Dark mysteries

in vitro and in vivo can be attributed to crowding? The answer to this question depends upon the particular reaction and the microscopic environment in which the reaction is taking place. Early demonstrations of the large effect of crowding on association equilibria and rates were based upon studies of the behavior of mutant and normal hemoglobins in erythrocytes. Hemolysate is a fairly simple fluid containing primarily hemoglobin, and it can be shown that volume exclusion is a dominating factor in this medium. However, in a more complex heterogeneous environment such as cytoplasm, crowding is probably just one of several nonspecific factors affecting reaction rates and equilibria, such as weak nonspecific associations with background molecules or large structures leading to possible sequestration or adsorption of reactants and/or products. Nevertheless it is essential to keep in mind that, in a crowded biological fluid, the effects of volume exclusion will always be present and play an important role in influencing macromolecular structure and function, independent of and in addition to the influences of other types of interactions. The ubiquity of this phenomenon in biological fluids has been compared to that of gravity. The influence of macromolecular crowding and macromolecular confinement on biochemical reactions in physiological media. Influence of macromolecular crowding upon the stability and state of association of proteins: predictions and associations. University at Baton Rouge, and Shannon Hackett at the Field Museum of Natural History in Chicago have studied a stretch of coding region in the MC1R gene in 23 individuals spanning the range of male plumage variation (published online in Proceedings B of the Royal Society). The only variable sites they detected were for synonymous substitutions, none of which were associated with either plumage type. Comparative analysis of the sequences at three amino acid sites thought to be functionally important in pigment variations in other species proved to be similar in these birds. This new study suggests that other mechanisms are at play. Many loci are known to influence pigmentation in vertebrates so " future studies exploring other candidate genes, especially those with regulatory functions, are likely to provide great insight into our understanding of the evolution of avian plumage colour, " the authors report. Bird plumage traits are the targets of both natural and sexual selection and fascinate evolutionary biologists as many species show quite dramatic variations within their range and between related species. So, genetic changes resulting in plumage …