Ultrastructure and taxonomy ofActinomonas pusilla, a heterotrophic member of the Pedinellales (Chrysophyceae)

Prey capture in protists utilizing microtubule filled processes and surface motility

Surface motility, which can be visualized by the movement of live prey organisms, polystyrene microspheres or other inert particles, has been shown to occur in a wide variety of microtubule-filled extensions of the protistan cell surface, although the associated functions remain enigmatic. This article integrates an extensive but poorly known body of literature showing that surface motility, associated with microtubule-filled cell extensions such as flagella, axopodia, actinopodia, reticulopodia, and haptonema, plays a crucial role in protistan prey capture. Surface motility has been most extensively studied in Chlamydomonas where it is responsible for flagella-dependent whole cell gliding motility. The force transduction machinery for gliding motility in Chlamydomonas is intraflagellar transport. Other than in Chlamydomonas, this field has not moved far beyond the descriptive to the mechanistic because of technical challenges associated with many of the protistan organisms that utilize surface motility for prey capture. The purpose of this article is to rekindle interest in the protistan systems that utilize surface motility for prey capture at a time when newly emerging molecular tools for working with protists are poised to reinvigorate a field that has been quiescent too long.

The Origin of Animal Multicellularity and Cell Differentiation

Over 600 million years ago, animals evolved from a unicellular or colonial organism whose cell(s) captured bacteria with a collar complex, a flagellum surrounded by a microvillar collar. Using principles from evolutionary cell biology, we reason that the transition to multicellularity required modification of pre-existing mechanisms for extracellular matrix synthesis and cytokinesis. We discuss two hypotheses for the origin of animal cell types: division of labor from ancient plurifunctional cells and conversion of temporally alternating phenotypes into spatially juxtaposed cell types. Mechanistic studies in diverse animals and their relatives promise to deepen our understanding of animal origins and cell biology.

Vestigial chloroplasts in heterotrophic stramenopiles Pteridomonas danica and Ciliophrys infusionum (Dictyochophyceae)

Two heterotrophic members of the Dictyochophyceae (stramenopiles), Pteridomonas danica and Ciliophrys infusionum, were investigated. An undescribed organelle bounded by four membranes and closely associated with the nucleus was detected in P. danica. The outermost membrane was continuous with the outer nuclear membrane. These features strongly suggested that this organelle was a vestigial chloroplast. A photosynthetic gene, rbcL, was successfully amplified by polymerase chain reaction (PCR) from P. danica and C. infusionum. These sequences were readily and well aligned with those of photosynthetic stramenopiles. Phylogenetic trees of 18S rDNA and rbcL were constructed. In all the trees obtained, P. danica and C. infusionum appeared in two different clades, the Pedinellales clade and the Ciliophryales/Rhizochromulinales clade, each of which contained photosynthetic members as well as heterotrophic members. The results indicated that the loss of photosynthetic ability occurred independently in P. danica and C. infusionum. This is the first report of the presence of a vestigial chloroplast (leucoplast) in colorless dictyochophytes.