Major transitions in the evolution of early land plants: a bryological perspective

Abstract

Background Molecular phylogeny has resolved the liverworts as the earliest-divergent clade of land plants and mosses as the sister group to hornworts plus tracheophytes, with alternative topologies resolving the hornworts as sister to mosses plus tracheophytes less well supported. The tracheophytes plus fossil plants putatively lacking lignified vascular tissue form the polysporangiophyte clade. Scope This paper reviews phylogenetic, developmental, anatomical, genetic and paleontological data with the aim of reconstructing the succession of events that shaped major land plant lineages. Conclusions Fundamental land plant characters primarily evolved in the bryophyte grade, and hence the key to a better understanding of the early evolution of land plants is in bryophytes. The last common ancestor of land plants was probably a leafless axial gametophyte bearing simple unisporangiate sporophytes. Water-conducting tissue, if present, was restricted to the gametophyte and presumably consisted of perforate cells similar to those in the early-divergent bryophytes Haplomitrium and Takakia. Stomata were a sporophyte innovation with the possible ancestral functions of producing a transpiration-driven flow of water and solutes from the parental gametophyte and facilitating spore separation before release. Stomata in mosses, hornworts and polysporangiophytes are viewed as homologous, and hence these three lineages are collectively referred to as the 'stomatophytes'. An indeterminate sporophyte body (the sporophyte shoot) developing from an apical meristem was the key innovation in polysporangiophytes. Poikilohydry is the ancestral condition in land plants; homoiohydry evolved in the sporophyte of polysporangiophytes. Fungal symbiotic associations ancestral to modern arbuscular mycorrhizas evolved in the gametophytic generation before the separation of major present-living lineages. Hydroids are imperforate water-conducting cells specific to advanced mosses. Xylem vascular cells in polysporangiophytes arose either from perforate cells or de novo. Food-conducting cells were a very early innovation in land plant evolution. The inferences presented here await testing by molecular genetics.

Conserved Gene Expression Programs in Developing Roots from Diverse Plants

“…Regarding the possibility of parallel recruitment, strong selective pressures and a limited genetic "toolkit" may have restricted the evolutionary path for root formation in both lineages. In this vein, it is notable that roots and shoots of extant plants deploy many of the same (or closely related) developmental genes (Benfey, 1999;Stahl and Simon, 2010), likely due to their common origin from a primitive telomic axis (Kenrick and Crane, 1997;Gensel and Berry, 2001;Friedman et al, 2004;Ligrone et al, 2012;Tomescu et al, 2014). Similarly, roots that evolved independently in separate lineages might still be expected to share a substantial fraction of their developmental program due to recruitment from a largely common pool of organ development genes.…”

Enigmatic Sebacinales

“…7 Mycobiont associations in liverworts and hornworts. Fungal phylogeny compiled after various authors, phylogeny of plants compiled after several authors in Ligrone et al (2012). Black lines refer to previous data (see accompanying text), red lines include new reports from Bidartondo et al (2011) Mycothalli in liverworts and Sebacinales mycobionts in jungermanniales (Fig.…”

Marchantia polymorpha L.: An Emerging Model Plant System to Study Contemporary Plant Biology – A Review

“…The beginning of embryophytes was a key event in evolutionary record that produced an incredible diversity in ecological, reproductive, morphological and physiological traits of land plants. The position of liverworts in the evolution of land plants has been reviewed in detail by several authors (Bennici, 2008;Kato, 2010;Ligrone, 2012;Shimamura, 2015;. However, the plant phylogeny study based on the fossil records (Kato 2010) and molecular data including plastid DNA (Nishiyama, 2004;Chang and Graham, 2011), mitochondrial genes (Qui et al, 1998;Qui et al, 2007), ribosomal genome sequence (Nickrent, 2000;Shaw et al, 2011) and nuclear genome sequence (Sanderson et al, 2004;Floyd and Bowman, 2007;Rensing et al, 2008;Banks et al, 2011;Nystedt et al, 2013) of the representative model species of the respective clade to study the events during plant evolution.…”