Intramolecular cooperativity in a protein binding site assessed by combinatorial shotgun scanning mutagenesis

Combinatorial shotgun alanine-scanning was used to assess intramolecular cooperativity in the high affinity site (site 1) of human growth hormone (hGH) for binding to its receptor. A total of 19 side-chains were analyzed and statistically significant data were obtained for 145 of the 171 side-chain pairs. The analysis revealed that 90% of the side-chain pairs exhibited no statistically significant pair interactions, and the remaining 10% of side-chain pairs exhibited only small interactions corresponding to cooperative interaction energies with magnitudes less than 0.4 kcal/mol. The statistical predictions were tested by measuring affinities for purified mutant proteins and were found to be accurate for five of six side-chain pairs tested. The results reveal that hGH site 1 behaves in a highly additive manner and suggest that shotgun scanning should be useful for assessing cooperative effects in other protein-protein interactions.     

NEUTRAL THEORY OF QUANTITATIVE GENETIC VARIANCE IN AN ISLAND MODEL WITH LOCAL EXTINCTION AND COLONIZATION

Additive genetic variance maintained by mutation in a selectively neutral quantitative character is analyzed for an ideal population distributed on n islands, each with local effective size N, that exchange migrants at a small rate, m. In a stable population structure, the expected genetic variance maintained within islands is identical to that in a panmictic population of the same total size, regardless of the migration rate (m > 0). This result contrasts with Wright's classical conclusion, based on inbreeding coefficients, that at least one immigrant per island every other generation (Nm > ½) is necessary for the genetic variance within local populations to approach that under panmixia. The expected genetic variance maintained among islands is inversely proportional to m and increases with the number of islands, but is independent of N. Local extinction and colonization diminish the genetic variance maintained within islands by reducing the effective size of island populations through the founder effect, although the expected genetic variance within islands is nearly as large as that in a panmictic population of the same total effective size. If the founders of new colonies originate from more than one island, rates of local extinction and colonization larger than about twice the migration rate will substantially reduce the genetic variance maintained among islands. These results indicate the importance of mutation and migration in maintaining quantitative genetic variance within small local populations.     

PHENOTYPIC AND GENETIC EFFECTS OF HYBRIDIZATION IN DARWIN'S FINCHES

Morphological consequences of hybridization were studied in a group of three interbreeding species of Darwin's finches on the small Galápagos island of Daphne Major in the inclusive years 1976 to 1992. Geospiza fortis bred with G. scandens and G. fuliginosa. Although interbreeding was always rare (< 5%), sufficient samples of measurements of hybrids and backcrosses were accumulated for analysis. Five beak and body dimensions and mass were measured, and from these two synthetic (principal-component) traits were constructed. All traits were heritable in two of the interbreeding species (G. fuliginosa were too rare to be analyzed) and in the combined samples of F, hybrids and backcrosses to G. fortis. In agreement with expectations from a model of polygenic inheritance, hybrid and backcross classes were generally phenotypically intermediate between the breeding groups that had produced them. Hybridization increased additive genetic and environmental variances, increased heritabilities to a moderate extent, and generally strengthened phenotypic and genetic correlations. New additive genetic variance introduced by hybridization is estimated to be two to three orders of magnitude greater than that introduced by mutation. Enhanced variation facilitates directional evolutionary change, subject to constraints arising from genetic correlations between characters. The Darwin's finch data suggest that these constraints become stronger when species with similar proportions hybridize, but some become weaker when the interbreeding species have different allometries. This latter effect of hybridization, together with an enhancement of genetic variation, facilitates evolutionary change in a new direction.     

Noncorrelated evolution between herbivore- and pollinator-linked features in Aristolochia chilensis (Aristolochiaceae)

The effect of mutualists (i.e. pollinators) and antagonists (i.e. herbivores) can have non-additive effects on plant fitness. This is often interpreted as evidence for correlated evolution on a suite of traits leading to an increase and decrease of the interaction of plants with mutualists and antagonists, respectively. This situation has been found to prevail in plants that have large floral and fruit displays but are not limited by pollinators for seed set. We suggest the alternative hypothesis, where plants limited by pollinators for seed set (e.g. deceit-pollinated plants) exhibit additive effects of pollinators and herbivores on fitness (i.e. noncorrelated evolution). Using a 2 × 2 factorial design, we tested this hypothesis by solely and simultaneously evaluating the effects of pollinators and the single herbivore, Battus polydamas archidamas , on female reproductive success of Aristolochia chilensis . Plants exposed to herbivores presented 2.6-fold greater herbivory than plants that excluded them. In addition, plants exposed to pollinators showed strong limitation by pollinators for seed set compared with other plants of the genus Aristolochia. However, only pollinators had a significant effect on fruit and seed set because plants that excluded them did not set fruits or seeds. Furthermore, herbivores and pollinators exerted additive effects on fruit and seed production. Collectively, these results indicate that herbivore- and pollinator-linked traits in A. chilensis exhibit noncorrelated evolution.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 239–245.     

The chemotaxonomy ofGeranium (Geraniaceae)

Relationships amongGeranium species constituting sectt.Anemonifolia, Lucida (monotypic) andRuberta together with representatives of sect.Unguiculata were investigated by gas-chromatographic study of essential oils, electrophoretic comparison of seed proteins and chromatographic separation of nectar amino acids. — Essential oil study gave little information.G. macrorrhizum (sect.Unguiculata) had far greater quantities of essential oils in its foliage than other species and differed from them qualitatively. — Species of sectt.Anemonifolia andRuberta, together withG. cataractarum (sect.Unguiculata), between them yielded 19 seed protein bands; the distribution of these indicated close relationship among the species and was consistent with hypotheses for the origin of certain species by alloand autopolyploidy partly within the group. Involvement of an unknown species in the origin of two allopolyploids was implied. Separate origins for the two octoploid species in this set are also inferred. In two instances there was evidence for the transformation of one band into another subsequent to the separation of related species. The inference of allopolyploidy was supported by the occurrence of additivity of parental bands shown by an artificial hybrid between two of the species. A model for the evolutionary divergence of the seed protein patterns is presented. Two species outside the above set appeared less closely related; they wereG. lucidum andG. macrorrhizum (sect.Unguiculata) and between them they showed 6 additional bands, four of which were shared. — Of 18 nectar amino acids found, 4 to 15 occurred in any one species, with low numbers (4 and 8) occurring in the most extreme inbreeders. The spectra of nectar amino acids of two hybrids showed additivity of those of the respective parents. The results echo rather closely those provided by seed proteins, but in the absence of data from outside the group their taxonomic significance is uncertain. However, the divergence between the two octoploids was again evident.     

Interrelationships between two-dimensional phase diagrams and mean molecular area-mole fraction curves in mixed monolayers

Both two-dimensional phase diagrams and mean molecular area (A)—mole fraction(x₂^π) curves have been investigated for various types of mixed monolayer systems. Two-dimensional phase diagram analysis and the behavior of A−x₂^π curves were found to be in good agreement with each other. From this result it was concluded that we should consider both phase diagrams and A−x₂^π curves of a mixed monolayer system in order to understand the behavior of the A−x₂^π curves.     

A generalized response surface model with varying relative potency for assessing drug interaction

When multiple drugs are administered simultaneously, investigators are often interested in assessing whether the drug combinations are synergistic, additive, or antagonistic. Based on the Loewe additivity reference model, many existing response surface models require constant relative potency and some of them use a single parameter to capture synergy, additivity, or antagonism. However, the assumption of constant relative potency is too restrictive, and these models using a single parameter to capture drug interaction are inadequate to describe the phenomenon when synergy, additivity, and antagonism are interspersed in different regions of drug combinations. We propose a generalized response surface model with a function of doses instead of one single parameter to identify and quantify departure from additivity. The proposed model can incorporate varying relative potencies among multiple drugs as well. Examples and simulations are given to demonstrate that the proposed model is effective in capturing different patterns of drug interaction.