and development also evolved independently in the embryophytes (land plants) in which the gametophyte stage was concomitantly reduced and the sporophyte dominant life cycle predominates (Niklas and Kutschera 2009).…”

Phylogeny and Molecular Evolution of the Green Algae

“…The unique sexual life cycle of land plants likely evolved through expansion of homeodomain gene networks (e.g. MADSbox genes) (Tanabe et al, 2005;Niklas & Kutschera, 2010). Ca 2+ -dependent signalling processes, which are important in the response to many developmental and environmental stimuli, have been found to be very different in green algae and land plants, with several Ca 2+ signalling mechanisms have apparently been lost in land plants after their divergence from charophyte algae (Wheeler & Brownlee, 2008).…”