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 environme ntally important genera of green seaweeds. They are widely regarded as easi ly distinguishable because of their dramatically different morphologies: Ul va species are flat, lettucelike blades two cell layers thick, and Enteromo rpha 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 purport ed species within these two genera. The results clearly indicate that the t wo genera are not respectively monophyletic and that the characteristic Ulv a and Enteromorpha morphologies have arisen independently several times thr oughout the evolutionary diversification of the group. The analyses demonst rate that this radical change in gross morphology can also happen within cl ades exhibiting sequence divergence typical of conspecific assemblages of t his group. We suggest that this morphological flexibility is the result of some form of developmental switch that results in either blades or tubes, b ut that this putative switch must be activated relatively infrequently, sin ce there is evidence that some lineages have retained their form for signif icant periods. This discovery suggests a possible new model system for stud y of the molecular mechanisms involved in the interplay between environment al stimuli and plant development.