Hybrid Lethal Systems in the Drosophila Melanogaster Species Complex. II. the Zygotic Hybrid Rescue (Zhr) Gene of D. Melanogaster

Hybrid females from Drosophila simulans females X Drosophila melanogaster males die as embryos while hybrid males from the reciprocal cross die as larvae. We have recovered a mutation in melanogaster that rescues the former hybrid females. It was located on the X chromosome at a position close to the centromere, and it was a zygotically acting gene, in contrast with mhr (maternal hybrid rescue) in simulans that rescues the same hybrids maternally. We named it Zhr (Zygotic hybrid rescue). The gene also rescues hybrid females from embryonic lethals in crosses of Drosophila mauritiana females X D. melanogaster males and of Drosophila sechellia females X D. melanogaster males. Independence of the hybrid embryonic lethality and the hybrid larval lethality suggested in a companion study was confirmed by employing two rescue genes, Zhr and Hmr (Hybrid male rescue), in doubly lethal hybrids. A model is proposed to explain the genetic mechanisms of hybrid lethalities as well as the evolutionary pathways.     

Insertional DNA and spontaneous mutation at the white locus in Drosophila simulans

A large body of data on molecular analyses of several multiallelic loci in Drosophila melanogaster has demonstrated a high incidence of mobile DNA element insertions among spontaneous mutations. In the sibling species D. simulans, the dispersed, middle repetitive, nomadic sequences are reduced to about one-seventh that of its sibling species (Dowsett and Young 1982). Does this reduced amount of middle repetitive DNA (or mobile DNA sequences) mean that in D. simulans the occurrence of insertion mutants will be rare compared with that of D. melanogaster? To test this possibility, we collected seven different spontaneous white mutants of D. simulans and studied their molecular gene structures. Five out of seven mutants had insertion sequences which varied in length from 0.4 kb to 16 kb. One bore a deletion spanning the w region and another showed no gross structural alteration. Thus the proportion of insertional mutations at the white locus in D. simulans is equivalent to that observed in D. melanogaster. Among the five insertional mutants, one, wmky, showed genetic instability; the other four were stable. wmky was found to mutate at a frequency of 2.1 x 10(-5) in meiotic cells and may also be unstable in somatic cells.     

Comparison of lethal mutations of Drosophila melanogaster with D. simulans when detected by the attached-X method.

The purpose of this study was to evaluate the attached-X method compared with the standard Basc method, and, using this method, to find out whether the observed differences in genetic polymorphisms are related to differences in lethal mutation rates in D. melanogaster and D. simulans. When EMS-treated Drosophila melanogaster males are mated to untreated attached-X females, a decrease in the progeny sex ratio (male/female + male) is observed due to the induced lethal mutations on the X chromosome. The decrease in the frequency of male progeny were shown as the attached-X index. The expected male number is calculated from the control sex ratio. The difference between the expected and the observed male numbers, expressed as the ratio to the expected male number, defines the attached-X index. The index values for various EMS concentrations were compared to the lethal frequencies obtained by the standard Basc method for the same EMS treatments, and gave a highly positive correlation (gamma = 0.993, p < 0.01, d.f. = 2), thus providing an alternative method for evaluation of possible mutagens. The attached-X method was applied to D. simulans, of which natural populations are known to have relatively low genetic variation, and frequencies of the EMS-induced X chromosome lethal mutations were estimated and compared with those in D. melanogaster. The results indicate that D. melanogaster is slightly more sensitive in the sperm and spermatogonial stages, but less susceptible in the spermatid stage when compared with D. simulans.(ABSTRACT TRUNCATED AT 250 WORDS)     

Misregulation of sex-lethal and disruption of male-specific lethal complex localization in Drosophila species hybrids

A major model system for the study of evolutionary divergence between closely related species has been the unisexual lethality resulting from reciprocal crosses of Drosophila melanogaster and D. simulans. Sex-lethal (Sxl), a critical gene for sex determination, is misregulated in these hybrids. In hybrid males from D. melanogaster mothers, there is an abnormal expression of Sxl and a failure of localization of the male-specific lethal (MSL) complex to the X chromosome, which causes changes in gene expression. Introduction of a Sxl mutation into this hybrid genotype will allow expression of the MSL complex but there is no sequestration to the X chromosome. Lethal hybrid rescue (Lhr), which allows hybrid males from this cross to survive, corrects the SXL and MSL defects. The reciprocal cross of D. simulans mothers by D. melanogaster males exhibits underexpression of Sxl in embryos.     

Why There is a One-way Crossability Between D. melanogaster and D. simulans?

In the Drosophila melanogaster complex, females D. melanogaster mate relatively easily with males Drosophila simulans but the reciprocal cross is rare. The species sexual isolation is mainly based on chemical and acoustic signal exchanges between partners. The male side of this communication is investigated in this paper in order to understand the asymmetry. In D. melanogaster the acoustic signature is highly significant, and is synergistically reinforced by the chemical signal. In D. simulans the importance of the two signaling channels seems to be reversed. This could explain why D. simulans males produce less precise interpulse interval (IPI) mean value in the courtship song, which can readily overlap those of D. melanogaster. As the males of the two species use the same chemical key, D. simulans males could be recognized by D. melanogaster females as a conspecific.     

Evolution of a desaturase involved in female pheromonal cuticular hydrocarbon biosynthesis and courtship behavior in Drosophila

Drosophila species exhibit polymorphism in female pheromonal cuticular hydrocarbons, with 7-monoenes produced in Drosophila simulans and 7,11-dienes in most populations of Drosophila melanogaster (5,9-dienes in several African populations). A female-biased desaturase, desatF, expressed only in D. melanogaster is involved in the synthesis of 7,11-dienes. We investigated the role of desatF in 5,9-diene flies. We constructed a 5,9-diene strain knock-down for desatF and showed that desatF is involved in 5,9-diene formation. We also studied D. melanogaster/D. simulans hybrids. These hybrid females produced dienes and received normal courtship from D. melanogaster males, but copulation success was reduced. With D. simulans males, courtship was decreased and no copulation occurred. Hybrids with a chromosomal deletion of the D. melanogaster desatF gene had no dienes and received normal courtship from D. simulans males; depending on the D. simulans parental strain, 7-19% of them succeeded in mating. D. simulans desatF promoter region shows 21-23% gaps and 86-89% identity with D. melanogaster promoter region, the coding region 93-94% identity, depending on the strain. These differences could explain the functional polymorphism of desatF observed between both species, contributing to different cuticular hydrocarbon profiles, that constitute an effective barrier between species.     

Identical satellite DNA sequences in sibling species of Drosophila

The evolution of simple satellite DNAs was examined by DNA-DNA hybridization of ten Drosophila melanogaster satellite sequences to DNAs of the sibling species, Drosophila simulans and Drosophila erecta. Seven of these repeat types are present in tandem arrays in D. simulans and each of the ten sequences is repeated in D. erecta. In thermal melts, six of the seven satellite sequences in D. simulans and seven of the ten sequences in D. erecta melted within 1 deg.C of the corresponding values in D. melanogaster. The remaining sequences melted within 3 deg.C of the homologous hybrids. Therefore, there is little or no alteration in those satellite sequences held in common, despite a period of about ten million years since the divergence of D. melanogaster and D. simulans from a common ancestor. Simple satellite sequences appear to be more highly conserved than coding regions of the genome, on a per nucleotide basis. Since multiple copies of three satellite sequences could not be detected in D. simulans yet are present in D. erecta, a species more distantly related to D. melanogaster than is D. simulans, these sequences show discontinuities in evolution. There were major quantitative variations between species, showing that satellite DNAs are prone to massive amplification or diminution events over timespans as short as those separating sibling species. In D. melanogaster, these sequences amount to 21% of the genome but only 5% in D. simulans and 0.4% in D. erecta. There was a general trend of lower abundance with evolutionary distance for most satellites, suggesting that the amounts of different satellite sequences do not vary independently during evolution.     

Incompatibility between X chromosome factor and pericentric heterochromatic region causes lethality in hybrids between Drosophila melanogaster and its sibling species

The Dobzhansky-Muller model posits that postzygotic reproductive isolation results from the evolution of incompatible epistatic interactions between species: alleles that function in the genetic background of one species can cause sterility or lethality in the genetic background of another species. Progress in identifying and characterizing factors involved in postzygotic isolation in Drosophila has remained slow, mainly because Drosophila melanogaster, with all of its genetic tools, forms dead or sterile hybrids when crossed to its sister species, D. simulans, D. sechellia, and D. mauritiana. To circumvent this problem, we used chromosome deletions and duplications from D. melanogaster to map two hybrid incompatibility loci in F(1) hybrids with its sister species. We mapped a recessive factor to the pericentromeric heterochromatin of the X chromosome in D. simulans and D. mauritiana, which we call heterochromatin hybrid lethal (hhl), which causes lethality in F(1) hybrid females with D. melanogaster. As F(1) hybrid males hemizygous for a D. mauritiana (or D. simulans) X chromosome are viable, the lethality of deficiency hybrid females implies that a dominant incompatible partner locus exists on the D. melanogaster X. Using small segments of the D. melanogaster X chromosome duplicated onto the Y chromosome, we mapped a dominant factor that causes hybrid lethality to a small 24-gene region of the D. melanogaster X. We provide evidence suggesting that it interacts with hhl(mau). The location of hhl is consistent with the emerging theme that hybrid incompatibilities in Drosophila involve heterochromatic regions and factors that interact with the heterochromatin.     

Genetics of Hybrid Inviability and Sterility in Drosophila: Dissection of Introgression of D. simulans Genes in D. melanogaster Genome

Interspecific crosses between Drosophila melanogaster and Drosophila simulans usually produce sterile unisexual hybrids. The barrier preventing genetic analysis of hybrid inviability and sterility has been taken away by the discovery of a D. simulans strain which produces fertile female hybrids. D. simulans genes in the cytological locations of 21A1 to 22C1-23B1 and 30F3-31C5 to 36A2-7 have been introgressed into the D. melanogaster genetic background by consecutive backcrosses. Flies heterozygous for the introgression are fertile, while homozygotes are sterile both in females and males. The genes responsible for the sterility have been mapped in the introgression. The male sterility is caused by the synergistic effect of multiple genes, while the female sterility genes have been localized to a 170 kb region (32D2 to 32E4) containing 20 open reading frames. Thus, the female sterility might be attributed to a single gene with a large effect. We have also found that the Lethal hybrid rescue mutation which prevents the inviability of male hybrids from the cross of D. melanogaster females and D. simulans males cannot rescue those carrying the introgression, suggesting that D. simulans genes maybe non-functional in this hybrid genotype. The genes responsible for the inviability have not been separated from the female sterility genes by recombination.     

Molecular identification of the active ninja retrotransposon and the inactive aurora element in Drosophila simulans and D. melanogaster.

How transposable elements evolve is a key facet in understanding of spontaneous mutation and genomic rearrangements in various organisms. One of the best ways to approach this question is to study a newly evolved transposable element whose presence is restricted to a specific population or strain. The retrotransposons ninja and aurora may provide insights into the process of their evolution, because of their contrasting characteristics, even though they show high sequence identity. The ninja retrotransposon was found in a Drosophila simulans strain in high copy number and is potent in transposition. On the other hand, aurora elements are distributed widely among the species belonging to the Drosophila melanogaster species complex, but are immobile at least in D. melanogaster. In order to distinguish the two closely resembled retrotransposons by molecular means, we determined and compared DNA sequence of the elements, and identified characteristic internal deletions and nucleotide substitutions in 5'-long terminal repeats (LTR). Analyses of the structure of ninja homologs and LTR sequences amplified from both genomic and cloned DNA revealed that the actively transposable ninja elements were present only in D. simulans strains, but inactive aurora elements exist in both D. melanogaster and D. simulans.     

A Genetic Basis for the Inviability of Hybrids between Sibling Species of Drosophila

A mutation of Drosophila melanogaster whose only known effect is the rescue of otherwise lethal interspecific hybrids has been characterized. This mutation, Hmr, maps to 1-31.84 (9D1-9E4). Hmr may be the consequence of a P element insertion. It rescues hybrid males from the cross of D. melanogaster females to males of its three sibling species, D. simulans, D. mauritiana and D. sechellia. This rescue is recessive, since hybrid males that carry both Hmr and a duplication expected to be Hmr+ are not rescued. Hmr also rescues the otherwise inviable female hybrids from the cross of compound-X D. melanogaster females to males of its sibling species. This rescue is also recessive, since a compound-X heterozygous for Hmr does not rescue. Another mutation, discovered on the In(1)AB chromosome of D. melanogaster, is also found to rescue normally inviable species hybrids: unlike Hmr, however, In(1)AB rescues hybrid females from the cross of In(1)AB/Y males to sibling females, as well as hybrid males from the cross of In(1)AB females to sibling males. These data are interpreted on the basis of a model for the genetic basis of hybrid inviability of complementary genes.     

Genetic and Maternal Variation for Heat Resistance in Drosophila from the Field

In Drosophila, field heritability estimates have focused on morphological traits and ignored maternal effects. This study considers heritable variation and maternal effects in a physiological trait, heat resistance. Drosophila were collected from the field in Melbourne, Australia. Resistance was determined using knock-down time at 37°. Drosophila melanogaster was more resistant than Drosophila simulans, and males tended to be more resistant than females. Field heritability and maternal effects were examined in D. simulans using the regression of laboratory-reared F(1) and F(2) onto field-collected parents. Males from the field were crossed to a laboratory stock to obtain progeny. The additive genetic component to variation in heat resistance was large and significant, and heritability was estimated to be around 0.5. A large maternal effect was also evident. Comparisons of regression coefficients suggested that the maternal effect was not associated with cytoplasmic factors. There was no correlation between body size (as measured by wing length) and heat resistance. Unlike in the case of morphological traits, the heritability for heat resistance in nature is not less than that measured in the laboratory.     

Nup96-dependent hybrid lethality occurs in a subset of species from the simulans clade of Drosophila

The cross of Drosophila melanogaster females to D. simulans males typically produces lethal F(1) hybrid males. F(1) male lethality is suppressed when the D. simulans Lhr(1) hybrid rescue strain is used. Viability of these F(1) males carrying Lhr(1) is in turn substantially reduced when the hybrids are heterozygous for some mutant alleles of the D. melanogaster Nup96 gene. I show here that similar patterns of Nup96-dependent lethality occur when other hybrid rescue mutations are used to create F(1) males, demonstrating that Nup96 does not reduce hybrid viability by suppressing the Lhr(1) rescue effect. The penetrance of this Nup96-dependent lethality does not correlate with the penetrance of the F(1) hybrid rescue, arguing that these two phenomena reflect genetically independent processes. D. simulans, together with two additional sister species, forms a clade that speciated after the divergence of their common ancestor from D. melanogaster. I report here that Nup96(-) reduces F(1) viability in D. melanogaster hybrids with one of these sister species, D. sechellia, but not with the other, D. mauritiana. These results suggest that Nup96-dependent lethality evolved after the speciation of D. melanogaster from the common ancestor of the simulans clade and is caused by an interaction among Nup96, unknown gene(s) on the D. melanogaster X chromosome, and unknown autosomal gene(s), at least some of which have diverged in D. simulans and D. sechellia but not in D. mauritiana. The genetic properties of Nup96 are also discussed relative to other hybrid lethal genes.     

Sensitivity of Drosophila melanogaster to low concentrations of gaseous mutagens. II. Chronic exposures.

Sex-linked recessive lethal mutations were induced in Drosophila melanogaster males by gaseous 1,2-dibromoethane at concentrations ranging from 0.2 to 2 parts per million. Significant numbers of mutations could be induced at all these concentrations. Pronounced germ-cell sensitivity differences were observed. For low exposures, spermatids and spermatocytes were about 10--20 times more sensitive than spermatozoa. The dose-effect relation was linear below 60 ppm . h for the 3 cell types. At higher exposures, sterility prevented mutation detection in spermatocytes and in spermatogonia. The lowest effective exposure for spermatozoa was 18 ppm . h (0.25 ppm for 72 h). In spermatids, the lowest exposure tested, 2.3 ppm . h (0.2 ppm for 11 h) induced 4 times the spontaneous mutation rate. Therefore, using prolonged exposure periods one may be able to detect concentrations in the range of parts per billion. Thus, Drosophila appears suitable as a system for detecting very low concentrations of gaseous mutagens in industrial, agricultural and environmental atmospheres.     

Ecological and Genetic Interactions in Drosophila–parasitoids Communities: A Case Study with D. Melanogaster, D. Simulans and their Common Leptopilina Parasitoids in Southe-astern France

Drosophila species are attacked by a number of parasitoid wasps, which constitute an important factor of population regulation. Since Drosophila melanogaster and Drosophila simulans share common parasitoid species, their ecology and evolution can hardly be understood without considering parasitoids. After a short review of data available on Drosophila-parasitoid interactions involving D. melanogaster and D. simulans as hosts, we report field and laboratory experiments investigating the ecological role of Leptopilina parasitoids in Drosophila communities of southern France. Seasonal survey of species abundance shows that strong interspecific interactions occur at both tropic levels. D. simulans progressively replaces D. melanogaster in southern areas suggesting competitive displacement. Parasitoids are responsible for very high Drosophila mortality (up to 90% in some fruits). Field data emphasize the importance of selective pressure that parasitoids exert on Drosophila communities. The two Leptopilina parasites (L. heterotoma and L boulardi) have different local abundances, which vary in time, and they also compete for hosts. We show that parasitoids can mediate the coexistence of D. melanogaster and D. simulans in the laboratory, and thus may contribute to their puzzling coexistence in the field. Conversely, hosts exert selective pressures on parasitoids, and development on either D. melanogaster or D. simulans strongly affects fitness of adult wasps in a temperature-dependent fashion. Local variation in host species abundance and diversity could thus account for the genetic differentiation we observed in one parasitoid species. Despite laboratory studies cannot fully explain complex field situations, it is clear that the ecology and evolution of Drosophila populations and communities, especially D. melanogaster and D. simulans, are strongly constrained by parasitoids, which should receive more attention.     

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.     

DNA from Drosophila melanogaster region 61C7/C8 has high evolution rate

The results of a comparative study of cloned DNA fragments of Drosophila simulans, D. mauritiana, D. teissieri, and D. erecta are presented. The fragments were amplified in PCR with primers specified to the region of D. melanogaster interband 61C7/C8. The uniqueness of all cloned fragments in the genomes of these species was confirmed. A comparative analysis of nucleotide sequences revealed that the rate of evolution of DNA from D. melanogaster interband 61C7/C8 is close to the rate of neutral evolution in the genus Drosophila.     

A New Hybrid Rescue Allele in Drosophila melanogaster

Crosses of Drosophila melanogaster females to males of its sibling species Drosophila simulans, Drosophila mauritiana and Drosophila sechellia produce no sons and daughters that are viable only at low temperatures. We describe here a novel rescue allele Df(1)EP307-1-2 isolated on the basis of its suppression of high temperature hybrid female lethality. Df(1)EP307-1-2 also rescues hybrid males to the pharate adult stage, the same stage at which it is lethal to D. melanogaster pure species males. Molecular analysis indicates that Df(1)EP307-1-2 is associated with a deletion of about 61 kb in the 9D region of the X chromosome. The structure of Df(1)EP307-1-2 suggests that it was formed by a process similar to P-element induced male recombination.     

Neutral and Non-Neutral Evolution of Drosophila Mitochondrial DNA

To test hypotheses of neutral evolution of mitochondrial DNA (mtDNA), nucleotide sequences were determined for 1515 base pairs of the NADH dehydrogenase subunit 5 (ND5) gene in the mitochondrial DNA of 29 lines of Drosophila melanogaster and 9 lines of its sibling species Drosophila simulans. In contrast to the patterns for nuclear genes, where D. melanogaster generally exhibits much less nucleotide polymorphism, the number of segregating sites was slightly higher in a global sample of nine ND5 sequences in D. melanogaster (s = 8) than in the nine lines of D. simulans (s = 6). When compared to variation at nuclear loci, the mtDNA variation in D. melanogaster does not depart from neutral expectations. The ND5 sequences in D. simulans, however, show fewer than half the number of variable sites expected under neutrality when compared to sequences from the period locus. While this reduction in variation is not significant at the 5% level, HKA tests with published restriction data for mtDNA in D. simulans do show a significant reduction of variation suggesting a selective sweep of variation in the mtDNA in this species. Tests of neutral evolution based on the ratios of synonymous and replacement polymorphism and divergence are generally consistent with neutral expectations, although a significant excess of amino acid polymorphism within both species is localized in one region of the protein. The rate of mtDNA evolution has been faster in D. melanogaster than in D. simulans and the population structure of mtDNA is distinct in these species. The data reveal how different rates of mtDNA evolution between species and different histories of neutral and adaptive evolution within species can compromise historical inferences in population and evolutionary biology.