Characterization of paramylon morphological diversity in photosynthetic euglenoids (Euglenales, Euglenophyta)

A characteristic feature of euglenoid cells is the presence of a beta 1-3 glucan storage product called paramylon. The euglenoid lineage is tremendously diverse, with a great deal of variation in paramylon grain morphology. Number, shape, location and external morphology of paramylon have been used as diagnostic features for several euglenoid species. The goals of this study were to examine and characterize the paramylon grains from a variety of different species in vivo and in vitro, provide a consistent, descriptive terminology that can be used to describe paramylon grain types, and discuss the potential phylogenetic utility of paramylon grain morphology. Over 1000 light microscopy images were examined to survey paramylon diversity across the Euglenales, and scanning electron microscopy was used to examine paramylon in 11 representative species (Colacium vesiculosum, Cryptoglena skujae, Discoplastis spathirhyncha, Euglena gracilis, Lepocinclis acus, L. OVUM, Monomorphina pyrum, Phacus orbicularis, P. pleuronectes, Strombomonas borystheniensis, and Trachelomonas ellipsoidalis). The various types of paramylon grains were separated into six distinct morphological categories: disk, ellipse, pyrenoid cap, plate, bobbin or rod, with further distinction for varieties found within each category. These categories were then applied to the diversity found in the genera and major lineages. This study was able to determine a high level of distinction among differing paramylon grain morphologies both within a species and among species, and variation was found in both large and small grains. Paramylon can be used at the generic level to support major clades and generic relationships and may provide insight into the taxonomic placement of euglenoids currently unavailable for sequencing.