The sporophytes of seed-free vascular plants - Major vegetative developmental features and molecular genetic pathways

Abstract

Seed-free vascular plants, collectively referred to as pteridophytes, include several distinct lineages, of which some have living representatives: the lycopsids, sphenopsids, ferns, and psilotopsids. Although the last three are included in a monophyletic group (the moniliformopses) by some workers, the most comprehensive phylogenies that include both extant and extinct taxa reject the monophyly of moniliformopses. The sporophytes of the main living groups of seed-free plants exhibit significantly divergent morphologies, both among the different groups, and between those and the seed plants. In terms of vegetative features, such differences are seen in embryo structure and development, body plan, stele architecture, branching, leaf development and phyllotaxis, and rooting structures. These divergent morphologies are determined by fundamental differences in development and are thought to reflect independent origins of major developmental features that are supported by the current understanding of plant phylogeny. In this context, it becomes highly enticing to search for shared pathways (process homologies) and homoplasy in the molecular genetic mechanisms that control development. Understanding the gene pathways that control fundamental developmental features in the different lineages will greatly improve the resolution of vascular plant phylogeny. In this chapter, I present a comparative survey of major vegetative features of sporophytes, emphasizing the differences among the various living seed-free lineages and between those and seed plants, and I review the state-of-the-art knowledge of molecular genetic pathways that control the development of seed-free plant sporophytes. Results published to date point, in some cases, to highly conserved pathways, such as the one shared between the control of rhizoid development in bryophytes, and that of root hairs in flowering plants; this broad taxonomic range brackets, phylogenetically, all seed-free plant lineages which are hence hypothesized to share the same pathway. In other cases, such as leaf development, -different lineages reveal complex mosaics of shared and divergent pathways. However, as molecular genetic studies of seed-free plants are still in their infancy compared to those of seed plants, and especially of angiosperms, most aspects of their vegetative sporophyte development have yet to be characterized from a molecular standpoint.