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.     

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.     

Direct Evidence of Genic Differentiation between Sex Ratio and Standard Gene Arrangements of X Chromosome in DROSOPHILA PSEUDOOBSCURA

Evidence of strong association of alleles at the Esterase-5 and adult Acid phosphatase-6 loci with the Sex Ratio and the Standard X chromosome arrangements in D. pseudoobscura is presented. There is no differentiation at these two loci, either in the Sex Ratio or the Standard arrangement from two widely separated geographic populations. At the Esterase-5 locus, alleles characteristic of one arrangement are found in low frequencies in the other arrangement. It is concluded, from these and other observations that the differentiation between the Sex Ratio and the Standard arrangement, at these two loci, is a result of natural selection favoring different coadapted gene blocks in these arrangements. The average heterozygosity at these two loci in different karyotypes is in the following order: ST/SR > ST/ST >> SR/SR which agrees very well with the fitness estimates of these karyotypes.     

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.     

Fitness of Karyotypes in DROSOPHILA PSEUDOOBSCURA

In the dynamics of the survival of chromosomal polymorphism selection may be operating at the genic level, at the chromosomal level or at the supergene level. Tests designed to distinguish between these levels were run on Drosophila pseudoobscura. There was no evidence for heterosis, a necessary requirement for gene-determined chromosomal polymorphism. A strong chromosmal selection was observed. No evidence was found for the presence within one locality of more than a single superallele for each supergene (= gene order). These results are compared to those found by others.     

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.     

Frequency-Dependent Selection at the PGM-1 Locus of DROSOPHILA PSEUDOOBSCURA

Frequency-dependent fitness was studied at the Pgm-1 locus of Drosophila pseudoobscura with respect to two fitness components: rate of development and larva-to-adult survival. The Pgm-1 locus is very polymorphic with only two alleles, Pgm-1¹⁰⁰ and Pgm-1¹⁰⁴, occurring at high frequencies. For each of these two alleles, 20 homozygous strains were obtained from a sample of 1,140 wild-inseminated females. First-instar larvae of the two genotypes were combined in a set of eight different frequencies: 0.0, 0.10, 0.25, 0.40, 0.60, 0.75, 0.90, and 1.0. Frequency-dependent fitness effects were observed for the two survival-related fitness components examined: larvae of the less common genotype develop faster and have a higher probability of survival than larvae of the more common genotype. The rate of survival at intermediate genotypic frequencies is similar to that in pure cultures. If selection acted solely as frequency-dependent effects on survival-related components of fitness, the equilibrium frequency of the Pgm-1¹⁰⁰ allele would be 0.615 for a two-genotype system, which fits an observed frequency range for this allele in nature between 0.55 and 0.71. Experimentally created linkage disequilibrium was excluded from the experiment by using a large number of independent strains. It is nevertheless possible that the frequency-dependent selection may not affect the Pgm-1 locus per se, but may reflect a linkage disequilibrium present in the natural population. Even if this were the case, the frequency-dependent selection could affect the frequency of the Pgm-1 alleles in nature.     

Origin,Differentiation, and Geographic Distribution of the Chenopodiaceae

Numbers of species and genera,endemic genera,extant primitive genera,relationship and distribution patterns of presently living Chenopodiaceae(two subfamilies,12 tribes,and 118 genera)are analyzed and compared for eight distributional areas,namely central Asia,Europe,the Mediterranean region,Africa,North America,South America, Australia and East Asia． The Central Asia,where the number of genera and diversity of taxa are greater than in other areas,appears to be the center of distribution of extant Chenopodiaceae．North America and Australia are two secondary centers of distribution． Eurasia has 11 tribes out of the 12,a total of 70 genera of extant chenopodiaceous plants,and it contains the most primitive genera of every tribe． Archiatriplex of Atripliceae,Hablitzia of Hablitzeae,Corispermum of Corispermeae,Camphorosma of Camphorosmaea,Kalidium of Salicornieae,Polecnemum of Polycnemeae,Alexandra of Suaedeae,and Nanophyton of Salsoleae,are all found in Eurasia,The Beteae is an Eurasian endemic tribe,demonstrating the antiquity of the Chenopodiaceae flora of Eurasia．Hence,Eurasia is likely the place of origin of chenopodiaceous plants． The presence of chenopodiaceous plants is correlated with an arid climate．During the Cretaceous Period,most places of the continent of Eurasia were occupied by the ancient precursor to the Mediterranean,the Tethys Sea．At that time the area of the Tethys Sea had a dry and warm climate．Therefore,primitive Chenopodiaceae were likely present on the beaches of this ancient land．This arid climatic condition resulted in differentiation of the tribes Chenopodieae,Atripliceae,Comphorosmeae,Salicornieae,etc．,the main primitive tribes of the subfamily Cyclolobeae. Then following continental drift and the Laurasian and Gondwanan disintegration, the Chenopodiaceae were brought to every continent to propagate and develop, and experience the vicissitudes of climates, forming the main characteristics and distribution patterns of recent continental floras. The tribes Atripliceae, Chenopodieae, Camphorosmeae, and Salicornieae of recent Chenopodiaceae in Eurasia, North America, South America, southern Africa, and Australia all became strongly differentiated. However, Australia and South America, have no genera of Spirolobeae except for a few maritime Suaeda species. The Salsoleae and Suaedeae have not arrived in Australia and South America, which indicates that the subfamily Spirolobeae developed in Eurasia after Australia separated from the ancient South America-Africa continent, and South America had left Africa. The endemic tribe of North America, the tribe Sarcobateae, has a origin different from the tribes Salsoleae and Suaedeae of the subfamily Spirolobeae. Sarcobateae flowers diverged into unisexuality and absence of bractlets. Clearly they originated in North America after North America had left the Eurasian continent. North America and southern Africa have a few species of Salsola, but none of them have become very much differentiated or developed, so they must have arrived through overland migration across ancient continental connections. India has no southern African Chenopodiaceae floristic components except for a few maritime taxa, which shows that when the Indian subcontinent left Africa in the Triassic period, the Chenopodiaceae had not yet developed in Africa. Therefore, the early Cretaceous Period about 120 million years ago, when the ancient Gondwanan and Laurasian continents disintegrated, could have been the time of origin of Chenopodiaceae plants.The Chinese flora of Chenopodiaceae is a part of Chenopodiaceae flora of central Asia. Cornulaca alaschnica was discovered from Gansu, China, showing that the Chinese Chenopodiaceae flora certainly has contact with the Mediterranean Chenopodiaceae flora. The contact of southeastern China with the Australia Chenopodiaceae flora, however, is very weak.     

Density- and Frequency-Dependent Selection at the Mdh-2 Locus in DROSOPHILA PSEUDOOBSCURA

We have studied differences in the number of Drosophila pseudoobscura produced in a culture when the flies differ with respect to two alleles (F and S) at the Mdh-2 locus, which codes for a malate dehydrogenase enzyme. The studies were done at low and at high density in two- and three-genotype combinations (S/S, F/F and S/F), with one-genotype cultures as controls.——Density affects the fitness of the Mdh-2 genotypes. Different genotypes are differently affected, and the genotype of the competitors also makes a difference on the fitness of a given genotype. When three genotypes are present in a culture, particularly at high density, intergenotypic competition is less intense than intragenotypic competition at several frequency combinations. That is, there is "overcompensation": the three genotypes together exploit the environmental resources better than one genotype alone.—The fitness of the genotypes is frequency dependent in both two-genotype and three-genotype combinations. An inverse relationship between frequency and fitness is observed at high density. This may lead to a stable polymorphism, because the fitness of a genotype increases as its frequency decreases.—Forty independent strains, sampled from a natural population, were used in the experiments. This ensures that more than 95% of the variation present in the genome in the natural population is also present is the experimental cultures. It also ensures that the genetic background of the Mdh-2 alleles is randomized in the same way as it is in nature. However, the possibility remains that Mdh-2 alleles in nature are nonrandomly associated with alleles at closely linked loci. If linkage disequilibrium is present in the experiments because it exists in nature, then the observed effects (such as frequency-dependent selection) would affect the Mdh-2 locus in nature as well.     

Polymorphism for Dimerizing Ability at the Esterase-5 Locus in DROSOPHILA PSEUDOOBSCURA

Three new alleles are reported at the esterase-5 locus of Drosophila pseudoobscura. All three of these alleles are different from those previously reported in their ability to dimerize. One allele will not form heterodimers or homodimers and exists only as a monomer. A second allele does form heterodimers but will not form homodimers. The third allele forms both hetero- and homodimers as well as forming monomers. Estimates of the frequency of these alleles in a natural population are given. The existence of these is discussed with respect to recently proposed models for a molecular mechanism for heterosis.     

Variation in Biochemical Properties of Allozymes of Xanthine Dehydrogenase in DROSOPHILA PSEUDOOBSCURA

Twenty-six D. pseudoobscura strains isogenic for xanthine dehydrogenase alleles from Mesa Verde, Colorado, were tested for differences in the biochemical properties of different allelic forms of xanthine dehydrogenase. No significant differences in binding affinity (K_m) or substrate specificity of the enzyme were found. Significant variation among strains, in activity (V_max) and among electromorphs, as well as among strains, in thermolability was found. For the few strains tested, the activity and thermolability differences were shown to co-segregate with the electrophoretic mobility of the variant allele.     

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.