Molecular phylogenetic evidence for a reversible morphogenetic switch controlling the gross morphology of two common genera of green seaweeds, Ulva and Enteromorpha.

Ulva and Enteromorpha are two of the most common, ubiquitous, and environmentally important genera of green seaweeds. They are widely regarded as easily distinguishable because of their dramatically different morphologies: Ulva species are flat, lettucelike blades two cell layers thick, and Enteromorpha species form hollow liquid- or gas-filled tubes one cell thick, which may also be highly branched. We present molecular phylogenetic analyses of nuclear ribosomal RNA ITS sequences from 39 samples representing 21 purported species within these two genera. The results clearly indicate that the two genera are not respectively monophyletic and that the characteristic Ulva and Enteromorpha morphologies have arisen independently several times throughout the evolutionary diversification of the group. The analyses demonstrate that this radical change in gross morphology can also happen within clades exhibiting sequence divergence typical of conspecific assemblages of this group. We suggest that this morphological flexibility is the result of some form of developmental switch that results in either blades or tubes, but that this putative switch must be activated relatively infrequently, since there is evidence that some lineages have retained their form for significant periods. This discovery suggests a possible new model system for study of the molecular mechanisms involved in the interplay between environmental stimuli and plant development.

A pathway of plasma membrane biogenesis bypassing the Golgi apparatus during cell division in the green alga Cylindrocapsa geminella.

Cell division in Cylindrocapsa geminella, in particular the mode of septum membrane biogenesis, has been studied with the transmission electron microscope. Septum formation takes place in a narrow layer of cytoplasm separating post-mitotic nuclei. First, each daughter nucleus develops a wide cytoplasmic pocket (invagination) containing numerous strands of rough endoplasmic reticulum (ER). Next, a proliferation of rough ER is observed in the equatorial zone of cytoplasm, which invariably contains a small number of widely scattered microtubules. The equatorially aligned cisternae of rough ER produce smooth-membraned vesicles, interpreted as smooth ER, which subsequently coalesce to form the membranous transverse septum. Thus, primary septum formation does not follow any of the two previously known basic cytokinetic patterns in green plants (i.e. plasma membrane furrowing and cell-plate formation), but instead represents a novel type of membrane flow, which effectively bypasses the Golgi apparatus. This pathway of membrane flow has remained largely ignored in current concepts of endomembrane structure and function in eukaryotes. However, it appears to be more widespread than has previously been recognized, especially in autospore-producing green algae and in red algae during the formation of tetraspores. It may represent an evolutionary intermediate type of cell division between the supposedly primitive method of plasma membrane furrowing and the more advanced cell-plate system.