Borate ester cross-linking of the cell wall pectic polysaccharide rhamnogalacturonan II (RG-II) is required for the growth and development of angiosperms and gymnosperms. Here, we report that the amounts of borate cross-linked RG-II present in the sporophyte primary walls of members of the most primitive extant vascular plant groups (Lycopsida, Filicopsida, Equisetopsida, and Psilopsida) are comparable with the amounts of RG-II in the primary walls of angiosperms. By contrast, the gametophyte generation of members of the avascular bryophytes (Bryopsida, Hepaticopsida, and Anthocerotopsida) have primary walls that contain small amounts (approximately 1% of the amounts of RG-II present in angiosperm walls) of an RG-II-like polysaccharide. The glycosyl sequence of RG-II is conserved in vascular plants, but these RG-IIs are not identical because the non-reducing l-rhamnosyl residue present on the aceric acid-containing side chain of RG-II of all previously studied plants is replaced by a 3-O-methyl rhamnosyl residue in the RG-IIs isolated from Lycopodium tristachyum, Ceratopteris thalictroides, Platycerium bifurcatum, and Psilotum nudum. Our data indicate that the amount of RG-II incorporated into the walls of plants increased during the evolution of vascular plants from their bryophyte-like ancestors. Thus, the acquisition of a boron-dependent growth habit may be correlated with the ability of vascular plants to maintain upright growth and to form lignified secondary walls. The conserved structures of pteridophyte, lycophyte, and angiosperm RG-IIs suggests that the genes and proteins responsible for the biosynthesis of this polysaccharide appeared early in land plant evolution and that RG-II has a fundamental role in wall structure.Plants first colonized the land approximately 480 million years ago. These early land plants, which are believed to be related to extant bryophytes (Kenrick and Crane, 1997;Qiu and Palmer, 1999), subsequently gave rise to the tracheophytes that now dominate the terrestrial environment. Many of the morphological and biochemical changes that allowed plants to adapt to life on land have been documented (Graham, 1993;Niklas, 1997). However, the limited amount of information available on the composition and architecture of non-flowering plant cell walls (Ligrone et al., 2002;Popper and Fry, 2003) is an impediment to understanding the evolutionary origins of cell walls and the changes in wall structure that occurred during the evolution of land plants.All growing plant cells are surrounded by a polysaccharide-rich primary wall. Primary walls regulate cell expansion and also have important roles in plant growth and development, in the defense against micro-organisms (Carpita and Gibeaut, 1993), and in intercellular signaling (Ridley et al., 2001). The primary walls of seed-bearing tracheophytes (gymnosperms and angiosperms) are composed predominantly of cellulose, hemicellulose, and pectin together with lesser amounts of structural glycoproteins, minerals, enzymes, and phenolic esters (Carpita ...

Species complexes consisting of ill-defined "species" are widely known among ferns, and their involvement with reticulate evolution is expected. Nevertheless approaches to reticulation history with DNA markers are not yet commonly adopted. We have successfully elucidated the biological status of the Vandenboschia radicans complex in East Asian islands by combining analyses of ploidy level, a cpDNA marker (rbcL), and a nuclear DNA marker (GapCp). The results based on 266 individuals collected from 174 localities throughout Japan and Taiwan suggest that complicated hybridizations have occurred involving at least three parental diploid species from within the V. radicans complex and Vandenboschia liukiuensis, which was formerly considered to be distinct from this complex. Triploids are the most common cytotype, but they show no evidence of apogamous reproduction, while all nonhybrid diploids are rare and have very limited distribution. Possible accounts of this phenomenon will be briefly discussed including the possibility of relict distribution and occasional apogamous reproduction.