The effects of high hydrostatic pressure on the microtubules of Tetrahymena pyriformis

Explaining bathymetric diversity patterns in marine benthic invertebrates and demersal fishes: physiological contributions to adaptation of life at depth

Bathymetric biodiversity patterns of marine benthic invertebrates and demersal fishes have been identified in the extant fauna of the deep continental margins. Depth zonation is widespread and evident through a transition between shelf and slope fauna from the shelf break to 1000 m, and a transition between slope and abyssal fauna from 2000 to 3000 m; these transitions are characterised by high species turnover. A unimodal pattern of diversity with depth peaks between 1000 and 3000 m, despite the relatively low area represented by these depths. Zonation is thought to result from the colonisation of the deep sea by shallow-water organisms following multiple mass extinction events throughout the Phanerozoic. The effects of low temperature and high pressure act across hierarchical levels of biological organisation and appear sufficient to limit the distributions of such shallow-water species. Hydrostatic pressures of bathyal depths have consistently been identified experimentally as the maximum tolerated by shallow-water and upper bathyal benthic invertebrates at in situ temperatures, and adaptation appears required for passage to deeper water in both benthic invertebrates and demersal fishes. Together, this suggests that a hyperbaric and thermal physiological bottleneck at bathyal depths contributes to bathymetric zonation. The peak of the unimodal diversity-depth pattern typically occurs at these depths even though the area represented by these depths is relatively low. Although it is recognised that, over long evolutionary time scales, shallow-water diversity patterns are driven by speciation, little consideration has been given to the potential implications for species distribution patterns with depth. Molecular and morphological evidence indicates that cool bathyal waters are the primary site of adaptive radiation in the deep sea, and we hypothesise that bathymetric variation in speciation rates could drive the unimodal diversity-depth pattern over time. Thermal effects on metabolic-rate-dependent mutation and on generation times have been proposed to drive differences in speciation rates, which result in modern latitudinal biodiversity patterns over time. Clearly, this thermal mechanism alone cannot explain bathymetric patterns since temperature generally decreases with depth. We hypothesise that demonstrated physiological effects of high hydrostatic pressure and low temperature at bathyal depths, acting on shallow-water taxa invading the deep sea, may invoke a stress-evolution mechanism by increasing mutagenic activity in germ cells, by inactivating canalisation during embryonic or larval development, by releasing hidden variation or mutagenic activity, or by activating or releasing transposable elements in larvae or adults. In this scenario, increased variation at a physiological bottleneck at bathyal depths results in elevated speciation rate. Adaptation that increases tolerance to high hydrostatic pressure and low temperature allows colonisation of abyssal depths and reduces the stress-evolution response, consequently returning speciation of deeper taxa to the background rate. Over time this mechanism could contribute to the unimodal diversity-depth pattern.

Two alpha-tubulin genes of Aspergillus nidulans encode divergent proteins

We have isolated and analyzed the tubA and tubB alpha-tubulin genes of Aspergillus nidulans. The nucleotide sequences of these genes predict polypeptides of 447 amino acids for tubA and 450 for tubB. The predicted amino acids sequences exhibit 28% divergence between the two polypeptides. This is the second known case of such high divergence between alpha-tubulins within the same species. The tubB gene is unique in that it codes for an extra glycine residue between what are usually the second and third amino acids. RNA blot analysis demonstrates that the tubA and tubB transcripts are each 1.8 kb long. The level of tubA transcript remains the same throughout the cell cycle. The level of tubB transcript does not change at any particular stage in the cell cycle but increases continuously during spore germination. The tubA gene was previously mapped to linkage group eight, and we have now mapped the tubB gene to linkage group four. Gene disruption in heterokaryons suggests that the phenotypic consequences of disruption are different for the tubA and tubB genes. Molecular disruption of tubA results in a block in nuclear division whereas in tubB it gives rise to abnormal cell and nuclear morphology.

Effects of hydrostatic pressure in the range 100-300 atmospheres on cell division and protein synthesis in synchronized Tetrahymena pyriformis: a comparison with cycloheximide and emetine

Heat-synchronized Tetrahymena pyriformis have been subjected to 5-, 15- and 30-minute pulses of hydrostatic pressure in the range 100-300 atm, without being simultaneously subjected to significant heats of compression. The pressure-induced division delays depend on (1) the level of pressure used, (2) the length of pressure pulse and (3) the time after the synchronizing treatment at which the pressure is applied. A pressure-dependent inhibition of 3H-leucine incorporation into protein was also measured. Comparison of the effects of pressure with those of pulse treatments of cycloheximide and emetine on cell division and protein synthesis revealed that the physical agent produced characteristically different responses from those of the chemical agents. Of particular interest was the fact that the division delays induced by pressures of 200 atm and above were greater than those observed after treatments with cyclohexmide and emetine which produced comparable levels of protein synthesis inhibition. Pressure also delayed cells if it was applied at a time when addition of chemical inhibitors had little effect on the first synchronous division. The results show that inhibition of protein synthesis by pressure cannot entirely account for pressure-induced effects on cell division. The possibility that pressure may also directly affect other processes, such as the assembly of proteins into structure required for division, is discussed.

Disagreement between calorimetric and van't Hoff enthalpies of assembly of protein supramolecular structures

The effect of temperature on the extent of association of self-assembling protein polymers is expressed mathematically in terms of the van't Hoff enthalpy of polymerization, deltaHV.H. This quantity has been experimentally defined in two ways--from the respective temperature derivatives of the critical polymerization concentration and of the fractional conversion of protein into polymer. These two definitions are shown not to be identical, except in certain limits. In terms of both definitions, it is shown that deltaHV.H. depends not only upon the enthalpy changes but also upon the corresponding equilibrium constants for the various equilibria involved in polymer formation. This has two consequences: (i) large deltaHV.H. values may result from reactions having small calorimetric enthalpy changes; and (ii) deltaHV.H. can depend strongly on temperature. These considerations are applied to two systems for which there exist considerable experimental data--namely, hemoglobin S and tubulin. The large discrepancy between the calorimetric and van't Hoff enthalpies for the polymerization of tubulin is shown to be explicable in terms of these considerations.

Longitudinal microtubules and their functions during asexual reproduction in Paramecium tetraurelia

Longitudinal microtubules appear inParamecium tetraureliawhen cells are ready to divide (ca.interfission age 0·9). They arise in the longitudinal cortical ridges between kineties and form, in each ridge, an incomplete circlet of 15–18 microtubules, disappearing 10–15 min after separation of the fission products (ca.interfission age 0·03–0·04). Vinblastine, colchicine, colcemid, mercaptoethanol, and cold treatments all result in loss of these microtubules, rounding up of the cells, and some suppression of cell division. The tubules are thought to play a role in cellular elongation, morphogenesis, and separation.

Stability of neuronal microtubules to high pressure in vivo and in vitro

Neuronal microtubules in a variety of nerve cell types are unaffected by high hydrostatic pressures over a range of 1400-10,000 pounds/inch(2) and periods of 10-45 min. Similarly, purified tubulin polymerized to form microtubules in vitro were not depolymerized by the same range of pressures. The depolymerization of microtubules in several types of non-neuronal cells, which has been reported, may have been over-generalized with regard to the direct action of pressure on microtubule stability.

Microtubule biogenesis and cell shape in Ochromonas. II. The role of nucleating sites in shape development

The proposal made in the preceding paper that the species-specific shape of Ochromonas is mediated by cytoplasmic microtubules which are related to two nucleating sites has been experimentally verified. Exposure of cells to colchicine or hydrostatic pressure causes microtubule disassembly and a correlative loss of cell shape in a posterior to anterior direction. Upon removal of colchicine or release of pressure, cell shape regenerates and microtubules reappear, first in association with the kineto-beak site concomitant with regeneration of the anterior asymmetry, and later at the rhizoplast site concomitant with formation of the posterior tail. It is concluded that two separate sets of cytoplasmic tubules function in formation and maintenance of specific portions of the total cell shape. On the basis of the following observations, we further suggest that the beak and rhizoplast sites could exert control over the position and timing of the appearance, the orientation, and the pattern of microtubule distribution in Ochromonas. (a) the two sites are accurately positioned in the cell relative to other cell organelles; (b) in regenerating cells microtubules reform first at these sites and appear to elongate to the cell posterior; (c) microtubules initially reappear in the orientation characteristic of the fully differentiated cell; (d) the two sets of tubules are polymerized at different times, in the same sequence, during reassembly or resynthesis of the microtubular system. Experiments using cycloheximide, after a treatment with colchicine, have demonstrated that Ochromonas cannot reassume its normal shape without new protein synthesis. This suggests that microtubule protein once exposed to colchicine cannot be reassembled into microtubules. Pressure-treated cells, on the other hand, reassemble tubules and regenerate the normal shape in the presence or absence of cycloheximide. The use of these two agents in analyzing nucleating site function and the independent processes of synthesis and assembly of microtubules is discussed.