Growth parameters and photosynthetic pigments in leaf segments of Zea mays exposed to cadmium, as related to protection mechanisms

The influence of cadmium on growth parameters and photosynthetic pigment content was studied in maize leaf segments differing in tissue maturity. Experiments were carried out with maize seedlings Zea mays L. cv. Hidosil treated with 25, 50, 100, 150 and 200 microM Cd for 14 days under low light conditions. Tissue age-dependent decrease of fresh mass, dry mass and area of the leaf segments was correlated with Cd concentration in the growth medium. Cd induced changes in chlorophyll and carotenoid contents, and the specific areas and densities of the leaf segments were dependent on age and metal concentration. Results are discussed with respect to the protection mechanisms in the leaf segments.     

Perspective: Reproductive isolation caused by natural selection against immigrants from divergent habitats

The classification of reproductive isolating barriers laid out by Dobzhansky and Mayr has motivated and structured decades of research on speciation. We argue, however, that this classification is incomplete and that the unique contributions of a major source of reproductive isolation have often been overlooked. Here, we describe reproductive barriers that derive from the reduced survival of immigrants upon reaching foreign habitats that are ecologically divergent from their native habitat. This selection against immigrants reduces encounters and thus mating opportunities between individuals from divergently adapted populations. It also reduces the likelihood that successfully mated immigrant females will survive long enough to produce their hybrid offspring. Thus, natural selection against immigrants results in distinctive elements of premating and postmating reproductive isolation that we hereby dub "immigrant inviability." We quantify the contributions of immigrant inviability to total reproductive isolation by examining study systems where multiple components of reproductive isolation have been measured and demonstrate that these contributions are frequently greater than those of traditionally recognized reproductive barriers. The relevance of immigrant inviability is further illustrated by a consideration of population-genetic theory, a review of selection against immigrant alleles in hybrid zone studies, and an examination of its participation in feedback loops that influence the evolution of additional reproductive barriers. Because some degree of immigrant inviability will commonly exist between populations that exhibit adaptive ecological divergence, we emphasize that these barriers play critical roles in ecological modes of speciation. We hope that the formal recognition of immigrant inviability and our demonstration of its evolutionary importance will stimulate more explicit empirical studies of its contributions to speciation.     

Molecular mechanisms of severe acute respiratory syndrome (SARS)

Background

The mechanisms during the initial phase of oxygen toxicity leading to pulmonary tissue damage are incompletely known. Increase of tumour necrosis factor alpha (TNFalpha) represents one of the first pulmonary responses to hyperoxia. We hypothesised that, in the initial phase of hyperoxia, TNFalpha activates the caspase cascade in type II pneumocytes (TIIcells).

Methods

Lung sections or freshly isolated TIIcells of control and hyperoxic treated rats (48 hrs) were used for the determination of TNFalpha (ELISA), TNF-receptor 1 (Western blot) and activity of caspases 8, 3, and 9 (colorimetrically). NF-kappaB activation was determined by EMSA, by increase of the p65 subunit in the nuclear fraction, and by immunocytochemistry using a monoclonal anti-NF-kappaB-antibody which selectively stained the activated, nuclear form of NF-kappa B. Apoptotic markers in lung tissue sections (TUNEL) and in TIIcells (cell death detection ELISA, Bax, Bcl-2, mitochondrial membrane potential, and late and early apoptotic cells) were measured using commercially available kits.

Results

In vivo, hyperoxia activated NF-kappaB and increased the expression of TNFalpha, TNF-receptor 1 and the activity of caspase 8 and 3 in freshly isolated TIIcells. Intratracheal application of anti-TNFalpha antibodies prevented the increase of TNFRI and of caspase 3 activity. Under hyperoxia, there was neither a significant change of cytosolic cytochrome C or of caspase 9 activity, nor an increase in apoptosis of TIIcells. Hyperoxia-induced activation of caspase 3 gradually decreased over two days of normoxia without increasing apoptosis. Therefore, activation of caspase 3 is a temporary effect in sublethal hyperoxia and did not mark the "point of no return" in TIIcells.

Conclusion

In the initiation phase of pulmonary oxygen toxicity, an increase of TNFalpha and its receptor TNFR1 leads to the activation of caspase 8 and 3 in TIIcells. Together with the hyperoxic induced increase of Bax and the decrease of the mitochondrial membrane potential, activation of caspase 3 can be seen as sensitisation for apoptosis. Eliminating the TNFalpha effect in vivo by anti-TNFalpha antibodies prevents the pro-apoptotic sensitisation of TIIcells.     

Analysis of ordered and disordered protein complexes reveals structural features discriminating between stable and unstable monomers

Most proteins exist in the cell as multi-component assemblies. However, which proteins need to be present simultaneously in order to perform a given function is frequently unknown. The first step toward this goal would be to predict proteins that can function only when in a complexed form. Here, we propose a scheme to distinguish whether the protein components are ordered (stable) or disordered when separated from their complexed partners. We analyze structural characteristics of several types of complexes, such as natively unstructured proteins, ribosomal proteins, two-state and three-state complexes, and crystal-packing dimers. Our analysis makes use of the fact that natively unstructured proteins, which undergo a disorder-to-order transition upon binding their partner, and stable monomeric proteins, which exist as dimers only in their crystal form, provide examples of two vastly different scenarios. We find that ordered monomers can be distinguished from disordered monomers on the basis of the per-residue surface and interface areas, which are significantly smaller for ordered proteins. With this scale, two-state dimers (where the monomers unfold upon dimer separation) and ribosomal proteins are shown to resemble disordered proteins. On the other hand, crystal-packing dimers, whose monomers are stable in solution, fall into the ordered protein category. While there should be a continuum in the distributions, nevertheless, the per-residue scale measures the confidence in the determination of whether a protein can exist as a stable monomer. Further analysis, focusing on the chemical and contact preferences at the interface, interior and exposed surface areas, reveals that disordered proteins lack a strong hydrophobic core and are composed of highly polar surface area. We discuss the implication of our results for de novo design of stable monomeric proteins and peptides.     

On the mechanisms of the reaction of dodecatungstophosphate with alkyl radicals in aqueous solutions

The reduced lacunary polyoxotungstate, [PW₁₁O₃₉]⁸⁻, reacts with the ^.CH₂CH(OH)CH₃ and ^.CH₂C(CH₃)₂OH radicals via a mechanism involving β-hydroxide elimination to yield propene and 2-methyl propene respectively, and [PW₁₁O₃₉]⁷⁻. [PW₁₁O₃₉]⁸⁻ is also oxidized by methyl radicals in a reaction which yields methane as the major product. It is proposed that the reactions proceed via the formation of short lived transients with W-C σ bonds.     

THE BIOLOGY OF SPECIATION

Since Darwin published the “Origin,” great progress has been made in our understanding of speciation mechanisms. The early investigations by Mayr and Dobzhansky linked Darwin's view of speciation by adaptive divergence to the evolution of reproductive isolation, and thus provided a framework for studying the origin of species. However, major controversies and questions remain, including: When is speciation nonecological? Under what conditions does geographic isolation constitute a reproductive isolating barrier? and How do we estimate the “importance” of different isolating barriers? Here, we address these questions, providing historical background and offering some new perspectives. A topic of great recent interest is the role of ecology in speciation. “Ecological speciation” is defined as the case in which divergent selection leads to reproductive isolation, with speciation under uniform selection, polyploid speciation, and speciation by genetic drift defined as “nonecological.” We review these proposed cases of nonecological speciation and conclude that speciation by uniform selection and polyploidy normally involve ecological processes. Furthermore, because selection can impart reproductive isolation both directly through traits under selection and indirectly through pleiotropy and linkage, it is much more effective in producing isolation than genetic drift. We thus argue that natural selection is a ubiquitous part of speciation, and given the many ways in which stochastic and deterministic factors may interact during divergence, we question whether the ecological speciation concept is useful. We also suggest that geographic isolation caused by adaptation to different habitats plays a major, and largely neglected, role in speciation. We thus provide a framework for incorporating geographic isolation into the biological species concept (BSC) by separating ecological from historical processes that govern species distributions, allowing for an estimate of geographic isolation based upon genetic differences between taxa. Finally, we suggest that the individual and relative contributions of all potential barriers be estimated for species pairs that have recently achieved species status under the criteria of the BSC. Only in this way will it be possible to distinguish those barriers that have actually contributed to speciation from those that have accumulated after speciation is complete. We conclude that ecological adaptation is the major driver of reproductive isolation, and that the term “biology of speciation,” as proposed by Mayr, remains an accurate and useful characterization of the diversity of speciation mechanisms.     

Acanthamoeba castellanii: Morphological analysis of the interaction with human cornea

The present study demonstrates that when Acanthamoeba castellanii trophozoites are co-cultivated with isolated human corneas, the amoeba can be invasive and cause damage to the intact corneal epithelium without the requirement of previous corneal abrasion. After adhesion, A. castellanii trophozoites migrate between cells forming bumps on the corneal cell layers and reaching Bowman´s membrane in 3 h, although no evidence of cell damage was observed until the phagocytic process was detected. Likewise, conditioned medium produced damage to the corneal cells that was proportional to the time of incubation, but this cytophatic effect involved only the most superficial layer of the human cornea and was not enough to explain amoebic invasion of Bowman´s membrane. As a result of our observations, we suggest that the mechanical action of the trophozoites and phagocytosis of corneal cells during the process of corneal invasion are more important than previously suggested.     

Effects of an invasive anuran [the cane toad (Bufo marinus)] on the invertebrate fauna of a tropical Australian floodplain

The ways in which invasive organisms influence native ecosystems remain poorly understood. For example, feral cane toads Bufo marinus have spread extensively through tropical Australia over the last 70 years, but assessments of their ecological impact remain largely anecdotal. We conducted experimental trials to examine the effect of cane toad presence on invertebrate fauna in relatively small (2.4 × 1.2 m) outdoor enclosures on a floodplain near Darwin in the wet–dry tropics. Toads significantly reduced invertebrate abundance and species richness, but only to about the same degree as did an equivalent biomass of native anurans. Thus, if toads simply replaced native anurans, the offtake of invertebrates might not be substantially different from that due to native anurans before toad invasion. However, our field surveys suggest that toads cause a massive (fourfold) increase in total amphibian biomass. The end result is that cane toads act as a massive nutrient sink in the floodplain ecosystem because they consume vast numbers of invertebrates but (unlike native frogs) are largely invulnerable to predation by frog-eating predators.