Novel cytoskeletal traits in the intestinal parasites (Squirmida, Platyproteum vivax) of Pacific peanut worms (Sipuncula, Phascolosoma agassizii)

The cytoskeletal organization of a squirmid, namely Platyproteum vivax, was investigated with confocal laser scanning microscopy (CLSM) to refine inferences about convergent evolution among intestinal parasites of marine invertebrates. Platyproteum inhabits Pacific peanut worms (Phascolosoma agassizii) and has traits that are similar to other lineages of myzozoan parasites, namely gregarine apicomplexans within Selenidium, such as conspicuous feeding stages, called “trophozoites,” capable of dynamic undulations. SEM and CLSM of P. vivax revealed an inconspicuous flagellar apparatus and a uniform array of longitudinal microtubules organized in bundles (LMBs). Extreme flattening of the trophozoites and a consistently oblique morphology of the anterior end provided a reliable way to distinguish dorsal and ventral surfaces. CLSM revealed a novel system of microtubules oriented in the flattened dorsoventral plane. Most of these dorsoventral microtubule bundles (DVMBs) had a punctate distribution and were evenly spaced along a curved line spanning the longitudinal axis of the trophozoites. This configuration of microtubules is inferred to function in maintaining the flattened shape of the trophozoites and facilitate dynamic undulations. The novel traits in Platyproteum are consistent with phylogenomic data showing that this lineage is only distantly related to Selenidium and other marine gregarine apicomplexans with dynamic intestinal trophozoites.