Gene Differences between the Sex Ratio and Standard Gene Arrangements of the X Chromosome and Linkage Disequilibrium between Loci in the Standard Gene Arrangement of the X Chromosome in DROSOPHILA PSEUDOOBSCURA

The Standard and Sex Ratio gene arrangements of the X chromosome of D. pseudobscura differ from each other in allele frequencies at the four X chromosome loci, esterase-5, adult acid phosphatase-6, phosphoglucomutase-1 and octanol dehydrogenase-3. The Standard arrangement which is the common arrangement in all populations is polymorphic at these loci in varying degrees, the geographically less widespread Sex Ratio arrangement has little polymorphism and is genically predominantly E-5(1.04) AP-6(-) Pgm1(1.0) ODH-3(1.0). The Sex Ratio arrangement from different populations is alike at all of the four loci, the Standard arrangement shows some gene frequency differences among populations. The Standard and Sex Ratio arrangements differ from each other by three inversions which suggests that the two arrangements are "old". Gene differences between these two chromosome arrangements can be explained due to differential natural selection of alleles in the Standard and Sex Ratio arrangments.-The order and percent recombination among these four loci in the Standard arrangement are: E-5-.294-AP-6-.335-Pgm-1-.024-ODH-3. The Standard X chromosomes from four different wild populations were analyzed for evidence of linkage disequilibrium between pairs of loci at these four loci. No evidence of linkage disequilibrium between pairs of loci was obtained. However, when linkages involving simultaneously three loci, E-5, AP-6 and Pgm-1 are considered, then significant departure from linkage equilibrium is observed.     

Gene Differences between the Sex Ratio and Standard Gene Arrangements of the X Chromosome in DROSOPHILA PERSIMILIS

The sibling species Drosophila pseudoobscura and D. persimilis each carry two gene arrangements in the right arm of the X chromosome; Standard (ST) and Sex Ratio (SR). The SR sequence of D. persimilis and the ST sequence of D. pseudoobscura have the same banding pattern. These cytologically identical arrangements carry different alleles at the esterase-5 (Est-5) and phosphoglucomutase-1 ( Pgm-1) loci. All the alleles on the SR arrangement of D. persimilis are also present on the ST arrangement of that species, and the same allele was most common in all cases. However, the allele frequencies at the Pgm-1 locus are significantly different between the two arrangements. Within the ST arrangement of D. persimilis, the alleles at the Est-5 and Pgm-1 loci are in linkage equilibrium.     

Genic Variation in Abundant Soluble Proteins of DROSOPHILA MELANOGASTER and DROSOPHILA PSEUDOOBSCURA

Genic variation was surveyed for 20 proteins of Drosophila melanogaster and 18 proteins of D. pseudoobscura. Analysis was by extraction and one-dimensional polyacrylamide gel electrophoresis under nondenaturing conditions, followed by staining with Coomassie Brilliant Blue to detect soluble proteins present in relatively large amounts ("abundant soluble proteins"). D. melanogaster was polymorphic for 65% of its protein loci and an individual was heterozygous for 10% of its loci. The respective figures for D. pseudoobscura were 61% and 11%. These estimates of genic variation fall between previously published estimates obtained for these species by one-dimensional electrophoresis of soluble enzymes and those obtained by two-dimensional electrophoresis of solubilized abundant proteins. However, variation for both species could be strongly partitioned between loci, on the basis of tissue and stage expression of the proteins. The results are discussed with respect to their bearing on the possibility that abundant proteins constitute a distinct class of proteins less polymorphic than soluble enzymes.     

Gene Differences between Third-Chromosome Inversions of DROSOPHILA PSEUDOOBSCURA

Associations of alleles of the acid phosphatase-3 locus with the different third-chromosome inversions from different populations of D. pseudoobscura are described. We observe only the allele AP-3 ^1.0 in the Standard and Arrow-head inversions and the allele AP-3^.98 in the Santa Cruz, Treeline, Cuernavaca and the Pikes Peak arrangements. The Chiricahua gene arrangement is polymorphic.     

Genic Heterogeneity at Two Alcohol Dehydrogenase Loci in DROSOPHILA PSEUDOOBSCURA and DROSOPHILA PERSIMILIS

A sequential electrophoretic survey of the second chromosome loci, alcohol dehydrogenase-6 (Adh-6) and octanol dehydrogenase ( Odh), was performed on 147 isochromosomal lines of Drosophila pseudoobscura and 60 lines of its sibling species, D. persimilis. Gels run with a variety of acrylamide concentrations and buffer pH's revealed the presence of 18 alleles of Adh-6 in the two species, where only eight had been previously detected by conventional electrophoretic methods. Only two alleles were added with our techniques to the previous total of nine in both species at the largely monomorphic Odh locus. Both enzymes show a predominance of one allele, with the other variants being fairly rare. There was no evidence of increased genetic divergence between the two species, but we found a striking increase in differentiation of Adh-6 alleles between the main body of D. pseudoobscura populations and the conspecific isolate from Bogotá, Colombia. These results are compared with our previous surveys of xanthine dehydrogenase in these species and discussed in reference to theories of genic polymorphism.     

Origin of Reproductive Isolation in the Absence of Apparent Genic Differentiation in a Geographic Isolate of DROSOPHILA PSEUDOOBSCURA

F(1) males obtained from the cross of D. pseudoobscura females from Bogotá (Colombia) x males of this species from mainland, i.e. populations from various locations in the United States and from Guatemala, are sterile. This sterility is due to genes located on the X chromosome and the autosomes; the Y chromosome is not involved. The percentage of sterile males in backcrosses can be explained by assuming an interaction between two loci on the Bogotá X chromosome and probably two loci, one each on two of the mainland autosomes. The role of founder events, inbreeding and geographic isolation in the development of reproductive isolation and the magnitude of gene differences responsible for the origin of reproductive isolation is discussed. It is concluded that founder events, inbreeding and geographic isolation play a major role in the development of reproductive isolation and that major adaptive incorporation of new alleles at a large number of structural loci is not necessary for the origin of reproductive isolation.     

Dosage Compensation of Genes on the Left and Right Arms of the X Chromosome of DROSOPHILA PSEUDOOBSCURA and DROSOPHILA WILLISTONI

We have investigated the occurrence of dosage compensation in D. willistoni and D. pseudoobscura, two species whose X chromosome is metacentric with one arm homologous to the X and the other homologous to the left arm of chromosome 3 of D. melanogaster. Crude extracts were assayed for isocitrate dehydrogenase (XR), glucose-6-phosphate dehydrogenase (XL?), 6-phosphogluconate dehydrogenase (XL?), and α-glycerophosphate dehydrogenase (chromosome 2) in D. willistoni, and for esterase-5 (XR), glucose-6-phosphate dehydrogenase (XL?), 6-phosphogluconate dehydrogenase (XL?) and amylase (chromosome 3) in D. pseudoobscura. Our results indicate that a mechanism for dosage compensation is operative in both arms of the X chromosome of these two species.     

Evidence for Extensive Genetic Differentiation between the Sex-Ratio and the Standard Arrangement of DROSOPHILA PSEUDOOBSCURA and D. PERSIMILIS and Identification of Hybrid Sterility Factors

This study deals with sex-ratio genes tightly linked within the Sex-Ratio inversion. By taking advantage of the fact that the Sex-Ratio chromosome of Drosophila persimilis [SR(B)] is homosequential to the Standard chromosome of D. pseudoobscura [ST(A)], we carried out two reciprocal introgression experiments. Individual segments of SR(B) or ST(A) were introgressed into the genome of D. pseudoobscura or D. persimilis, respectively. Males possessing a hybrid SR(B)-ST(A) X chromosome and a genetic background derived from either of the two species were tested for fertility and sex-ratio expression.—It was found that, in terms of the meiotic drive genes, the Sex-Ratio chromosome differs extensively from the Standard chromosome. Because recombinations of these genes result in a complete loss of sex-ratio expression, this finding lends strong support to the hypothesis of gene coadaptation. Coadaptation, in this context, is the advantage of being transmitted preferentially. In light of this finding, the evolution of the sex-ratio system in these two sibling species is discussed.—Introgression experiments also yielded information about hybrid sterility. With reciprocal introgression, sterility interactions were found to be "asymmetric." The asymmetry is fully expected from the viewpoint of evolution of postmating reproductive isolation.     

Distinctions among Allelic Variants Associated with Chromosome 3 Inversions in DROSOPHILA PSEUDOOBSCURA and DROSOPHILA PERSIMILIS

Efforts were made to discriminate new genetic variants among electrophoretic alleles that are associated with chromosome 3 inversions of Drosophila pseudoobscura and D. persimilis. Apparent genetic similarities for electrophoretic alleles between these two species and among the common inversions they carry were reexamined by altering gel concentration and buffer pH. At the amylase locus, the 1.09 electrophoretic allele could be further separated into two allelic classes that differentiated the WT and KL arrangements. Similarly, the 0.84 electrophoretic allele was divided into two allelic classes, one characteristic of the Santa Cruz phylad arrangements, TL and SC, and the other found in strains of the Standard phylad arrangements and CH. Uncommon amylase alleles proved to be different alleles in the two species. No new allelic variants, however, could be found among strains with the amylase 1.00 allele, the commonest allele in the Standard phylad of both species. No major new allelic variation was detected for acid phosphatase-3 and larval protein-10 that revealed any further differentiation among species or inversions. Variation at all three loci in strains of the Bogota population remained genetically similar to variation in strains of mainland D. pseudoobscura.     

Components of Selection in X Chromosome Lines of DROSOPHILA MELANOGASTER: Sex Ratio Modification by Meiotic Drive and Viability Selection

Selection coefficients and segregation parameters have been estimated in 18 randomly chosen lines carrying wild X chromosomes on the cn bw genetic background. Each line was studied in replicated crosses of four types, with approximately 100 replications per line per cross. Crosses in which male X chromosomes differed exhibited significant sex ratio heterogeneity. Maximum likelihood estimation of segregation parameters revealed two lines in which the proportion of X-bearing gametes produced by males was significantly different from Mendelian expectations. These observations suggest that segregation distortion is a common feature of naturally occurring genetic variation. Non-Mendelian segregation has important evolutionary implications.     

Normal Female Germ Cell Differentiation Requires the Female X Chromosome to Autosome Ratio and Expression of Sex-Lethal in DROSOPHILA MELANOGASTER

In somatic cells of Drosophila, the ratio of X chromosomes to autosomes (X:A ratio) determines sex and dosage compensation. The present paper addresses the question of whether germ cells also use the X:A ratio for sex determination and dosage compensation. Triploid female embryos were generated which, through the loss of an unstable ring-X chromosome, contained some germ cells of 2X;3A constitution in their ovaries. Such germ cells were shown to differentiate along one of two alternative pathways: a minority developed into normal female oocytes and eggs; the majority developed into abnormal multicellular cysts. An X:A ratio of 1 is, therefore, required in female germ cell development, at least in the mature ovary after stem cell division. Abnormal development of female germ cells was also observed when 2X;2A germ cells which were homozygous or trans-heterozygous for mutant alleles at the Sex-lethal locus were transplanted into normal female host embryos at the blastoderm stage. Germ cells homozygous for amorphic alleles failed to give rise to normal eggs. Instead, they formed multicellular cysts, very similar to those formed by 2X;3A cells. Zygotic Sxl+ activity is, therefore, also necessary for the development of normal female germ cells. No abnormalities were detected in transplanted germ cells from female embryos whose mothers had been homozygous for the mutation daughterless. When normal XY germ cells were transplanted into female embryos, no traces of such cells could be found in the adult ovary. XY germ cells seem, therefore, not to develop as far as 2X;3A or Sxl homozygous cells in a female gonad. This indicates that neither 2X;3A nor Sxl homozygous germ cells are equivalent to normal XY germ cells.     

Genic Heterogeneity within Electrophoretic "Alleles" and the Pattern of Variation among Loci in DROSOPHILA PSEUDOOBSCURA

An investigation, similar to our previously reported xanthine dehydrogenase study, was undertaken to examine the extent of hidden genic variation at nine loci (five larval proteins, three esterases and one aldehyde oxidase) by sequential application of various electrophoretic criteria employing pH, gel concentration and buffer variation. Polymorphic loci appear to fall into two distinct groups: weakly polymorphic, including larval protein 6, 7, 8, 10 and 13 and esterase-1 and -6; and highly polymorphic, including esterase-5, Xdh and possibly Ao. Monomorphic loci may belong to a third group different from all polymorphic loci. Bogota, a geographical isolate that is reproductively isolated from the mainland population, was found to be genetically distinct at four of the ten loci examined in detail so far, including Xdh, whereas previously it was found to be genetically distinct at none. These results are discussed in the light of balancing selection, neutral and mutation-selection hypotheses of genic variation in natural populations.     

Gene Flow and the Geographical Distribution of a Molecular Polymorphism in DROSOPHILA PSEUDOOBSCURA

This paper discusses the relation between the geographical distribution of an enzyme polymorphism and population structure in Drosophila pseudoobscura. California populations of this species living in very different montane and lowland habitats separated by several kilometers are similar to each other in the frequency of an esterase allele. Previous estimates suggest that gene flow is too limited to account for this homogeneity of genetic structure, so that it must reflect some balancing force of natural selection. We show, however, that dispersal over unfavorable habitats is much greater than earlier supposed. Isolated populations of D. pseudoobscura separated by 15 km from other populations are subject to large amounts of immigration. This is shown by changes in the seasonal abundance of this species and in the annual pattern of lethal alleles in such populations. The genetic structure of an experimentally perturbed isolated population in an oasis returned to normal within a single year, suggesting that such populations are ephemeral and that the oasis is subject to annual recolonization by distant migrants. Direct assessment of marked flies shows that they can move at least 10 km in 24 hours over a desert. Such extensive gene flow may help explain the distribution of the esterase allele, and is relevant to the high level of molecular polymorphism and its general lack of geographic differentiation throughout the range of D. pseudoobscura.     

Correlations between Chromosome Segments and Fitness in DROSOPHILA MELANOGASTER. I. the X Chromosome and Egg Production

Unmarked segments within the X chromosomes of four different Drosophila melanogaster isogenic lines were assessed with respect to egg production. By making a series of crosses among original and derived recombinant lines, it was possible to estimate parameters representing additive, dominance and interaction effects of the segments. It was shown that whereas most of the segments were additive for egg production when homozygous, they all displayed dominance in the heterozygous condition. Two of the strains were characterized by intersegmental interaction. A possible position effect was detected for these same two strains, with flies in the coupling phase laying more eggs than those in the repulsion configuration. There was no apparent relationship between the number of eggs laid and the amount of heterozygosity within the X chromosome.     

Gene Frequency Changes at the alpha-Amylase Locus in Experimental Populations of DROSOPHILA PSEUDOOBSCURA

The frequencies of alleles at the α-Amylase locus of D. pseudoobscura were followed in both large and small experimental populations. No evidence for balancing or directional selection was found, although our ability to detect weak selection is limited. The gene frequency changes in our experimental populations were consistent with the hypothesis of selective neutrality and genetic drift due to sampling error.