Studies on the nature and function of polygenic loci in Drosophila. I. Comparison of genomes from selection lines

Organization of polygenic systems: Cell death modifiers from natural populations ofDrosophila melanogaster

Expression of three wing-cell death mutants inDrosophilamelanogaster was used to survey cryptic polygenic modifiers of cell death in wild type strains. Females carrying the X-linked mutants Beadex-3, notchoid and scalloped were crossed to males from each of 20 isofemale strains. Phenotypic variation in the amount of cell death was measured in F1mutant males that were heterozygous for polygenic loci segregating in the wild strains. As expected, each mutant uncovered a broad range of polygenic variation among strains. Yet, when cluster analyses were used to evaluate the degree of correlation among the expressions ofBx3,sdandnd, the isofemale strains could be partitioned into a small number of groups that were similar in the effects they had upon the severity of cell death. Chromosome mapping of one cell death suppressor strain demonstrated that different polygenic loci could produce the same phenotype in different mutant backgrounds.

Differences in the selection response of serially repeated color pattern characters: standing variation, development, and evolution

Background

There is spectacular morphological diversity in nature but lineages typically display a limited range of phenotypes. Because developmental processes generate the phenotypic variation that fuels natural selection, they are a likely source of evolutionary biases, facilitating some changes and limiting others. Although shifts in developmental regulation are associated with morphological differences between taxa, it is unclear how underlying mechanisms affect the rate and direction of evolutionary change within populations under selection.

Here we focus on two ecologically relevant features of butterfly wing color patterns, eyespot size and color composition, which are similarly and strongly correlated across the serially repeated eyespots. Though these two characters show similar patterns of standing variation and covariation within a population, they differ in key features of their underlying development. We targeted pairs of eyespots with artificial selection for coordinated (concerted selection) versus independent (antagonistic selection) change in their color composition and size and compared evolutionary responses of the two color pattern characters.

Results

The two characters respond to selection in strikingly different ways despite initially similar patterns of variation in all directions present in the starting population. Size (determined by local properties of a diffusing inductive signal) evolves flexibly in all selected directions. However, color composition (determined by a tissue-level response to the signal concentration gradient) evolves only in the direction of coordinated change. There was no independent evolutionary change in the color composition of two eyespots in response to antagonistic selection. Moreover, these differences in the directions of short-term evolutionary change in eyespot size and color composition within a single species are consistent with the observed wing pattern diversity in the genus.

Conclusion

Both characters respond rapidly to selection for coordinated change, but there are striking differences in their response to selection for antagonistic, independent change across eyespots. While many additional factors may contribute to both short- and long-term evolutionary response, we argue that the compartmentalization of developmental processes can influence the diversification of serial repeats such as butterfly eyespots, even under strong selection.

Sources of behavioral deviation modeled by early color preferences in quail. II. Diathetic genes and the leftover variance "V(e)"

The sources of deviation from population-typical norms of variability were studied in relation to early approach preferences between colors in newly hatched Japanese quail chicks (Coturnix coturnix japonica). Subjects were drawn from an unselected genetic control line, two genetic lines that were artificially selected for extreme blue and red preferences, their hybrids, and a large set of backcross and assortatively derived generations. Large progressive increases were observed with selection in the leftover (V(e)) component of F1 variance and consistent additional variance increases in F2 hybrids. Backcross generations exhibited greater than expected expression of the blue and red preference genes. Discussion differentiates between diathetic and normative influences in preference deviations and relates the increased Ve to relaxed normative canalization of preference development by directional selection.

How small are the smallest selectable domains of form?

Two lines of Drosophila melanogaster from the same base population were selected in opposite directions to produce simultaneous antagonistic changes in two very small (less than 0.2 mm) and closely adjacent (less than 0.3 mm) dimensions within the base of the wing. Wing dimensions near the targeted area became differentiated by large positive and negative percentage differences, while only small homogeneous percentage changes occurred in the remainder of the wing. If very small regions of morphology (less than 100 cells across) can respond to selection almost independently, even in small population samples, then the control of developmental detail must involve many genes, and the diversity of possible outcomes in development and adaptation must be large.

Genetic variation in a modifier system affecting the expression of bare mutant of Drosophila subobscura

Genetic variation affecting the expression of Bare (Ba), a morphological mutant of Drosophila subobscura that reduces the number of bristles and is located on the O chromosome of this species, is reported. Our results show that O chromosomes extracted from a natural population (El Pedroso, Santiago de Compostela, Spain) show considerable genetic variation in modifier effect upon Ba expression. The amount of modifier variability is dependent on the chromosomal arrangement (OST and O3 + 4 + 7), since modifier variation is higher in OST than in O3 + 4 + 7 chromosomes. Investigations of the genetic architecture of this modifier system on the O chromosome carried out using biometrical methods indicate that a relatively small number of genetic "factors" can explain the differences in modifier effect between a wild O chromosome of high modifier effect and a marker chromosome of low score. In addition, the modifier effects show a non-uniform distribution along the O chromosome and some indication of clustering of modifier "factors" around the major locus (Ba) is also observed.

Detection of simple polygenic segregations in a natural population

Penetrance frequencies were used to quantify segregating polygenic effects in a natural population of Drosophila melanogaster. When males from a series of 100 isofemale strains were crossed to females from a veinlet (ve) line that had been selected for shortened veins, gaps commonly appeared in the fifth longitudinal (L5) vein in the ve/+ heterozygotes. We were able to assign each strain to one of six significantly different clusters, based upon the pattern of polygenic modifiers of ve dominance segregating in each strain. We conclude that the extensive range of phenotypic variation in vein-forming ability is actually based upon a relatively simple underlying polygenic structure that is consistent with segregation of only a small number of alleles or allele combinations in the wild population.

Selection acting upon slow-migrating ADH alleles differing in enzyme activity

Relative egg to adult viability was compared in three stocks of Drosophila melanogaster differing in alcohol dehydrogenase (ADH) activity. Two of these, Grell's standard ADHS and a mutant ADHS strain producing half the normal number of ADH molecules, had the same electrophoretic mobility. The experiment demonstrated a correlation between survival in alcohol-supplemented media and enzyme activity. This supports the hypothesis that enzyme activity per se, rather than some other attribute of allelic differences, may be responsible for the previously observed selective advantage of high activity alleles on alcohol media.

A test of the influence of isoallelic variation upon a quantitative character

In order to test the assumption that isoalleles can contribute to polygenic variation, several isoalleles of cubitus interruptus were examined for their influence upon the quantitative expression of a related venation mutant, veinlet. In spite of the fact that veinlet and cubitus interruptus affect closely related developmental processes, the ci isoalleles had no effect upon the expression of veinlet. This test does not, of course, disprove the hypothesis that isoalleles play a part in polygenic variation. It is clear, however, that not all seemingly relevant isoalleles are necessarily effective elements in a given polygenic system.

Genetic assimilation of part of a mutant phenotype

When the mutantveinletwas removed from aDrosophila melanogasterline selected for shortened vein length it was found that the selected polygenic background produced vein gaps in the absence of the major mutant. This example of genetic assimilation was unusual, however, in that the phenotype of the assimilated line was not exactly the same as the phenotype of the original selection line. It differed in two respects: only one of the longitudinal veins was affected in the assimilation line whereas all veins are shortened in the selectedveline, and vein gaps were sub-terminal in the assimilated line whereas they are terminal inveinlet. Modifiers of L 4 vein length were found to be located on both chromosomes II and III, though the chromosome II factor had a larger effect and was required for gap formation. The chromosome II gene was mapped to the same position as a similar L 4 vein length polygene (PL(2)L4a) found in anotherveinletselection line and reported elsewhere. The pattern of expression of thevemutant and the assimilated line genotype were compared using profiles of vein frequency at given points. The vein pattern profiles are discussed in relation to an hypothesis to explain the observed selection response and the effectiveness which these polygene combinations showed in modifying the much more complexveinletphenotype.