Cell differentiation and the cytoskeleton in Acetabularia

Abstract

summary In multicellular organisms, differentiation of individual cells is typically linked to the development of the whole organism. As cells acquire tissue‐specific morphologies and become functionally specialized they lose in turn a number of other functions. A free living, single celled organism, however, maintains all such functions. Compartmentalization and intracellular communication are two basic principles by which expression of specialized features is achieved within a unicell. Both in turn depend on the structure and dynamics of the cytoskeleton. Giant algal unicells lend themselves as experimental models for the study of the cytoskeleton, because the cytoskeletal arrays inside these cells become equally enormous in size. Some of these organisms are large enough to be mistaken for multicellular plants, equipped with holdfast, stem and assimilatory organ. The marine green alga Acetabularia is one of these giant cells, which has already been well known to phycologists and cell biologists for several decades. The current review discusses recent progress in the study of the cytoskeleton in Acetabularia and examines classic concepts of cell morphogenesis from the perspective of cytoskeletal function.

Differential Messenger RNA Gradients in the Unicellular AlgaAcetabularia acetabulum. Role of the Cytoskeleton

“…ADP-Glc-P mRNA specifies ADPGlc pyrophosphorylase, an enzyme involved in starch synthesis that is at least partly located in the plastids. A similar localization is expected for the actin isoform-1 mRNA, because actin microfilaments run through the entire cell (see Menzel, 1994). The distribution of tubulin mRNAs deserves some special consideration.…”

“…Sawitzky (1997) established that tubulin mRNAs are present in the A. acetabulum cell at all developmental ages, and in this study, we found these mRNAs to be nearly ubiquitous in all parts of the cell. However, neither microtubules nor unassembled tubulin proteins were detected during vegetative development until after meiosis (Menzel, 1994;Sawitzky, 1997). These findings suggest that tubulin mRNAs are synthesized and stored in the A. acetabulum cytoplasm long before they are actually used for synthesis of the corresponding tubulin proteins.…”

“…the spatial redistribution of poly(A) RNA appears to be dependent only on the presence of microfilaments (Bouget et al, 1995). In A. acetabulum, microtubules or tubulin proteins are undetectable during the vegetative growth, but microfilaments and intermediate filaments form a welldeveloped network (Menzel, 1994;Berger et al, 1998). We found that, without exception, cytochalasin D completely prevented gradient formation of all the mRNAs exhibiting apical/basal distribution ( Fig.…”

Phylogeny and Molecular Evolution of the Green Algae

“…Many Dasycladales, however, remain uninucleate throughout much of their life cycle with a giant diploid nucleus that only divides at the onset of reproduction (Berger & Kaever, 1992). In contrast to the siphonocladous type, the cytoplasm of siphonous algae exhibits vigorous streaming, enabling transportation of transcripts across the plant (Menzel, 1987;Menzel, 1994;Mine et al, 2001). Although some siphonous algae are tiny microscopic siphons, many form large and complex seaweeds that exhibit morphological differentiation into structures that resemble the roots, stems, and leaves of land plants and even have similar functions (Chisholm et al, 1996).…”

“…Transplant experiments with the giant-celled Acetabularia, conducted by Joachim Hämmerling, demonstrated that the nucleus of a cell contains the genetic information that directs cellular development, and postulated the existence of messenger RNA before its structure was determined (Hämmerling, 1953). Acetabularia, along with other giant-celled green algae (Valonia, Chara and Nitella), has also served as an experimental organism for electro-physiological research and studies of cell morphogenesis (Menzel, 1994;Mandoli, 1998;Shepherd et al, 2004;Bisson et al, 2006;Mine et al, 2008). The charophyte Mougeotia played a key role in outlining the role of phytochrome in plant development (Winands & Wagner, 1996).…”