hobo transposable elements in Drosophila melanogaster and D. simulans.

Genomic patterns of occurrence of the transposable element hobo are polymorphic in the sibling species Drosophila melanogaster and D. simulans. Most tested strains of both species have apparently complete (3.0 kb) and smaller hobo elements (H lines), but in both species some strains completely lack such canonical hobo elements (E lines). The occurrence of H and E lines in D. simulans as well as in D. melanogaster implies that an hypothesis of recent introduction in the latter species is inadequate to explain the phylogenetic occurrence of hobo. Particular internally deleted elements, the approximately 1.5 kb Th1 and Th2 elements, are abundant in many lines of D. melanogaster, and an analogous 1.1 kb internally deleted element, h del sim, is abundant in most lines of D. simulans. Besides the canonical hobo sequences, both species (and their sibling species D. sechellia and D. mauritiana) have many hobo-hybridizing sequences per genome that do not appear to be closely related to the canonical hobo sequence.     

Inviability of hybrids between D. melanogaster and D. simulans results from the absence of simulans X not the presence of simulans Y chromosome.

Interspecific crosses between D. melanogaster and D. simulans or its sibling species result in unisexual inviability of the hybrids. Mostly, crosses of D. melanogaster females x D. simulans males produce hybrid females. On the other hand, only hybrid males are viable in the reciprocal crosses. A classical question is the cause of the unisexual hybrid inviability on the chromosomal level. Is it due to the absence of a D. simulans X chromosome or is it due to the presence of a D. simulans Y chromosome? A lack of adequate chromosomal rearrangements available in D. simulans has made it difficult to answer this question. However, it has been assumed that the lethality results from the absence of the D. simulans X rather than the presence of the D. simulans Y. Recently I synthesized the first D. simulans compound-XY chromosome that consists of almost the entire X and Y chromosomes. Males carrying the compound-XY and no free Y chromosome are fertile. By utilizing the compound-XY chromosome, the viability of hybrids with various constitutions of cytoplasm and sex chromosomes has been examined. The results consistently demonstrate that the absence of a D. simulans X chromosome in hybrid genome, and not the presence of the Y chromosome, is a determinant of the hybrid inviability.     

Inviability of hybrids between D. melanogaster and D. simulans results from the absence of simulans X not the presence of simulans Y chromosome

Interspecific crosses between D. melanogaster and D. simulans or its sibling species result in unisexual inviability of the hybrids. Mostly, crosses of D. melanogaster females X D. simulans males produce hybrid females. On the other hand, only hybrid males are viable in the reciprocal crosses. A classical question is the cause of the unisexual hybrid inviability on the chromosomal level. Is it due to the absence of a D. simulans X chromosome or is it due to the presence of a D. simulans Y chromosome? A lack of adequate chromosomal rearrangements available in D. simulans has made it difficult to answer this question. However, it has been assumed that the lethality results from the absence of the D. simulans X rather than the presence of the D. simulans Y. Recently I synthesized the first D. simulans compound-XY chromosome that consists of almost the entire X and Y chromosomes. Males carrying the compound-XY and no free Y chromosome are fertile. By utilizing the compound-XY chromosome, the viability of hybrids with various constitutions of cytoplasm and sex chromosomes has been examined. The results consistently demonstrate that the absence of a D. simulans X chromosome in hybrid genome, and not the presence of the Y chromosome, is a determinant of the hybrid inviability.     

The Regulation of Aldehyde Oxidase in Imaginal Wing Discs of Drosophila Hybrids: Evidence for cis- and trans-Acting Control Elements

The aldehyde oxidase (Aldox) distribution pattern was determined for wing discs of partial hybrids between D. melanogaster and D. simulans. In these animals the regulation of Aldox activity is not uniform over the disc epithelium as both cis-dominant and trans -acting control were evident in different regions of the disc. The Aldox expression was shown to be regulated by loci on the X chromosome, 2L and 3R of D. melanogaster and 2R and 3R of D. simulans.     

Comparison of alcohol dehydrogenase expression in Drosophila melanogaster and D. simulans

Alcohol dehydrogenase (ADH) gene expression was analyzed in Drosophila melanogaster and its sibling species D. simulans. The levels of ADH activity, ADH-cross-reacting material (CRM), and ADH-mRNA were analyzed for several strains of each species, which derive from diverse geographic locations around the world. There is considerable quantitative variation in ADH activity, CRM level, and RNA level among strains within species at all developmental stages. However, the only consistent differences between the two species are in pupal RNA level and in late-adult activity and CRM level. Late-adult melanogaster flies that are homozygous for the Slow allozyme have approximately twice the level of ADH activity and CRM as do simulans flies. The regression of activity on CRM over strains is highly significant and essentially the same for each species, which means that most, if not all, of the activity difference between the species is due to a difference in concentration of the ADH protein. In contrast, there is no significant regression of CRM level on mRNA level in adults of either species; nor is there a significant difference in RNA level between species. Therefore, the difference in ADH protein concentration is not due to RNA template availability. Thus, the interspecific difference in ADH level in adults must be due either to a difference in the rate of translation of the two RNAs or to a difference in protein stability.     

Polymorphism and divergence at the prune locus in Drosophila melanogaster and D. simulans

The prune locus of Drosophila melanogaster lies at the tip of the X chromosome, in a region of reduced recombination in which nearby loci show reduced variation relative to evolutionary divergence from D. simulans. DNA sequencing of prune alleles from D. melanogaster and D. simulans reveals extremely low variation in D. melanogaster but greater variation in D. simulans. Divergence between the two species is not reduced. This pattern may be explained by either positive selection leading to hitchhiking of neutral variation or background selection against deleterious mutations. The pattern of silent versus replacement polymorphism and divergence at prune is consistent with either a model of weakly deleterious selection against amino acid substitutions or balancing selection.      

Comparative Life Histories and Ecophysiology of Drosophila melanogaster and D. simulans

Numerous laboratory investigations have compared Drosophila melanogaster and D. simulans for various life history traits and fitness related ecophysiological parameters. From presently available information, it is however difficult to get a general comparative pattern describing the divergence of their ecological niches and understanding their demographic success. Two environmental factors seem however to have played a major role: temperature and alcoholic resources. From an ecophysiological approach, D. simulans may be described as generally more sensitive to stresses; other results point to this species as more cold adapted than its sibling; in some cases, however, D. simulans may appear as better adapted to a warm environment. When investigated, ecophysiological traits show a lesser geographic variability in D. simulans than in D. melanogaster. Presently available information does not explain the ecological prevalence of D. simulans in many places with a mild temperate or subtropical climate. This is presumably due to the fact that most comparisons have been done at a single, standard temperature of 25 degrees C. Comparative studies should be undertaken, spanning the thermal ranges of the two species, and the phenotypic plasticity of ecophysiological traits should now be considered.     

Variation in response to CO2 in Drosophila melanogaster and D. simulans

A type of slow-recovery from high temperature CO2 exposure is a wild-type characteristic of Drosophila simulans. Species hybrids of wild-type D. melanogaster and D. simulans have intermediate recovery times. The response to nitrogen exposure at both high and low temperatures is the same for the two species suggesting that the observed CO2 response is more than just anoxia. Since there is no female bias in hybrids, and since injections of D. simulans extract into D. melanogaster produced no increase in CO2 sensitivity in the recipients, we conclude that the prolonged recovery time results from genomic differences between D. simulans and D. melanogaster, and is not due to a sigma-like infectious agent.     

Low genic variation in male-reproductive-tract proteins of Drosophila melanogaster and D. simulans

We report results, using two-dimensional gel electrophoresis (2DE), of natural population surveys of allelic variation in approximately 300 male-reproductive-tract polypeptides in both Drosophila melanogaster and its sibling species, D. simulans. Despite our efforts to maximize operational sensitivity of our 2DE gels to polymorphism, variation estimates in both species were low (proportion of polymorphic loci [P] = 9%, and average heterozygosity [H] = 1%-3%), compared with those by one-dimensional gel electrophoresis (1DE) (P = 29%-55%; H = 8%-19%) in the same populations. However, H of polymorphic loci was very similar for 2DE and 1DE proteins; and for 17 of a total of 54 polymorphic proteins, 2DE detected three or four distinct alleles. The results suggest that the differing levels of variability widely seen with 1DE and 2DE are real and reflect differing intensities of functional constraint between different classes of structural loci. However, the alternative possibility remains that 2DE has a greater between-locus unevenness of variant detection sensitivity than does 1DE.     

Variable post‐zygotic isolation in Drosophila melanogaster/D. simulans hybrids

The study of hybrid inviability reveals cryptic divergence between the genetic interactions that maintain stable phenotypes in the pure species . We characterized the effects of natural variation on the penetrance of hybrid inviability phenotypes in crosses between Drosophila melanogaster and two species of the D. simulans subcomplex, D. simulans and D. sechellia. Using a panel of wild‐caught lines, we studied the levels of genetic variance present in D. simulans and D. sechellia affecting prezygotic and post‐zygotic isolation in hybridizations with D. melanogaster females. We observed extensive variability in the viability of hybrid individuals, dependent on the genotype of the parents, suggesting that intraspecific natural variation manifests directly in hybrid phenotypes. Furthermore, we found that genetic background significantly affects the penetrance of a well‐studied determinant of hybrid inviability: the interaction between Hmr^mel–Lhr^sim. Our results suggest that hybrid inviability – and reproductive isolation generally – can be modified by polymorphisms at multiple loci segregating within the parental species. Just as the penetrance of most mutant phenotypes can be modified by the genetic background within the pure species, the penetrance of hybrid inviability phenotypes is highly influenced by the parental genotypes.     

Polymorphism in structure of the retrotransposable element 412 in Drosophila simulans and D. melanogaster populations.

The structure of the 412 retrotransposable element was investigated in various natural populations of D. melanogaster and D. simulans by a restriction enzyme analysis. We show that although the canonical structure of the 412 element was the same in both species, a high structural polymorphism existed with various rearranged elements. A 412 family was thus composed of heterogeneous copies of different sizes, with a large proportion of full-size copies. D. simulans had more rearranged copies than D. melanogaster, with some specific copies, such as a 5.6-kb BsrBI fragment, present in all populations of D. simulans. Full-size and rearranged copies were detected in both the euchromatin and the heterochromatin, with many rearranged copies in D. simulans, suggesting a recent mobilization of the 412 element in this species.     

Genetics of Drosophila simulans male mating discrimination in crosses with D. melanogaster

The genetic bases of sexual isolation between Drosophila melanogaster and D. simulans have been mainly studied in females, and there is little information about the role of the males in interspecific mating discrimination. Using D. simulans synthetic lines with compound chromosomes from a population of the Seychelles Islands (high frequency of interspecific mating) and a multimarker strain (low frequency), we show that D. simulans males play an important role in discriminating D. melanogaster females. The genetics of male discrimination fits well with the inheritance mode of a single locus, dominant for sexual isolation, located in chromosome II near the net mutation (2L-0.0). The heterospecific mating success of the male was not related to his sexual vigor. The specific load of male cuticular hydrocarbons was counted as a possible source of discrimination used by the D. melanogaster female.     

Response to selection for increased hybridization between Drosophila melanogaster females and D. simulans males.

Females of Drosophila melanogaster and males of D. simulans hybridizing in a nonchoice condition were artificially selected for 12 generations. The frequency of hybridization increased from 10% to 79%. Response to selection occurred in both species, particularly in D. melanogaster. Female receptivity was the primary sexual trait that accounted for breaking up sexual isolation in these species, but it remained unclear which elements of the D. simulans male courtship were involved.     

Contrasting molecular population genetics of four hexokinases in Drosophila melanogaster, D. simulans and D. yakuba

As part of a larger study contrasting patterns of variation in regulatory and nonregulatory enzymes of the central metabolic pathways we have examined the molecular variation in four uncharacterized hexokinase genes unique to muscle, fat body, and testis in Drosophila melanogaster, D. simulans, and D. yakuba. Earlier isoenzyme studies had designated these genes as Hex-A, Hex-C, and Hex-t. There are two tightly linked testes-specific genes designated here as Hex-t1 and Hex-t2. Substantial and concordant differences across species are seen in levels of both amino acid and silent polymorphism. The flight muscle form Hex-A is the most conserved followed by the fat body hexokinase Hex-C and testis-specific hexokinases Hex-t1 and Hex-t2. While constraints acting at the amino acid level are expected, the silent polymorphisms follow this pattern as well. All genes are in regions of normal recombination, therefore hitchhiking and background selection are not likely causes of interlocus differences. In D. melanogaster latitudinal clines are seen for amino acid polymorphisms at the Hex-C and Hex-t2 loci. There is evidence for accelerated amino acid substitution in Hex-t1 that has lost residues known to be associated with glucose and glucose-6-phosphate binding. D. simulans shows substantial linkage phase structuring that suggests historical population subdivision.     

Male sterility thermal thresholds in Drosophila: D. simulans appears more cold-adapted than its sibling D. melanogaster

Numerous different criteria may be used for analysing species thermal adaptation. We compared male sterility thresholds in the two most investigated cosmopolitan siblings, D. melanogaster and D. simulans. A survey of various populations from Europe and North Africa evidenced consistent differences between the two species, and a detailed analysis was made on flies from Marrakech. Sharp sterility thresholds were observed in both species but at different temperatures: D. simulans appeared more tolerant to cold than its sibling (difference 1°C) but more sensitive to heat (difference 1.5°C). When transferred to an optimum temperature of 21°C, D. simulans males, sterilized by a low temperature, recovered more rapidly than males of D. melanogaster; the reverse was true on the high temperature side. The analysis of progeny number also revealed the better tolerance of D. simulans males to cold but a lesser tolerance to heat. From these observations, we might expect that D. simulans should be more successful in cold temperate countries than its sibling, while ecological observations point to the contrary. Our data clearly show the difficulty of comparing ecophysiological data to field observations, and also the need of extensive comparative life history studies in closely related species.