PMC

Three processes from the telome theory proposed to be involved in megaphyll evolution (redrawn from Zimmermann, ). These processes may have occurred in various sequences in different groups of vascular plants.

Stages in the evolution of the megaphyll as documented by Zimmermann's telome theory (), with representative fossil plants for each stage illustrated below. Upper schematic: , images of Rhynia, Psilophyton and Archaeopteris after , image of Actinoxylon after .

Schematic representation of the expression patterns of integument development genes, in three different species. (a) Arabidopsis thaliana (WUS by Grobeta-Hardt; BELL1 by Robinson-Beers et al.; ANT by Elliot et al.; KAN by Leon-Kloosterziel et al.; Eshed et al.; UCN by Enuguttii et al.). (b) Gnetum gnemon previously published (WUS by Nardmannn et al. 2009; Melbel1, GneANT, GnmoKANs and GnmoUCNs by Zumajo-Cardona and Ambrose). (c) Ginkgo results presented here. (d) Illustration of telome theory, synangial hypothesis and neo-synangial hypothesis for the origin of the seed. Notably, the telome theory indicates the evolution of integuments from sterile structures while both the synangial and neo-synangial hypotheses indicate the evolution of integuments from fertile (sporangia) structures.

A diagram of two events closely related to the “retention” theory regarding seed origin. (A–D) Putative process of integument origin during evolution according to the “telome theory.” (A) Oval-shaped megasporangia are located at the tips of the telomes/branches. (B,C) Several telomes/branches were coopted as surrounding tissues to enclose the dominant/fertile megasporangium. (D) The surrounding tissues merged into a structure designated as the integument (often consisting of two layers in angiosperms; modified from ). (E) Illustration of a stalked ovule of Callospermarion. “m” refers to a megaspore in the megasporangium (indicated by the dotted region) enclosed by the integument with vascular bundles represented by the black strips (modified from ).