Selection for canalization at two extra dorsocentral bristles in Drosophila melanogaster.

From 10 isofemale lines of D. melanogaster, the D2 line was established with the aim of obtaining an invariant phenotype at two extra dorsocentral bristles. Line D2 was also subdivided into two other lines, SA and ASD, based on their different bristle patterns. The SA line was selected for two symmetrical anterior extra bristles, and the ASD line was selected for two asymmetrical extra bristles, one anterior and one posterior. Only the SA line showed any canalizing response (estimated by the width of the probit transformation) at the two-extra-bristle class. Nevertheless, the results from the different lines were more consistent with the independent ones of both the anterior and posterior regions of the extra dorsocentral bristles. This analysis showed some independent genetic systems for each region, developmental canalization being at two extra symmetrical bristles per region in all the selected lines (D2, ASD, and SA). Therefore, this canalization did not depend directly on the extra-bristle positional pattern used in the selection. The wild-type canalizing system is suggested to explain the fast canalizing response in a phenotype that had not been previously canalized by natural selection.     

Selection and temperature effects on extra dorsocentral bristles inDrosophila melanogaster

Starting from base populations which showed a tendency to form supernumerary dorsocentral bristles, selection for high numbers of dorsocentrals was carried out. In all, 9 lines were selected 6 to 14 generations. Selection proved to be effective in all but two lines. Selection for reduced numbers in one of the lines also proved effective.Mean values of over 10 were reached in the females of certain high lines. Average counts in males were always lower than those of females. This sex difference is not a consequence of the difference in size between the sexes.The base populations were of mutant stock origin, and some of them segregated for mutant genes which proved to be correlated with extrabristle phenotype. Supernumerary bristles were not distributed in the same manner in all of the selection lines, nor was the reaction of phenotypic expression to temperature the same in various lines.     

Selection for six scutellar chaetae in Drosophila melanogaster

Summary 1. From a base population showing some flies with more than the normal 4 scutellar chaeta phenotype directional selection was carried out and lead to a line with many flies having 6 chaetae. 2. Selection was then practised for 6 chaetae such that the extra 2 chaetae were in the anterior left and anterior right positions on the scutellum. This lead to a line with most flies having this chaeta number and pattern, therefore showing some canalization. 3. Additive genetic activity controlling the increased chaeta number was found on all the 3 major chromosomes.     

Some apparently simple relationships involving missing scutellar bristles in scute mutants of Drosophila melanogaster

An analysis of the presence or absence of the anterior and posterior scutellar bristles on the left and right sides of scute-bearing Drosophila melanogaster and of the distribution of the 16 possible patterns of the four scutellar bristles among 2893 males suggests the following: (1) that scutellar bristles are commonly present or absent in frequencies involving powers of 1/2; and (2) that observed excesses of symmetrical classes can be accounted for by the detection of potentially asymmetrical flies and their subsequent correction. Corrected asymmetrical flies seem to be converted with equal frequencies into one or the other of the corresponding symmetrical ones. An explanatory model has not been constructed; the precise ratios, however, suggest that correspondingly precise chromosomal events are involved.Work reported here has been supported under Grants GM24850 and GM29810 from the National Institute of General Medical Sciences, U.S. Public Health Services.     

Canalization at two extra scutellar bristles: artificial selection, chromosome analysis, and effect of temperature.

Two isofemale lines (P1 and P2) from a wild-type population of Drosophila melanogaster were selected for an invariant phenotype, two symmetrical and anterior scutellar extra bristles. Both P1 and P2 lines responded quickly to direct selection for two extra bristles, but although this phenotype was canalized in the P2 line, it was not in the P1 line. This lack of canalization at two extra bristles, measured by probit width, is the first reported in the literature. Analysis of chromosomal contribution showed the P1 decanalized phenotype to be due to chromosome 2. Synthetic chromosomal combinations were tested at four development temperatures (17 degrees, 21 degrees, 24 degrees, and 30 degrees C), and no correlation was observed between canalization at two extra bristles measured by probit width and minimum phenotypic change due to different temperatures. In this test, development at the highest temperatures was accompanied by an increased tendency to return to the wild phenotype in the canalized phenotypes only, suggesting that this drastic change could be accounted for by the action of the wild-type canalizing system. Decanalized genotypes, or those canalizing at phenotypes other than the wild type, could be explained by mutations which, to a greater or lesser degree, alter the normal genetic system of canalization.     

Chromosomal polymorphism and extra bristles of Drosophila melanogaster: joint variation under selection in isofemale lines

Selection for increased numbers of dorsocentral and scutellar bristles produced an increase of In(3R)C heterozygotes in isofemale lines of D. melanogaster. The influence of polygenic selection in the maintenance of the chromosomal polymorphism is discussed.     

Evidence for canalization of Distal-less function in the leg of Drosophila melanogaster

A considerable body of theory pertaining to the evolution of canalization has emerged recently, yet there have been few empirical investigations of their predictions. To address this, patterns of canalization and trait correlation were investigated under the individual and joint effects of the introgression of a loss-of-function allele of the Distal-less gene and high-temperature stress on a panel of iso-female lines. Variation was examined for number of sex comb teeth and the length of the basi-tarsus on the pro-thoracic leg of male Drosophila melanogaster. I demonstrate that whereas there is evidence for trait canalization, there is no evidence to support the hypothesis of the evolution of genetic canalization as a response to microenvironmental canalization. Furthermore, I demonstrate that although there are genetic correlations between these traits, there is no association between their measures of canalization. I discuss the prospects of the evolutionary lability of the Distal-less gene within the context of changes in genetic variation and covariation.     

Selection for malathion-resistance in Drosophila melanogaster

Results from a long-term selection experiment for malathion-resistance in Drosophila melanogaster are described. A polled population of 40 locally-caught, iso-female lines was exposed to increasing concentrations of malathion in the food at both a high selection intensity (MH) and a lower intensity (ML). The response was consistent with a polygenic system. Both adult and larval resistance increased in parallel. Changes in the dose-response curve of adults could be approximately described by a dose-modification factor. Larval resistance was more complex; both selected populations showed a maternal effect which could not be explained by sex-linked genes. Larval resistance in the selected populations behaved as a co-dominant trait with respect to the susceptible controls. Adult resistance was dominant in the ML and co-dominant in the MH population, suggesting that different genes conferring resistance were selected. The selection procedure also produced a developmental delay in both populations, dependent on malathion concentration, but present even in its absence.     

Genes involved in the development of bristles and hairs in Drosophila melanogaster

Mutations in three loci influencing the development of bristles and hairs were detected in experiments with strains containing either a mobilized Stalker or a mobilized P-element. The mutations in two genes, suppressor of scute and putative microchaete, modify phenotypic expression of mutations in the scute locus. In particular, su(sc) mutations suppress the sc-phenotype in the scutellum and enhance the Hw-phenotype in the thorax. Mutations in the third gene, pseudoscute, lead to reduction of all bristles and hairs. The latter locus seems to control the development of bristles independently of the achaete-scute complex control.     

The differential genetic and environmental canalization of fitness components in Drosophila melanogaster

Canalization describes the process by which phenotypic variation is reduced by developmental mechanisms. A trait can be canalized against environmental or genetic perturbations. Stabilizing selelction should favor improved canalization, and the degree of a trait's canalization should be positively correlated with its impact on fitness. Here we report, for Drosophila melanogaster, measurements of environmental canalization for five fitness components. We compare them with measurements of genetic canalization, and we discuss the impact of inbreeding on both. In three experiments we measured the variation of fitness components within lines nested within temperature, treatment, and experiment. Lines differed in the position of a P element insert or in genetic background. Within lines flies were genetically nearly identical. We designated trait variation within lines as environmental canalization. The canalization of the traits increased with their impact on fitness, and the pattern was similar to that found for the canalization of fitness components against genetic differences, measured as the variation among lines nested within temperature, treatment, and experiment. This suggests that developmental mechanisms buffer the phenotype against both genetic and environmental disturbance. The results also suggest, less strongly, that inbreeding weakens canalization.