Frequency-Dependent Selection at the LAP Locus in DROSOPHILA MELANOGASTER

Results of fitness estimates for the Lap locus in Drosophila melanogaster revealed that under crowded media conditions gene frequency equilibrium was maintained by frequency-dependent selection. Evidence was obtained that indicated that mating and egg-to-adult viability were frequency dependent.     

Selection for Male Recombination in DROSOPHILA MELANOGASTER

Two-way selection for male recombination over seven intervals of the third chromosome in Drosophila melanogaster was practiced for nine generations followed by relaxed selection for five generations. Significant responses in both directions were observed but these mainly occurred in early generations in the low line and in later generations in the high line. Divergence of male recombination frequencies between the two selection lines was not restricted to any specific region but occurred in every measured interval of the chromosome. However, right-arm intervals showed a more pronounced response than either left-arm intervals or the centromeric region. Correlated responses in sterility and distortion of transmission ratios occurred as a result of selection for male recombination. Cluster distributions of male recombinants suggested a mixture of meiotic and late gonial events but relative map distances more closely resembled those of the salivary chromosome than standard meiotic or mitotic distances. Patterns of male recombination over time in both second and third chromosomes strongly suggested a major effect associated with the presence of third chromosomes from the Harwich strain. Evidence was also found for modifiers with relatively small effects located in other regions of the genome. The overall results are interpreted in terms of a two-component model of hybrid dysgenesis.     

Genetic Analysis of the Hairy Locus in DROSOPHILA MELANOGASTER

Mutations of the hairy locus in Drosophila may affect both adult chaeta differentiation and embryonic segmentation. In an effort to understand this phenotypic complexity, we have analyzed 30 mutant alleles of the locus. We find that the alleles fall into four groups according to their complementation properties, suggesting a structurally complex locus in which two distinct functions share a common coding region.     

Recombination and Response to Selection in DROSOPHILA MELANOGASTER

Most biologists beleive that recombination speeds response to selection for traits determined by polygenic loci. To test this hypothesis, sixteen Drosophila melanogaster populations were selected for positive phototaxis for twenty-one generations. In some populations, balancer chromosomes were used to suppress autosomal recombination, and in others the autosomes were free to recombine. Suppression of recombination had no effect on mean rate of response to selection, though it may have increased variability in the rate of response among replicate lines. Suppressed recombination lines did not shift selection response to the freely recombining X chromosomes, despite fairly large increased in X chromosome recombination. The results suggest that in populations of moderate size, sex does not accelerate short term response to selection.     

Selection for Reduced Crossing over in DROSOPHILA MELANOGASTER

Selection was practiced for reducing crossing over between the third chromosome genes Sb and H(2) of Drosophila melanogaster; the method employed was to select the repulsion double heterozygotes Sb+/+H(2) every generation. Two replicate selection lines were maintained. After 24 generations of selection, Line 1 showed no significant difference from the control, although the regression of recombination value on generation was significant. In generation 20, Line 2 had a significantly lower recombination value than the control, as well as having a highly significant regression coefficient. No chromosome rearrangements were involved in the response. It was concluded that there was substantial genic variability in the frequency of crossing over between Sb and H(2) in the base population.     

Genetic Control of Adh Expression in DROSOPHILA MELANOGASTER

Natural variants displaying different levels of expression of the gene for alcohol dehydrogenase (Adh) were subjected to genetic mapping experiments. The strains studied carry one of the two common electrophoretic forms of the enzyme. The difference among Adh-fast strains appears to be due to multiple loci with trans-acting effects. Differences among Adh-slow strains are due to modifiers or quantitative sites located very close to the structural gene (less than 0.05 map unit) or part of it. The modifiers detected in the Adh^s strains seem to operate only on the structural allele in the cis-position.—A modifier that affects the ratio of ADH levels in larvae and adults was also detected in the Adh^s strains. This modifier is also closely linked to Adh and is cis-acting.     

Genetic Change of Recombination Value in DROSOPHILA MELANOGASTER. II. Simulated Natural Selection

Selection of Gl-Sb coupling heterozygotes was carried out for more than one hundred generations commencing with six independent lines drawn from a common base population. Population sizes were eight, sixteen and forty-eight parents per generation. The effect of natural selection on recombination value was measured by sampling and testing females at varying intervals of time. There was a significant reduction in percentage recombination between Gl and Sb from fifteen to a level between five and ten in four out of six of the original lines. In most cases this reduction occurred rather rapidly after the initiation of the experiment. In the remaining two lines there was no significant decrease in recombination value; there was, however, a significant increase in at least one subline of this group. The rapid rate of change of recombination value is most readily explained by the presence of a recombination modifying gene which is linked to the modified region. Genetic random drift was again shown to have an important effect on changes in recombination value in small populations. High recombination was almost completely recessive to low recombination in the one case examined. Lethal genes were fixed in sheltered regions of unmarked third chromosomes in five lines or sublines. These results are discussed in relation to the mode of development of permanent heterozygosity in some species of plants.     

Genetic Analysis of the Achaete-Scute System of DROSOPHILA MELANOGASTER

Several mutations in the achaete-scute region of Drosophila have been analyzed phenotypically and cytologically. One group of them corresponds to point mutations, another to rearrangements with one breakpoint in this region. Trans heterozygotes of the different point mutations or of the different rearrangements show poor complementation or fail to complement; therefore, they could be interpreted as mutations affecting the same gene product. However, left-right inversion recombinants and duplication-deficiency combinations between rearrangements with different cytological breakpoints uncover a complex organization of the achaete-scute region. This region seems to contain several independent achaete and scute functions, as well as a lethal function, arranged as a tandem reverse repeat at both sides of a lethal locus. Since all of the mutants show the same phenotype qualitatively, though different quantitatively, we suggest that these functions are of a reiterative nature. The achaete-scute wild-type condition may well be dependent on a multimeric gene product made of several evolutionary related monomers.     

Density-Dependent Fertility Selection in Experimental Populations of DROSOPHILA MELANOGASTER

The effects of larval density on components of fertility fitness were investigated with two mutant lines of Drosophila melanogaster. The differences in adult body weight, wing length, larval survivorship and development time verified that flies reared at high density were resource limited. Experimental results indicate that: (1) relative fecundities of both sexes show density-dependent effects, (2) there is a strong density effect on male and female mating success, and (3) in general, there is a reduction in fecundity differences between genotypes at high density. These results imply that it may be important to consider fertility in models of density-dependent natural selection.     

Fine-Structure Analysis and Genetic Organization at the Base of the X Chromosome in DROSOPHILA MELANOGASTER

Genetic organization at the base of the X chromosome was studied through the analysis of X-ray-induced deficiencies. Deficiencies were recovered so as to have a preselected right end "anchored" in the centric heterochromatin to the right of the su(f) locus. "Free" ends of deficiencies occurred at any of 22 intervals in Section 20 and in the proximal portion of Section 19 of Bridges' (1938) polytene chromosome map. The distribution of 130 such free ends of deficiencies induced in normal, In(1)sc ⁸, and In(1)w^m4 chromosomes suggests that on the single section level, genes are flanked by "hot" or "cold" sites for X-ray-induced breaks, and that occurrence of the hot spots is dependent on their interaction with the fixed-end sites in the centric heterochromatin. In the light of these results, it is argued that long heterochromatic sequences separate the relatively few genes in Section 20, and thus endow it with several characteristics typical of heterochromatic regions. Section 20 is considered to be a transition region between the mostly heterochromatic and mostly euchromatic regions of the X chromosome; the differences between them are suggested as being merely quantitative.     

Transposable Element-Induced Response to Artificial Selection in DROSOPHILA MELANOGASTER

The P family of transposable elements in Drosophila melanogaster transpose with exceptionally high frequency when males from P strains carrying multiple copies of these elements are crossed to females from M strains that lack P elements, but with substantially lower frequency in the reciprocal cross. Transposition is associated with enhanced mutation rates, caused by insertion and deletion of P elements, and chromosome rearrangements. If P element mutagenesis creates additional variation for quantitative traits, accelerated response to artificial selection of progeny of M female female X P male male strain crosses is expected, compared with that from progeny of P female female X M male male strain crosses.--Divergent artificial selection for number of bristles on the last abdominal tergite was carried out for 16 generations among the progeny of P-strain males (Harwich) and M-strain females (Canton-S) and also of M-strain males (Canton-S) and P-strain females (Harwich). Each cross was replicated four times. Average realized heritability of abdominal bristle score for the crosses in which P transposition was expected was 0.244 +/- 0.017, 1.5 times greater than average heritability estimated from crosses in which transposition was expected to be rare (0.163 +/- 0.010). Phenotypic variance of abdominal bristle score increased by a factor of four in lines selected from M female female X P male male crosses when compared with those selected from P female female X M male male hybrids. Not all quantitative genetic variation induced by P elements is additive. A substantial fraction of nonadditive genetic variation is implicated by chromosomal analysis, which demonstrates deleterious fitness effects of the mutations when homozygous.--Several putative "quantitative" mutations were identified from chromosomes extracted from the selected lines; these will form the basis for further investigation at the molecular level of the genes controlling quantitative inheritance.     

A Genetic Analysis of Phototactic Behavior in DROSOPHILA MELANOGASTER I. Selection in the Presence of Inversions

The effectiveness of selection for positive and negative phototactic behavior in populations of Drosophila melanogaster heterozygous for various multiple inversions was compared using the method of realized heritability. Selection in the presence of FM6, SM1 or TM3 alone was as effective as in populations carrying no inversions. However, the presence of FM6 and TM3 together reduced the effectiveness of selection for photopositive behavior and FM6 and SM1 and TM3 restricted the response to selection for negative phototactic behavior. The results are discussed in terms of the organization of genes influencing phototactic behavior in this species.