The Ultrastructure of Polytomella agilis*

Abstract

SYNOPSIS Motile cells and cysts of Polytomella agilis, obtained over the entire growth cycle, were examined by electron microscopy. In typical late log phase cells there is a concentric arrangement of the internal organelles around the centrally located nucleus. Lying just beneath the plasma membrane is a peripheral band of elongate mitochondria. Numerous well defined Golgi bodies are also distributed around the nucleus. Vesicles associated with the Golgi body increase in size with distance from the secretory edges of the organelle. Cytoplasmic membranes with associated ribosomes are found between the mitochondrial and Golgi regions. A layer of slender membrane‐limited structures is located near the mitochondrial layer. These organelles, which resemble proplastids, become highly branched during late log and early stationary phase, reaching maximum development in late stationary and early pre‐cyst stages. Large storage granules of varying density are found within the cell. The PAS‐positive granules have been isolated and shown to contain starch. There is an increase in the amount of this storage material as the cells enter the stationary phase. The remainder of the cytoplasmic matrix is finely granular and contains numerous free ribosomes except in the region of the anterior papilla. Four flagella arise from basal bodies at the anterior end of the cell. The cyst is characterized by a thick multi‐layered cell wall whose electron density obscures the limiting plasma membrane. Large storage granules are located close to and often in contact with the periphery of the cell, suggesting their involvement in the process of cell wall deposition. Altho mitochondria can still be seen in the mature cyst, other cytoplasmic organelles often appear atypical. The mature cyst has an irregular profile possibly due to shrinkage from dehydration.

A Plastid without a Genome: Evidence from the Nonphotosynthetic Green Algal Genus Polytomella

“…cells with the DNA-binding fluorescent dye Synergy Brands Green I, which readily visualizes both plastid and mitochondrial nucleotides in C. reinhardtii (Nishimura et al, 1998). Using this dye, we were able to detect nucleoids in P. parva that seem to be associated with highly reticulated mitochondrial structures, which are known to layer over its plastid (Moore et al, 1970). However, we were unable to eliminate the possibility that some of these signals might be located in the plastid.…”

“…The plastid of Polytomella spp., which derives from a primary endosymbiosis, has been characterized by ultrastructural studies and typically contains copious amounts of starch (Moore et al, 1970;Brown et al, 1976). However, experiments searching for evidence of a plastid gene expression system within it have proven unsuccessful (Nedelcu et al, 1996;Nedelcu, 2001).…”

Enzymes of the Heme Biosynthetic Pathway in the Nonphotosynthetic Alga Polytomella sp

“…The role of the LHC motif in FeCs, found to date only in the forms of FeC expressed in photosynthetic tissues (51), is not understood. The possibility that the LHC motif plays a role in light regulation or targeting to the thylakoid membranes is strengthened by its absence from the FeC of Polytomella sp., which does not contain thylakoid membranes (33). The Cterminal domains of plant and animal FeCs are responsible for dimerization (11).…”

Next‐generation sequencing data suggest that certain nonphotosynthetic green plants have lost their plastid genomes

“…). Although nonphotosynthetic, Polytomella algae have a plastid (Moore et al ., ), but early attempts to identify a gene expression system within it were unsuccessful (Nedelcu et al ., ; Nedelcu, ), even though similar techniques identified one in the plastids of other colorless green algae, such as Prototheca wickerhamii and Polytoma uvella , both of which lost photosynthesis independently of Polytomella (Fig. ).…”