Does functional redundancy exist?

Functional redundancy has often been assumed as an intuitive null hypothesis in biodiversity experiments, but theory based on the classical Lotka-Volterra competition model shows that functional redundancy sensu stricto is incompatible with stable coexistence. Stable coexistence requires differences between species which lead to functional complementarity and differences between the yields of mixtures and monocultures. Only a weaker version of functional redundancy, i.e. that mixture yields lie within the range of variation of monoculture yields, is compatible with stable coexistence in Lotka-Volterra systems. Spatial and temporal environmental variability may provide room for functional redundancy at small spatial and temporal scales, but is not expected to do so at the larger scales at which environmental variations help maintain coexistence. Neutral coexistence of equivalent competitors, non-linear per capita growth rates, and lack of correlation between functional impact and biomass may provide the basis for the existence of functional redundancy in natural ecosystems. Overall, there is a striking parallel between the conditions that allow stable coexistence and those that allow overyielding.     

Does active transport exist?

Conclusion Since its publication, the two-membrane theory has been generally accepted as a prototype of a transporting epithelium. It should be remembered, however, that whereas it gives a quantitative measure of the active sodium transport, the coupling ratio between Na transport and K recycling has to be estimated by other methods (see Skou, 1957). We are reminded of our initial conclusion: that active transport can only be observed with certainty in systems where net transport of the species in question is going on.Also, the procedures described in the foregoing are powerful only for tight epithelia where paracellular shunt paths can be neglected.The finding (Ussing, 1978; Sten-Knudsen & Ussing, 1981) that the flux ratio is time independent, i.e., constant from the first appearance of the tracers on the receiving side, makes it possible, at least theoretically, to analyze multipathway systems like leaky epithelia (see, for example, Ussing & Eskesen, 1989 and Ussing & Nedergaard, 1993).     

Does a proton-pumping ATPase exist in the tonoplast?

In order to account for the accumulation of metabolites in plant vacuoles, the existence of a proton-pumping ATPase has been widely suggested in the literature. The demonstration of such a tonoplast-bound ATPase was merely based on the characterization of a nitrate-sensitive microsomal fraction. In some examples, this ATPase activity has been evidenced on vacuole preparations obtained under conditions which were criticized by Boller. The application of the reverse phase high-performance liquid chromatography method (RP-HPLC) to the simultaneous separation of adenine nucleotides, in the presence of tonoplast vesicles isolated from Catharanthus roseus, showed results not necessarily correlated with the ATPase hypothesis. Moreover, in light of the H+-quenching of quinacrine fluorescence observed during ATP hydrolysis by vacuoles or tonoplast vesicles, the existence of a proton-pumping ATPase may be questioned.     

The skeletal muscle satellite cell: stem cell or son of stem cell?

The concept of the adult tissue stem cell is fundamental to models of persistent renewal in functionally post-mitotic tissues. Although relatively ignored by stem cell biology, skeletal muscle is a prime example of an adult tissue that can generate terminally differentiated cells uniquely specialized to carry out tissue-specific functions. This capacity is attributed to satellite cells, a population of undifferentiated, quiescent precursors that become activated to divide and differentiate in response to the demands of growth or damage. The aim of this review is to discuss the role of the satellite cell as an adult tissue-specific stem cell. We examine evidence for the presence of behaviourally and phenotypically distinct subpopulations of precursor within the satellite cell pool. Further, we speculate on the possible identity, origins and relevance of multipotent muscle stem cells, a population with both myogenic and hematopoietic potentials that has been isolated from whole muscle. Taken together, current evidence suggests the possibility that the regenerative compartment of adult skeletal muscle may conform to an archetypal stem cell-based hierarchy, maintained within a stem cell niche. It therefore remains to be seen whether all satellite cells are skeletal muscle-specific stem cells, or whether some or all are the progeny of an as yet unidentified muscle stem cell.     

Does the natriuretic peptide system exist throughout the animal and plant kingdom?

Natriuretic peptides (NPs) and their receptors have been identified in vertebrate species ranging from elasmobranchs to mammals. Atrial, brain and ventricular NP (ANP, BNP and VNP) are endocrine hormones secreted from the heart, while C-type NP (CNP) is principally a paracrine factor in the brain and periphery. In elasmobranchs, only CNP is present in the heart and brain and it functions as a circulating hormone as well as a paracrine factor. Four types of NP receptors are cloned in vertebrates. NPR-A and NPR-B are guanylyl cyclase-coupled receptors, whereas NPR-C and NPR-D have only a short cytoplasmic domain. NPs are hormones important for volume regulation in mammals, while they act more specifically for Na(+) regulation in fishes. The presence of NP and its receptor has also been suggested in the most primitive vertebrate group, cyclostomes, and its molecular identification is in progress. The presence of ANP or its mRNA has been reported in the hearts and ganglia of various invertebrate species such as mollusks and arthropods using either antisera raised against mammalian ANP or rat ANP cDNA as probes. Immunoreactive ANP has also been detected in the unicellular Paramecium and in various species of plants including Metasequoia. Furthermore, the N-terminal prosegments of ANP, whose sequences are scarcely conserved even in vertebrates, have also been detected by the radioimmunoassay for human ANP prosegments in all invertebrate and plant species examined including Paramecium. Although these data are highly attractive, the current evidence is too circumstantial to be convincing that the immunoreactivity truly originates from ANP and its prosegments in such diverse organisms. The caution that has to be exercised in identification of vertebrate hormones from phylogenetically distant organisms is discussed.     

Does a tree-like phylogeny only exist at the tips in the prokaryotes?

The extent to which prokaryotic evolution has been influenced by horizontal gene transfer (HGT) and therefore might be more of a network than a tree is unclear. Here we use supertree methods to ask whether a definitive prokaryotic phylogenetic tree exists and whether it can be confidently inferred using orthologous genes. We analysed an 11-taxon dataset spanning the deepest divisions of prokaryotic relationships, a 10-taxon dataset spanning the relatively recent gamma-proteobacteria and a 61-taxon dataset spanning both, using species for which complete genomes are available. Congruence among gene trees spanning deep relationships is not better than random. By contrast, a strong, almost perfect phylogenetic signal exists in gamma-proteobacterial genes. Deep-level prokaryotic relationships are difficult to infer because of signal erosion, systematic bias, hidden paralogy and/or HGT. Our results do not preclude levels of HGT that would be inconsistent with the notion of a prokaryotic phylogeny. This approach will help decide the extent to which we can say that there is a prokaryotic phylogeny and where in the phylogeny a cohesive genomic signal exists.     

Neurogenesis: is the adult stem cell young or old?

Stem cell biology is one of the most exciting, controversial, and debated fields in science today. It has been suggested that neuronal replacement therapy using stem cell transplants may be one possible answer to a host of neuropathological disorders including spinal cord injury, stroke, and neurodegenerative diseases. Important sources for stem cells include the developing embryo and adult central nervous system, but will these populations of cells exhibit similar behavior and responses to stimuli? This review will discuss some important similarities and differences between the embryonic and adult stem cell, as well as the basis for developing therapeutic approaches for stem cell replacement.     

Does adipocyte hypercellularity in obesity exist?

Adipose tissue samples were biopsied from three subcutaneous sites in 80 obese and 27 non-obese patients. Additional samples were taken from intra-abdominal sites in 44 of the patients. There was a small increase in the calculated number of fat cells in the more obese patients, but there was no relation between fat-cell number and obesity of childhood onset. Omental fat cells were one-third the size of subcutaneous cells. Thus the calculated number of fat cells, usually based solely on subcutaneous samples, is an underestimate of the true number, and most obese patients can accommodate their fat without needing to recruit new cells. The diagnosis of "hyperplastic" obesity--that is, an excess number of fat cells--is unreliable and its relation to infantile obesity doubtful.     

Does recycling in germinal centres exist?

A general criterion is formulated to decide if the recycling of B cells exists in germinal centre (GC) reactions. The criterion is independent of the selection and affinity maturation process and based solely on total centroblast population arguments. An experimental test is proposed to verify whether the criterion is fulfilled.     

Can science resolve the ethical impasse in stem cell research?

Advances in our understanding of the control of early human embryonic development could offer solutions to the moral dilemmas associated with human embryonic stem cell research.