Structural Similarities between Surface Layers of Selected Charophycean Algae and Bryophytes and the Cuticles of Vascular Plants

Abstract

Land plants are a monophyletic group that arose from a common ancestor that would be classified with the charophycean green algae. Thus early divergent plants and charophycean algae can be used as simple model systems for studying the evolution of more complex features of higher plants. We used TEM and SEM to examine surface layers of the charophycean green algae Nitella gracilis, Chara zeylanica, and Coleochaete orbicularis and of gametophytes of the putatively early divergent bryophytes Monoclea gottschei (liverwort), Notothylas orbicularis (hornwort), and Sphagnum fimbriatum (moss) in an attempt to learn how the cuticle of vascular plants may have originated In all of the taxa we studied, TEM data indicate that there is an osmiophilic layer on the outer cell wall that bears some structural resemblance to early developmental stages of vascular plant cuticles. SEM shows further that the surface layer of Monoclea is nodular, that Notothylas may have either a nodular or a sheetlike surface, and that Coleochaete and Sphagnum bear sheetlike surface layers with regular ridges running parallel to the edges of their thalli. The numerous ridges at the edge of the thallus in Coleochaete resemble those at the base of branches in charalean algae. These algae have a distinctive sheetlike or ridged surface layer, composed of strands, which appears to protect young, unexpanded shoots and is then pulled apart as growth takes place This may be a specialization resulting from the charalean lifestyle. Based on our results, it appears that some structures that are at least positionally analogous to the plant cuticle arose before the charophycean algae and bryophytes diverged from a common ancestor. While structural correlation is not proof of homology, it indicates that further examination at the biochemical level is warranted.