Selection at the 'Esterase-2' Locus of 'Drosophila buzzatii'? Perturbation-Reperturbation Experiments
Apparent selection affecting starch gel electrophoretic alleles at the 'Esterase-2' locus of 'Drosophila buzzatii' has been detected in laboratory and natural populations. Perturbation-reperturbation of allele frequencies in replicated laboratory populations attempts to test direct selective effects at the locus versus effects of linked loci. Sequential gel electrophoresis has identified more alleles within starch classes, and three of these alleles (within the a, b and c starch alleles) were used in cage population experiments. Allele 'a/1.00/1.00/1.00' was set up in 10 replicate populations with allele 'c/1.00/1.00/1.00', and in an independent 10 replicate populations with allele 'b/0.99/1.01/1.00'. For each set, three reperturbations were done. Replicate populations generally showed similar patterns of allele frequency change and clear directionality: effects of selection, not drift. However, four populations deviated from their replicates, indicating issipation of linkage disequilibrium. Estimates of pre-adult viability in the F2 of pair-wise crosses among 12 sequential gel electrophoretic alleles showed very variable modes of inheritance and relative viability fitnesses. Together with the diversity of patterns of allele frequency change in the cage populations, these results suggest a gene complex, with selection acting on an interacting set of loci which may include 'Esterase'-2.
The phenotypic variance gradient - a novel concept
Evolutionary ecologists commonly use reaction norms, which show the range of phenotypes produced by a set of genotypes exposed to different environments, to quantify the degree of phenotypic variance and the magnitude of plasticity of morphometric and life-history traits. Significant differences among the values of the slopes of the reaction norms are interpreted as significant differences in phenotypic plasticity, whereas significant differences among phenotypic variances (variance or coefficient of variation) are interpreted as differences in the degree of developmental instability or canalization. We highlight some potential problems with this approach to quantifying phenotypic variance and suggest a novel and more informative way to plot reaction norms: namely "a plot of log (variance) on the y-axis versus log (mean) on the x-axis, with a reference line added". This approach gives an immediate impression of how the degree of phenotypic variance varies across an environmental gradient, taking into account the consequences of the scaling effect of the variance with the mean. The evolutionary implications of the variation in the degree of phenotypic variance, which we call a "phenotypic variance gradient", are discussed together with its potential interactions with variation in the degree of phenotypic plasticity and canalization.
Effective population size of natural populations of 'Drosophila buzzatii', with a comparative evaluation of nine methods of estimation
Allozyme and microsatellite data from numerous populations of 'Drosophila buzzatii' have been used (i) to determine to what degree Ne varies among generations within populations, and among populations, and (ii) to evaluate the congruence of four temporal and five single-sample estimators of Ne. Effective size of different populations varied over two orders of magnitude, most populations are not temporally stable in genetic composition, and Ne showed large variation over generations in some populations. Short-term Ne estimates from the temporal methods were highly correlated, but the smallest estimates were the most precise for all four methods, and the most consistent across methods. Except for one population, Ne estimates were lower when assuming gene flow than when assuming populations that were closed. However, attempts to jointly estimate Ne and immigration rate were of little value because the source of migrants was unknown. Correlations among the estimates from the single-sample methods generally were not significant although, as for the temporal methods, estimates were most consistent when they were small. These single-sample estimates of current Ne are generally smaller than the short-term temporal estimates. Nevertheless, population genetic variation is not being depleted, presumably because of past or ongoing migration. A clearer picture of current and short-term effective population sizes will only follow with better knowledge of migration rates between populations. Different methods are not necessarily estimating the same Ne, they are subject to different bias, and the biology, demography and history of the population(s) may affect different estimators differently.
The Swamp Buffalo: Domestication, Dispersal, and Genetic Differentiation
Water buffalo is an important livestock species in Asia as well as in the world. The swamp type buffalo, found throughout southeast Asia, from Assam and Nepal in the west to the Yangtse valley of China, is traditionally reared to produce draft power and meat. In this study, microsatellite markers were analyzed to determine genetic origin and population relationships of swamp buffalo in China and south-east Asia. Results showed that populations in south-east Asia and southwest of China had highest level of genetic variability. Differentiation among the Chinese swamp populations was much less than among the southeast Asian. Relationships among the swamp populations (DA genetic distances and STRUCTURE analyses) show the southeast Asian populations separated into two groups by the Chinese populations. Given these relationships and the patterns of genetic variability, we postulate that the swamp buffalo was domesticated in the region of the far south of China, northern Thailand and Indochina. Following domestication, it spread south through peninsular Malaysia to Sumatra, Java and Sulawesi, and north through China, and then to Taiwan, the Philippines and Borneo.