Visualizing the complex substructure of euglenid pellicle strips with SEM

Comparative analysis of cytoskeletal diversity within the Euglenophyceae has provided important context for understanding the phylogenetic relationships and major evolutionary transitions within the group (e.g. switches in modes of nutrition and motility). Some ultrastructural characters used in earlier cladistic analyses of euglenids involved different states for the lateral projections that extend from the frame of each pellicle strip in photosynthetic lineages. Previously, the overall structure of 'strip projections' in different lineages was (arduously) reconstructed from a series of ultra-thin sections viewed with transmission electron microscopy (TEM). In this study, we were able to determine the structure of strip projections with greater precision, and without the laborious protocols associated with TEM (e.g. ultramicrotomy), by examining disrupted pellicles from three photosynthetic euglenids (Lepocinclis fusiformis, Phacus longicauda var. tortus, and P. segretii) using scanning electron microscopy (SEM). The structure of the strip projections observed here demonstrated that either (1) previous TEM studies of the pellicle overlooked certain ultrastructural features in some taxa or (2) the (prearticular) strip projections in L. fusiformis, P. segretii, and P. longicauda var. tortus represent a novel character state that could be phylogenetically informative.