Molecular Biology and Evolution, Vol 16, 1011-1018, Copyright © 1999 by Society for Molecular Biology and Evolution
IH Tan, J Blomster, G Hansen, E Leskinen, CA Maggs, DG Mann, HJ Sluiman and MJ Stanhope
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.
ORIGINAL ARTICLE
Molecular phylogenetic evidence for a reversible morphogenetic switch controlling the gross morphology of two common genera of green seaweeds, Ulva and Enteromorpha
Royal Botanic Garden Edinburgh, U.K.
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