Contrasting patterns of geographic variation in the cosmopolitan sibling species Drosophila melanogaster and Drosophila simulans

An electrophoretic study was carried out to compare the geographic pattern of genetic variation in Drosophila simulans with that of its sibling species, Drosophila melanogaster. An identical set of 32 gene-protein loci was studied in four geographically distant populations of D. simulans and two populations of D. melanogaster, all originating from Europe and Africa. The comparison yielded the following results: (1) tropical populations of D. simulans were, in terms of the number of unique alleles, average heterozygosity per locus, and percentage of loci polymorphic, more variable than conspecific-temperate populations; (2) some loci in both species showed interpopulation differences in allele frequencies that suggest latitudinal clines; and (3) temperate-tropical genetic differentiation between populations was much less in D. simulans than in D. melanogaster. Similar differences between these two species have previously been shown for chromosomal, quantitative, physiological, and middle-repetitive DNA variation. Estimates of N _m (number of migrants per generation) from the spatial distribution of rare alleles suggest that both species have similar levels of interpopulation gene flow. These observations lead us to propose two competing hypotheses: the low level of geographic differentiation in D. simulans is due to its evolutionarily recent worldwide colonization and, alternatively, D. simulans has a narrower niche than D. melanogaster. Geographic variation data on different genetic elements (e.g., mitochondrial DNA, two-dimensional proteins, etc.) are required before these hypotheses can be adequately tested.We thank the Natural Science and Engineering Research Council of Canada for financial support (Grant A0235 to R.S.S.).     

Genetic,ontogenetic, and tissue-specific variation of dipeptidases in Drosophila melanogaster

Three dipeptidases in Drosophila melanogaster are under independent genetic control and their structural genes have been localized, Dip-A to 2R and Dip-B and Dip-C to 3R (Voelker and Langley, 1978; Ohnishi and Voelker, 1981). These enzymes were characterized with respect to their substrate specificities, genetic variability (electrophoretic mobility and quantitative activity level), ontogeny (activity and isozyme pattern), and tissue localization. The dipeptide substrate specificities of DIP-A and DIP-B overlap each other considerably, but do not overlap with DIP-C. In natural populations, DIP-B and DIP-C are essentially monomorphic electrophoretically whereas DIP-A is polymorphic for three allozymes. Both DIP-A and DIP-B show quantitative genetic variation of activity level within an allozyme class. All three enzymes are expressed at all stages in the life cycle, but DIP-A and DIP-B activities vary considerably according to developmental stage and sex of adult. The tissue localizations of DIP-A and DIP-B activities show similar patterns and a nearly ubiquitous occurrence of both enzymes, but with particularly high values in larval and adult midguts and in the adult female reproductive system. These results suggest a general metabolic role for the enzymes, such as regulation of the concentrated pools of amino acids and oligopeptides found in Drosophila tissues.This work was supported by Public Health Service Grant GM 11546.Paper No. 7066 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh.     

Electrophoretic variability in natural populations of Drosophila melanogaster and Drosophila simulans

Genetic variation at 59 gene loci coding for enzymes (50) and larval proteins (9) has been studied in sympatric populations of Drosophila melanogaster and D. simulans from insular and continental origin. The average number of alleles per locus, the mean proportion of polymorphic loci and the mean heterozygosity are similar both within and between species. There are however some significant differences between D. simulans populations in the genotypic frequencies for four polymorphic loci.     

Clonal analysis in hybrids between Drosophila melanogaster and Drosophila simulans

We have analysed the viability of cellular clones induced by mitotic recombination in Drosophila melanogaster/D. simulans hybrid females during larval growth. These clones contain a portion of either melanogaster or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome arm. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a significant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome that is homozygous, the less viable is the recombinant melano-gaster/simulans hybrid clone. The few clones that appeared may represent cases in which mitotic recombination took place in distal chromosome intervals, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster/simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe more) that cause hybrid lethality. Therefore, the two species, D. melanogaster and D. simulans, have diverged to such a degree that the absence of part of the genome of one species cannot be substituted by the corresponding part of the genome of the other, probably due to the formation of co-adapted gene complexes in both species following their divergent evolution after speciation. The disruption of those coadapted gene complexes would cause the lethality of the recombinant hybrid clones.     

Three purine auxotrophic loci on the second chromosome of Drosophila melanogaster

Mutations at three second-chromosomal loci of Drosophila melanogaster have been isolated, mapped, and shown to be purine nucleoside auxotrophs. Two of the loci, adenosine2 and adenosine3, located at map positions 18.4 and 20, respectively, produce mutations which are supplementable with adenine, adenosine, and inosine. Guanosine supplements mutations at the burgundy locus (55.7); this locus was described previously through a pteridine eye-color defect but identified as an auxotrophic locus after the isolation of a new allele, bur ^gua2-1 . The mutation ade2-1 also has defective pteridine metabolism.This work was supported by NSERC Grant A3269 to D. Nash and by an AHFMR graduate studentship to M. E. Johnstone.     

Dipeptidase-A: A polymorphic locus in Drosophila melanogaster

Dipeptidase A (Dip-A), a new peptidase locus in Drosophila melanogaster, is located on the second chromosome at map position 55.2 and in the 41A-B; 42A2-3 interval in the salivary gland chromosomes. Three alleles are reported. In the Carpenter, North Carolina population the allele frequencies are: Dip-A ⁶ (fastest)=0.064; Dip-A ⁴ (intermediate)=0.920; and Dip-A ² (slowest)=0.015.     

A Comprehensive Study of Genic Variation in Natural Populations of Drosophila melanogaster. III. Variations in Genetic Structure and Their Causes between Drosophila melanogaster and Its Sibling Species Drosophila simulans

The natural populations of Drosophila melanogaster and Drosophila simulans were compared for their genetic structure. A total of 114 gene-protein loci were studied in four mainland (from Europe and Africa) and an island (Seychelle) populations of D. simulans and the results were compared with those obtained on the same set of homologous loci in fifteen worldwide populations of D. melanogaster. The main results are as follows: (1) D. melanogaster shows a significantly higher proportion of loci polymorphic than D. simulans (52% vs. 39%, P<0.05), (2) both species have similar mean heterozygosity and mean number of alleles per locus, (3) the two species share some highly polymorphic loci but they do not share loci that show high geographic differentiation, and (4) D. simulans shows significantly less geographic differentiation than D. melanogaster. The differences in genetic differentiation between the two species are limited to loci located on the X and second chromosomes only; loci on the third chromosome show similar level of geographic differentiation in both species. These two species have previously been shown to differ in their pattern of variation for chromosomal polymorphisms, quantitative and physiological characters, two-dimensional electrophoretic (2DE) proteins, middle repetitive DNA and mitochondrial DNA. Variation in niche-widths and/or genetic "strategies" of adaptation appear to be the main causes of differences in the genetic structure of these two species.     

Patterns of puffing activity in the salivary gland chromosomes of Drosophila

Salivary gland X chromosome puffing patterns are described for the Oregon stock of Drosophila melanogaster and for the Berkeley stock of D. simulans. In D. melanogaster regular phase specific puffing was recorded at 21 loci in the third larval instar and subsequent prepupal stage. A comparison of the X chromosome puffing patterns of male and female larvae failed to show any qualitative differences although in the males a group of puffs were active for a longer time during development than in females. The X chromosome puffing patterns of D. simulans are similar to those described for D. melanogaster although two puffs (4F 1–4 and 7B 1–3) were active in D. simulans but not in D. melanogaster. The sex differences in puffing observed in D. melanogaster were also observed in D. simulans.     

Genetic and cytogenetic studies of four glycolytic enzymes in Drosophila melanogaster: Aldolase,triosephosphate isomerase, 3-phosphoglycerate kinase,and phosphoglucomutase

Four glycolytic enzymes in Drosophila melanogaster have been genetically and/or cytogenetically mapped. The structural gene for aldolase (Ald) has been genetically mapped to 3-91.5 and cytogenetically localized to 97A-B. Tpi, the structural gene for triosephosphate isomerase, has been genetically mapped to 3-101.3 and cytogenetically localized to 99B-E. Utilizing closer-flanking markers than the previous mapping, Pgk, the structural gene for 3-phosphoglycerate kinase, has been mapped to 2-5.9; cytogenetically it was found to lie in the interval between 22D and 23E3. The cytogenetic location of Pgm, the structural gene for phosphoglucomutase which has been located genetically at 3-43.4, was determined to be in 72D1-5.     

Structural and genetic studies of the proliferation disrupter genes of Drosophila simulans and D. melanogaster

To examine whether structural and functional differences exist in the proliferation disrupter (prod) genes between Drosophila simulans and D. melanogaster, we analyzed and compared both genes. The exon–intron structure of the genes was found to be the same – three exons were interrupted by two introns, although a previous report suggested that only one intron existed in D. melanogaster. The prod genes of D. simulans and D. melanogaster both turn out to encode 346 amino acids, not 301 as previously reported for D. melanogaster. The numbers of nucleotide substitutions in the prod genes was 0.0747 ±  per synonymous site and 0.0116 ± 0.0039 per replacement site, both comparable to those previously known for homologous genes between D. simulans and D. melanogaster. Genetic analysis demonstrated that D. simulans PROD can compensate for a deficiency of D. melanogaster PROD in hybrids. The PRODs of D. simulans and D. melanogaster presumably share the same function and a conserved working mechanism. The prod gene showed no significant interaction with the lethality of the male hybrid between these species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparative population genomics of latitudinal variation in Drosophila simulans and Drosophila melanogaster

Examples of clinal variation in phenotypes and genotypes across latitudinal transects have served as important models for understanding how spatially varying selection and demographic forces shape variation within species. Here, we examine the selective and demographic contributions to latitudinal variation through the largest comparative genomic study to date of Drosophila simulans and Drosophila melanogaster, with genomic sequence data from 382 individual fruit flies, collected across a spatial transect of 19 degrees latitude and at multiple time points over 2 years. Consistent with phenotypic studies, we find less clinal variation in D. simulans than D. melanogaster, particularly for the autosomes. Moreover, we find that clinally varying loci in D. simulans are less stable over multiple years than comparable clines in D. melanogaster. D. simulans shows a significantly weaker pattern of isolation by distance than D. melanogaster and we find evidence for a stronger contribution of migration to D. simulans population genetic structure. While population bottlenecks and migration can plausibly explain the differences in stability of clinal variation between the two species, we also observe a significant enrichment of shared clinal genes, suggesting that the selective forces associated with climate are acting on the same genes and phenotypes in D. simulans and D. melanogaster.     

Genetic and biochemical characterization of little isoxanthopterin (lix), a gene controlling dihydropterin oxidase activity in Drosophila melanogaster.

Summary Dihydropterin oxidase catalyses the oxidation of 7,8-dihydropteridines into their fully oxidized products, and is involved in the biosynthesis of isoxanthopterin. Fifteen Drosophila melanogaster mutants, selected for their low pterin and isoxanthopterin content, were assayed for dihydropterin oxidase activity. The activity was around 100% in most mutants tested, slightly reduced in red, g and dke, and undetectable in lix. In flies carrying various doses of the lix ⁺ allele, a correlation was found between enzyme activity and the number of lix ⁺ copies in the genome. The results suggest that lix is the structural gene for the dihydropterin oxidase enzyme. Isoxanthopterin was quantitated in strains carrying deficiencies for the region in which lix has been mapped by recombination. This allowed us to assign the lix locus to the 7D10-7171-2 segment of the X chromosome.     

Drosophila melanogaster males respond differently at the behavioural and genome‐wide levels to Drosophila melanogaster and Drosophila simulans females

Drosophila melanogaster are found in sympatry with Drosophila simulans, and matings between the species produce nonfertile hybrid offspring at low frequency. Evolutionary theory predicts that females choose mates, so males should alter their behaviour in response to female cues. We show that D. melanogaster males quickly decrease courtship towards D. simulans females. Courtship levels are reduced within 5 min of exposure to a heterospecific female, and overall courtship is significantly lower than courtship towards conspecific females. To understand changes at the molecular level during mate choice, we performed microarray analysis on D. melanogaster males that courted heterospecific D. simulans females and found nine genes have altered expression compared with controls. In contrast, males that court conspecific females alter expression of at least 35 loci. The changes elicited by conspecific courtship likely modulate nervous system function to reinforce positive conspecific signals and dampen the response to heterospecific signals.     

Genetic and biochemical studies on glutamate-pyruvate transaminase from Drosophila melanogaster

We have used electrophoretic variants of glutamate-pyruvate transaminase (GPT, E.C. 2.6.1.2) in Drosophila melanogaster to genetically map the structural gene to position 42.6 on the X chromosome. By pseudodominance tests over several deficiencies we have localized it cytogenetically to the interval 11Fl-2 to 12Al-2. The sedimentation constant (s _20,w) of the native enzyme was determined in sucrose density gradients to be 5.9 and the native molecular weight approximately 87,000. The similarity in physical properties to mammalian enzymes suggests that the enzyme may also be dimeric in D. melanogaster.     

A Genomic Comparison of Faster-Sex, Faster-X, and Faster-Male Evolution Between Drosophila melanogaster and Drosophila pseudoobscura

A genomic comparison of Drosophila melanogaster and Drosophila pseudoobscura provides a unique opportunity to investigate factors involved in sequence divergence. The chromosomal arrangements of these species include an autosomal segment in D. melanogaster which is homologous to part of the X chromosome in D. pseudoobscura. Using orthologues to calculate rates of nonsynonymous (d_N) substitutions, we found genes on the X chromosome to be significantly more diverged than those on the autosomes, but it is not true for segment 3L-XR which is autosomal in D. melanogaster (3L) and X-linked in D. pseudoobscura (XR). We also found that the median d_N values for genes having reproductive functions in either the male, the female, or both sexes are higher than those for sequences without reproductive function and even higher for sequences involved in male-specific function. These estimates of divergence for male sex-related sequences are most likely underestimates, as the very rapidly evolving reproductive genes would tend to lose homology sooner and thus not be included in the comparison of orthologues. We also noticed a high proportion of male reproductive genes among the othologous genes with the highest rates of d_N. Reproductive genes with and without an orthologue in D. pseudoobscura were compared among D. melanogaster, D. simulans, and D. yakuba and it was found that there were in fact higher rates of divergence in the group without a D. pseudoobscura orthologue. These results, from widely separated taxa, bolster the thesis that sexual system genes experience accelerated rates of change in comparison to nonsexual genes in evolution and speciation. [Reviewing Editor: Dr. Willie J. Swanson]     

Modifiers of position-effect variegation in the region from 86C to 88B of the Drosophila melanogaster third chromosome

Summary Four dominant suppressor and one enhancer of variegation loci were mapped in the polytene chromosome region extending from section 86C to section 88B of the Drosophila melanogaster third chromosome using a set of deficiencies. The suppressor locus Su-var(3) 14 maps in 86CD, Su-var(3) 13 in 86F4-7, Su-var(3)6 in 87B4-7 and Su-var(3)7 in 87E4-5. The enhancer locus E-var(3)3 maps in 87E12-F11. Su-var(3)13, Su-var(3)6 and Su-var(3)7 are also defined by point mutant alleles originally identified by other criteria (Reuter et al. 1986). Duplications covering the suppressor loci Su-var(3)14, Su-var(3)13, Su-var(3)6 and Su-var(3)7 were found to reduce considerably the haplo-abnormal effect of heterozygous point mutants of the corresponding loci. One suppressor locus, Su-var(3)7, maps within a region which has previously been cloned. The positions of deficiency breakpoints delimiting the suppressor locus indicate that all the necessary sequences for its function are located within 10 kb of cloned DNA.     

Studies on the Ribosomal RNA Cistrons in Interspecific Drosophila Hybrids. II. Heterochromatic Regions Mediating Nucleolar Dominance

Interspecific hybrids of D. melanogaster and D. simulans normally exhibit a secondary constriction only at the D. melanogaster nucleolus organizer (NO). This phenomenon, termed nucleolar dominance, occurs only when the NO-bearing sex chromosomes of both species are present in conjunction. Experiments were initiated to localize regions on the sex chromosomes of D. melanogaster involved in mediating this suppression. Sex chromosome heterochromatic rearrangements and deficiencies were introduced into F₁ hybrids and their corresponding effect on simulans NO constriction formation was examined in hybrid mitotic neuroblast tissue. Sex chromosomes deficient for both the D. melanogaster NO and adjacent heterochromatin were unable to restrict the formation of a constriction at the D. simulans NO. The presence of a D. melanogaster NO, however, was not sufficient for the establishment of nucleolar dominance. Results from an array of NO-bearing X and Y chromosome rearrangements and deficiencies indicate that at least one heterochromatic region, proximal to the NO on the D. melanogaster X and distal to the NO on the D. melanogaster Y, affects the induction of this interchromosomal phenomenon.     

Identification and mapping of three flower colour loci of potato (S. tuberosum L.) by RFLP analysis

Summary The inheritance of flower colour in diploid potato (2 n = 2x = 24), was found to be controlled by three unlinked loci D, F and P. To determine the allelism with previously described loci and to dissect this oligogenic trait, a set of tester clones with well-defined genotypes was developed. By backcrossing the mapping population with these tester clones it was possible to obtain monogenic segregation ratios. These were required to detect linkage with RFLP loci and, despite distorted Mendelian ratios, the inheritance and mapping of the D, F and P loci could be unambiguously determined. Locus D, involved in the biosynthesis of red anthocyanins, was mapped on chromosome 2, while locus P, involved in the production of blue anthocyanins, was mapped on chromosome 11. Locus F, involved in the flower-specific expression of gene(s) accommodated by the D and P loci, was mapped on chromosome 10. The tester clones and the map position of the D, F and P loci may be of considerable value in simplifying the genetics of anthocyanin pigmentation.     

Identification of One Intron Loss and Phylogenetic Evolution of Dfak Gene in the Drosophila melanogaster Species Group

Intron loss and its evolutionary significance have been noted in Drosophila. The current study provides another example of intron loss within a single-copy Dfak gene in Drosophila. By using polymerase chain reaction (PCR), we amplified about 1.3 kb fragment spanning intron 5–10, located in the position of Tyr kinase (TyK) domain of Dfak gene from Drosophila melanogaster species group, and observed size difference among the amplified DNA fragments from different species. Further sequencing analysis revealed that D. melanogaster and D. simulans deleted an about 60 bp of DNA fragment relative to other 7 Drosophila species, such as D. elegans, D. ficusphila, D. biarmipes, D. takahashii, D. jambulina, D. prostipennis and D. pseudoobscura, and the deleted fragment located precisely in the position of one intron. The data suggested that intron loss might have occurred in the Dfak gene evolutionary process of D. melanogaster and D. simulans of Drosophila melanogaster species group. In addition, the constructed phylogenetic tree based on the Dfak TyK domains clearly revealed the evolutionary relationships between subgroups of Drosophila melanogaster species group, and the intron loss identified from D. melanogaster and D. simulans provides a unique diagnostic tool for taxonomic classification of the melanogaster subgroup from other group of genus Drosophila.