DELWICHE, CHARLES F.1*, KENNETH G. KAROL1, and RICHARD M. MCCOURT2. 1Cell Biology & Molecular Genetics, University of Maryland, College Park MD 20742; 2Department of Botany, Academy of Natural Sciences, Philadelphia, PA 19103. - One small step: why did the charophytes have the right stuff?
The origin and evolution of land plants (i.e., embryophytes)
represents one of the seminal events in the history of life on earth.
When the land plant lineage took hold upon the continents, there were
profound changes in the global environment including dramatic
modification of the erosion regime, correlated changes in marine
nutrient inputs, and striking fluctuations in atmospheric chemistry.
These changes took place not only because of the ability of land
plants to survive on land, but also because their structure and
physiology permitted them to make efficient use of resources, anchor
themselves effectively and in so doing stabilize the substrate and
permit the formation of modern soils, and maintain metabolic activity
for long periods of time even in the absence of rainfall and surface
moisture. Molecular phylogenetic analyses show the order Charales is
the sister group to all land plants, with the Coleochaetales sister to
the land plant/Charales lineage. Thus in a very real sense, the
embryophytes are “drier algae.” Many groups of green algae live in the
terrestrial environment, but only one of these – the land plants – has
radiated into a wide range of habitats. It is not known why the land
plant lineage has been so successful. Traits that are likely to have
played a role in their success include cell wall biochemistry,
desiccation resistance and tolerance, structural complexity, and
various reproductive strategies. In all probability the success of the
land plant lineage was not the result of a single “key innovation,”
but an emergent property resulting from complex interactions among
these and other features of the lineage. Comparative study of the
properties of diverse aquatic and terrestrial algae can be used to
identify properties that were important in the colonization of the
land and how they interact.
Key words: Charophyta, Chlorophya, Embryophyte, land plant, molecular phylogenetics, paleobotany