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T Cavalier-Smith

Publications and source records attributed to T Cavalier-Smith.

7 recordsLinked to original sources
PubMed1992

Origins of secondary metabolism.

Secondary metabolites generally benefit their producers as poisons that protect them against competitors, predators or parasites. They are produced from universally present precursors (most often acetyl-CoA, amino acids or shikimate) by specific enzymes that probably arose by the duplication and divergence of genes originally coding for primary metabolism. Most secondary metabolites are restricted to single major taxa on the universal phylogenetic tree and so probably originated only once. But different secondary metabolic pathways have originated from different ancestral enzymes at radically different times in evolution. Secondary metabolites are most abundantly produced by microorganisms in crowded habitats and by plants, fungi and sessile animals like sponges, where chemical defence and attack rather than physical escape or fighting are at a premium. The first secondary metabolites were probably antibiotics produced in microbial mats over 3500 million years ago. These first ecosystems probably consisted entirely of eubacteria: archaebacteria and eukaryotes arose much later. As a phylogenetic context for considering the earliest origins of antibiotics I summarize a cladistic analysis of the explosive eubacterial primary diversification. This suggests that the most primitive surviving cells are the photosynthetic heliobacteria. Study of these and of the nearly as primitive chloroflexibacteria, spirochaetes and deinobacteria may provide the best evidence on the origins of secondary and primary metabolism.

Anti-Bacterial Agents↗
PubMed1992

The number of symbiotic origins of organelles.

Mitochondria and chloroplasts both originated from bacterial endosymbionts. The available evidence strongly supports a single origin for mitochondria and only somewhat less strongly a single, slightly later, origin for chloroplasts. The arguments and evidence that have sometimes been presented in favor of the alternative theories of the multiple or polyphyletic origins of these two organelles are evaluated and the kinds of data that are needed to test more rigorously the monophyletic theory are discussed. Although chloroplasts probably originated only once, eukaryotic algae are polyphyletic because chloroplasts have been secondarily transferred to new lineages by the permanent incorporation of a photosynthetic eukaryotic algal cell into a phagotrophic protozoan host. How often this has happened is much less clear. It is particularly unclear whether or not the chloroplasts of typical dinoflagellates and euglenoids originated in this way from a eukaryotic symbiont: their direct divergence from the ancestral chloroplast cannot be ruled out and indeed has several arguments in its favor. The evidence for and against the view that the chloroplast of the kingdom Chromista was acquired in a single endosymbiotic event is discussed. The possibility that even the chloroplast of Chlorarachnion might have been acquired during the same symbiosis that created the cryptomonad cell, if the symbiont was a primitive alga that had chlorophyll a, b and c as well as phycobilins, is also considered. An alga with such a combination of pigments might have been ancestral to all eukaryote algae.

Biological Evolution↗
PubMed1978

Nuclear volume control by nucleoskeletal DNA, selection for cell volume and cell growth rate, and the solution of the DNA C-value paradox.

The 40,000-fold variation in eukaryote haploid DNA content is unrelated to organismic complexity or to the numbers of protein-coding genes. In eukaryote microorganisms, as well as in animals and plants, DNA content is strongly correlated with cell volume and nuclear volume, and with cell cycle length and minimum generation time. These correlations are simply explained by postulating that DNA has 2 major functions unrelated to its protein-coding capacity: (1) the control of cell volume by the number of replicon origins, and (2) the determination of nuclear volume by the overall bulk of the DNA: cell growth rates are determined by the cell volume and by the area of the nuclear envelope available for nucleocytoplasmic transport of RNA, which in turn depends on the nuclear volume and therefore on the DNA content. During evolution nuclear volume, and therefore DNA content, has to be adjusted to the cell volume to allow reasonable growth rates. The great diversity of cell volumes and growth rates, and therefore of DNA contents, among eukaryotes results from a varying balance in different species between r-selection, which favours small cells and rapid growth rates and therefore low DNA C-values, and K-selection which favours large cells and slow growth rates and therefore high DNA C-values. In multicellular organisms cell size needs to vary in different tissues: size differences between somatic cells result from polyteny, endopolyploidy, or the synthesis of nucleoskeletal RNA. Conflict between the need for large ova and small somatic cells explains why lampbrush chromosomes, nurse cells, chromatin diminution and chromosome elimination evolved. Similar evolutionary considerations clarify the nature of polygenes, the significance of the distribution of haploidy, diploidy and dikaryosis in life cycles and of double fertilization in angiosperms, and of heteroploidy despite DNA constancy in cultured cells, and other puzzles in eukaryote chromosome biology. Eukaryote DNA can be divided into genic DNA (G-DNA), which codes for proteins (or serves as recognition sites for proteins involved in transcription, replication and recombination), and nucleoskeletal DNA (S-DNA) which exists only because of its nucleoskeletal role in determining the nuclear volume (which it shares with G-DNA, and performs not only directly, but also indirectly by coding for nucleoskeletal RNA). Mechanistic and evolutionary implications of this are discussed.

Animals↗
PubMed1977

Mitosis and microtuble assembly.

Microtubules reconstituted in vitro are identical in helical structure with tubules making up the mitotic spindle and both are formed by a helical condensation-polymerization mechanism. The protomer of microtubules is a heterodimer (alphabeta), of mol.wt. 110000 and sedimentation coefficient, SO25,w, 6S. This dimer has one binding site for colchicine or podophyllotoxin, two sites for Vinca alkaloids, two sites for guanine nucleotides and Ca2+-binding sites. There is also a beta-chain phosphoserine. Modulation of these properties is discussed as a possible way of regulating the competence of tubulin to polymerize. Reconstituted microtubules depolymerize to a mixture of 30-36S oligomeric tubulin and 6S dimer molecules. The 30-36S tubulin appears as a ring or disc when made visible in the electron microscope by negative staining. Three pathways of microtubule assembly have been proposed involving this ring as an intermediate: (1) the uncoiling of these rings into protofilaments; (2) the stacking of rings into macrotubules; (3) the rings as a scaffold for the assembly of a short segment of microtubule helix. Finally, the regulation of mitosis is discussed in terms of recent studies of tubulin and its polymerization, and studies in vitro and in vivo of the process of mitotic-spindle formation and disassembly.

Alkaloids↗