Evolution of Distorted Pellicle Patterns in Rigid Photosynthetic Euglenids (Phacus Dujardin)

Department of Botany, University of British Columbia, 6270 University Boulevard, Vancouver, Canada V6T 1Z4.
Journal of Eukaryotic Microbiology (Impact Factor: 3.22). 09/2009; 57(1):19-32. DOI: 10.1111/j.1550-7408.2009.00447.x
Source: PubMed


Members of the euglenid genus Phacus are morphologically differentiated from other photosynthetic species by the presence of a rigid cytoskeleton (pellicle) and predominantly dorsoventrally flattened, leaf-shaped cells. In order to better understand the evolutionary history of this lineage, we used scanning electron microscopy to examine patterns of pellicle strips in Phacus acuminatus, Phacus longicauda var. tortus, Phacus triqueter, Phacus segretii, Phacus pleuronectes, Phacus similis, Phacus pusillus, Phacus orbicularis, Phacus warszewiczii, and Discoplastis spathirhyncha, a putative close relative of Phacus and Lepocinclis. Our observations showed that while the earliest diverging species in our analyses, namely P. warszewiczii, has three whorls of exponential reduction, most members of Phacus have clustered patterns of posterior strip reduction that are bilaterally symmetrical distortions of the radially symmetrical "whorled" patterns found in other photosynthetic euglenids. Comparative morphology, interpreted within the context of molecular phylogenetic analyses of combined nuclear small subunit rDNA and partial nuclear large subunit rDNA sequences, demonstrates that clustered patterns of posterior strip reduction arose after the divergence of Phacus from other photosynthetic euglenids and are the result of developmental processes that govern individual strip length. Clustered patterns of pellicle strips in Phacus do not appear to be adaptively significant themselves; they evolved in association with the origin of cell flattening and cell rigidity, which may be adaptations to a planktonic lifestyle.

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Available from: Heather J Esson
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    • "The genus Phacus Dujard. comprises photosynthetic euglenoids and has a very large variety of cell shapes and pellicle surface patterns (Starmach 1983; Kočárková et al. 2005; Essen & Leander 2010). "
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    • "One member of the Eutreptiales (Eutreptia pertyi) has been shown to possess two linear subwhorls of posterior strip reduction that corresponds to one exponential whorl of strip reduction [13] (Figure 7). Aside from a few derived species of Phacus [40], R. viridis is the first and only euglenid described so far with one clear exponential whorl of strip reduction; this precisely corresponds to the expected character state that was present in the most recent ancestor of all euglenids possessing posterior whorls of strip reduction (i.e., all phototrophic species) [9,13,19]. The eight strips that terminate on the whorl of strip reduction in R. viridis form four distinct pairs that, when connected by lines, define the equidistant corners of a square (Figure 2D). "
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