Cells of Chlorella fusca var. vacuolata (Cambridge 211/8p) resisted efforts aimed at producing naked protoplasts by enzymatic degradation of the cell wall, and a study of the development and composition of the wall was therefore undertaken.

1.

After cytokinesis has produced naked autospores within the mother cell wall, cell wall formation commences outside the autospore plasma membrane with the appearance of small trilaminar plaques. These enlarge while inter-autospore granular material diminishes in quantity, and they eventually fuse to produce a complete trilaminar sheath around each autospore.




2.

A microfibrillar, cellulase digestible, layer is deposited between the trilaminar component and the plasma membrane. Meanwhile the corresponding microfibrillar component of the mother cell wall is digested leaving only its resistant trilaminar component.




3.

The trilaminar component includes a substance considered to be the polymerized carotenoid, sporopollenin, on the basis of its resistance to extreme extraction procedures including acetolysis, and its infra red absorption spectrum.




4.

Two phases of sporopollenin biosynthesis were detected by means of pulse and pulse-chase treatments with 14C-acetate at intervals during the cell cycle in synchronous cultures. One coincides with the formation of the sporopollenin-containing trilaminar wall component, and the other is 6–8 hours earlier while the cells are in karyokinesis. The former yields labelled sporopollenin directly and the latter probably represents formation of a precursor.




5.

Of five other strains of Chlorella tested, only one possesses sporopollenin, and so does one Scenedesmus and two out of three strains of Prototheca.




6.

Examination of the wall structure of the above algae suggest a relationship between the presence of sporopollenin and the development of an outer, trilaminar wall component.




7.

A survey of the literature gives support to this hypothesis and further suggests that the ability to synthesise sporopollenin is related to the ability to produce secondary carotenoids.




8.

The significance of the findings is discussed.

Sporopollenin in the cell wall of Chlorella and other algae: Ultrastructure, chemistry, and incorporation of 14 C-acetate, studied in synchronous cultures

We report an extensive scanning electron microscope (SEM) study of cleavage planes, cell shape changes, and cell lineages during cleavage of the asexual embryo of Volvox carteri f. nagariensis. Although our data generally confirm the basic description of cleavage developed by others using light microscopy, there is one important exception. We observed that the fourth cleavage plane is much more oblique than had previously been recognized. We show that, as a result, the four tiers of cells in the 16-cell embryo overlap extensively, and the new generation of asexual reproductive cells, or gonidia, are derived from three of these tiers (rather than two, as previously believed). Our study focused on the development of the highly organized system of cytoplasmic bridges that appears during cleavage. Hundreds of cytoplasmic bridges are formed in each division cycle as a result of incomplete cytokinesis. Existing bridges are conserved and divided between daughter cells while new bridges are formed at each division. Hence, the number of bridges per embryo increases regularly even though the number per cell declines from the fourth cleavage on. The bridges are organized into bands that girdle the cells at a predictable level and exhibit a regular 500-nm interbridge spacing; bridge bands of adjacent cells are in register and form a structural continuum throughout the embryo which we term "The cytoplasmic bridge system." The only place where bridges are not present is along a pair of intersecting slits, called the phialopore. We describe in detail the development of this bridge-free region.

https://rupress.org/jcb/article-pdf/91/3/743/1075632/743.pdf

Cleavage patterns, cell lineages, and development of a cytoplasmic bridge system in Volvox embryos.

Hypotheses for the functions of intercellular bridges in male germ cell development and its cellular mechanisms

Publisher Summary This chapter discusses various genetic, biochemical, and molecular approaches for the development and evolution of Volvox . All members of the genus Volvox possess two, and only two, distinctive cell types—mortal somatic cells and immortal germ cells. But it is in Volvox carteri that this distinction is most clearly established in very early development, and hence the challenge of elucidating the mechanisms for establishing such a dichotomy is fundamentally presented in the chapter. The existence of two entirely different and interdependent cell types—somatic cells specialized for motility and germ cells specialized for reproduction—neither of which could succeed by itself in perpetuating the germ plasm in nature, raises Volvox to the level of a multicellular organism with complete division of labor. The chapter discusses the qualities of V. carteri f . Nagariensis, which was recommended as a model for exploring the molecular-genetic control of development. Two asexual spheroids (individuals) of V. carferi strain HK 10 are illustrated in the chapter. The chapter also describes the visible events of embryonic development and reviews what has been learned from experimental and genetic analysis concerning the control of various important component steps. Six kinds of probes that hybridize with distinct families of middle repetitive DNA sequences have been used to probe Southern blots of genomic DNA in a search for restriction fragment-length polymorphisms (RFLPs) that might be useful for the sort of genetic analysis described in the chapter.

Genetic, Biochemical, and Molecular Approaches to Volvox Development and Evolution

Volvox: A simple algal model for embryogenesis, morphogenesis and cellular differentiation.

The development of sperm packets of the M5 strain of Volvox aureus.

1.

The synthesis of both DNA and RNA are shown to follow the same general pattern during the vegetative life cycle of Volvox aureus M5. Nucleic acid synthesis is minimal during the first 12 h of development and maximal at the 36–48 h phase.




2.

Mitomycin C at 45 μg/ml and actinomycin D at 4 μg/ml are shown to selectively inhibit DNA and RNA synthesis respectively.




3.

The continued synthesis of DNA and RNA throughout the period of cell division is required for normal completion of development. Gonidial enlargement is not inhibited by either antibiotic.

Nucleic acid synthesis during the vegetative life cycle of Volvox aureus M5.

For some time biologists have been aware that “genes” control the characteristics of living organisms. Only in recent years, however, through the tremendous explosion of knowledge derived from biochemical genetics and molecular biology, have the more exact relationships of genes to characteristics been elucidated. These relationships are now so well established that they have become, in the words of James Bonner,l almost “neo-classical biology.” With the clarification of these relationships, one of the most important problems confronting biologists is that of the differential control of the gene activity that is responsible for cellular differentiation. A variety of methods and experimental organisms are used by the numerous investigators involved with this problem. These investigators may be roughly divided into two broad categories: (1) those working with higher organisms, both plant and animal, and ( 2 ) those working with microorganisms, such as bacteria and fungi. In work with higher organisms one major problem is the complexity of the organism itself. In such organisms the multiplicity of cell types and the complex interactions that occur make a precise analysis of the system difficult. Although extremely valuable information has been obtained, the problem of complexity cannot be minimized. This is reemphasized by the fact that most of the basic principles of biochemical genetics and molecular biology have come not from work on higher organisms, but rather from bacteria and bacterial viruses. The colonial flagellate Volvox is exceptionally well suited for the study of the basic principles of differentiation and development. Volvox shows true cellular differentiation and division of labor, but contains a maximum of four basic cell types: asexual reproductive cells (gonidia), male initials, eggs, and sterile vegetative cells. The cell diversity is, therefore, among the least complex of all multicellular organisms. In addition, chemical induction systems for males, females, and parenthenospores are known for several species. The developmental patterns can therefore be specifically and predictably altered at will.

William H. Darden

Papers

The development of sperm packets of the M5 strain of Volvox aureus.

Nucleic acid synthesis during the vegetative life cycle of Volvox aureus M5.

Hormonal control of sexuality in the the genus volvox

A new system of male induction in Volvox aureus M5.