Inevitability or contingency: how many chromosomes do we really need?

<正>In an essay written by the evolutionary biologist Theodosius Dobzhansky in 1973, he pointed out that "Nothing in Biology Makes Sense Except in the Light of Evolution." An interesting phenomenon in biology is the presence of variable numbers of chromosomes in different organisms. Besides several species, which possess multiple circular chromosomes or simply linear chromosomes (Baril et al.1989; Suwanto and Kaplan, 1989; Jumas-Bilak et al., 1998),most prokaryotes only possess one circular chromosome. In contrast, the genomes of eukaryotic species are usually packaged into linear chromosomes with numbers varying     

SNARE complexes and neuroexocytosis: how many, how close?

Regulated secretion is an essential process in all eukaryotic cells. The release of molecules contained inside exocytic granules and synaptic vesicles is mediated by the assembly of a SNARE complex formed by the coil-coiling of three proteins: SNAP-25, syntaxin and VAMP/synaptobrevin. It seems that SNARE complexes assemble together in rosette-shaped super-complexes but there is controversy on the actual number (N) of copies of SNARE complexes that are necessary to mediate exocytosis. We discuss attempts to determine the value of N and suggest that N varies with the type of exocytic vesicles. In addition, we propose that the N value in neuroexocytosis can be estimated by the comparative use of different types of botulinum neurotoxins.     

Recombination-suppression: how many mechanisms for chromosomal speciation?

Over the past decade several theoretical and empirical studies have revived interest in the role of chromosomes in speciation. The resulting models do not suffer from the problems experienced by previously proposed mechanisms of chromosomal speciation, because they invoke suppression of recombination rather than a reduction in the fitness of heterokaryotypes as their core process. However, they are not free from difficulties. The evidence for recombination-suppression models is discussed here. The general conclusion is that a consensus opinion on which models best describe the real-world situation is currently unlikely because of an inability of the available empirical evidence to fully distinguish between them, which may be due in part to a lack of exclusivity. I argue that future work should take this lack of exclusivity into account. Resolving the biogeography of speciation is also suggested in order to tell the various models apart. Further study is needed which focuses on confirming the operation of individual elements of the various models, rather than attempting to validate any single mechanism as a whole.     

Canine babesiosis in Europe: how many diseases?

Babesiosis, recognized since ancient times as an important disease of livestock and more recently as an emerging disease in dogs worldwide, is caused by intraerythrocytic protozoa of the genus Babesia and is transmitted by ticks. The pathophysiology of canine babesiosis has been extensively studied but many questions remain unanswered, especially regarding the diversity of disease manifestations in different European countries. Continued investigation of the similarities and differences in host-parasite interplay in canine babesiosis in different European countries should lead to a better understanding of the disease process, potentially leading to better prediction of disease outcome and the development of new treatment modalities. From the European point of view it is important to conduct these studies on Babesia canis.     

Retrotransposition of Alu elements: how many sources?

It is generally thought that only a few Alu elements are capable of retrotransposition and that these 'master' sources produce inactive copies. Here, we use a network phylogenetic approach to demonstrate that recently integrated human-specific Alu subfamilies typically contain 10-20% of secondary source elements that contributed 20-40% of all subfamily members. This multiplicity of source elements provides new insight into the remarkably successful amplification strategy of the Alu family.     

Evolving endosomes: how many varieties and why?

The cell biologist's insight into endosomal diversity, in terms of both form and function, has increased dramatically in the past few years. This understanding has been promoted by the availability of powerful new techniques that allow imaging of both cargo and machinery in the endocytic process in real time, and by our ability to inhibit components of this machinery by RNA interference. The emerging picture from these studies is of a highly complex, dynamic and adaptable endosomal system that interacts at various points with the secretory system of the cell.     

How many, how old, how soon?

This article highlights the population projections for the UK to 2066 and their health implications. The changes in the demographic age structure of human populations have dual implications. Although they represent accomplishments in social and health development considering the fact that more individuals are able to live until old age, these changes also bring new demands that affect all aspects of society, including employment, taxation, pension, education, and health. Age specific population projections presented were obtained from the Office for National Statistics. It is projected that the total number of people aged 60 years and older will rise from 12 million in 2001 to 18.6 million in 2031 and the number of individuals suffering with chronic diseases and disabilities will see a three-fold increase. Although these projections rely on assumptions and are subject to some changes, it is made clear that the proportion of the population aged 65 and above will continue to increase substantially over the next decades. These population trends will have a large impact on the health and health care needs of the population. Thus, there is a need to develop and plan institutions and policies that address the demands of the aging population. Understanding and identification of the causes and prevention of conditions that could result to serious disabilities must be a high priority.     

Nuclear targeting of proteins: how many different signals?

The nuclear import of proteins into the cell nucleus involves the recognition of a nuclear localization signal sequence, borne by the protein to be transported, by complex molecules called importins, that will subsequently mediate the crossing over of the nuclear envelope. The most frequently encountered signal sequence is made up of short stretches of basic amino acid residues and is recognized by importins alpha and/or beta. Other signal sequences have been described, and some have been shown to mediate the association with importins other than importin alpha or beta. Recently, approaches have been developed that allow the cloning, on a functional basis, of sequences able to specify the nuclear localization of proteins. A variety of peptidic motifs of limited size which do not contain previously described signal sequences were isolated in such assays. It reveals that the spectrum of sequences that are able to target a protein to the cell nucleus may be wider than currently expected. It will probably also lead to the identification of novel target sequences for importins and will demonstrate the implication of additional members of this family of proteins in nuclear transport.     

Numerosity perception: how many speckles on the hen?

How do we tell how many objects there are in a visual scene? A recent study has shown that the numerousness of objects is a 'primary visual property' of the scene, just like the objects' colour, shape or location.     

Mitochondrial permeability transitions: how many doors to the house?

The inner mitochondrial membrane is famously impermeable to solutes not provided with a specific carrier. When this impermeability is lost, either in a developmental context or under stress, the consequences for the cell can be far-reaching. Permeabilization of isolated mitochondria, studied since the early days of the field, is often discussed as if it were a biochemically well-defined phenomenon, occurring by a unique mechanism. On the contrary, evidence has been accumulating that it may be the common outcome of several distinct processes, involving different proteins or protein complexes, depending on circumstances. A clear definition of this putative variety is a prerequisite for an understanding of mitochondrial permeabilization within cells, of its roles in the life of organisms, and of the possibilities for pharmacological intervention.     

Cellulose: how many cellulose synthases to make a plant?

Many questions remain about the biosynthesis of cellulose, the major plant cell wall component, not least of which is why plants have so many genes for the cellulose synthase catalytic subunit. Perhaps multiple isoforms of cellulose synthase are needed in the same cell for the formation of functional dimeric complexes.     

Sorting of proteins to storage vacuoles: how many mechanisms?

Vacuoles receive their proteins through the secretory pathway, this requires protein sorting signals and molecular machineries that, until recently, have been believed to be markedly distinct for lytic and storage vacuoles. However, new biochemical, morphological and genetic data indicate that the only known class of vacuolar sorting receptors, believed to be specific for lytic vacuoles, might also be involved in the sorting of certain storage proteins. Furthermore, storage vacuoles can have a complex multimembrane structure that is difficult to explain based on a single trafficking mechanism. A new array of possible molecular interactions is thus emerging that no longer supports a clear-cut distinction between the two types of vacuoles based on sorting signals and putative receptors.     

Evidence for Fisher's dominance theory: how many 'special cases'?

Dominance, its genetic basis and evolution has been at the heart of one of the most intense controversies in the history of genetics. For more than eighty years the existence of dominance modifiers, genetic elements controlling dominance-recessivity interactions, has been suggested as a theoretical possibility, but the modifier elements themselves have remained elusive. A recent study of the self-incompatibility locus in flowering plants provided the first empirical evidence for such genetic elements: small non-coding RNAs that control dominance-recessivity by mediating methylation of the promoter of the recessive allele. Theory has shown that several biological situations are favorable for the evolution of dominance modifiers. We argue that the elucidation of this mechanism of dominance opens up new research avenues that could lead to uncovering dominance modifiers in other genetic systems, such as genes controlling Batesian and Müllerian mimicry or host-parasite interactions, thereby shedding light on the generality of the proposed mechanism.