Simulans Males Research Articles

Indirect evidence of alteration in the expression of the rDNA genes in interspecific hybrids between Drosophila melanogaster and Drosophila simulans.

Crosses between Drosophila melanogaster females and D. simulans males produce viable hybrid females, while males are lethal. These males are rescued if they carry the D. simulans Lhr gene. This paper reports that females of the wild-type D. melanogaster population Staket do not produce viable hybrid males when crossed with D. simulans Lhr males, a phenomenon which we designate as the Staket phenotype. The agent responsible for this phenomenon was found to be the Staket X chromosome (Xmel, Stk). Analysis of the Staket phenotype showed that it is suppressed by extra copies of D. melanogaster rDNA genes and that the Xmel, Stk chromosome manifests a weak bobbed phenotype in D. melanogaster Xmel, Stk/0 males. The numbers of functional rDNA genes in Xmel, Stk and Xmel, y w (control) chromosomes were found not to differ significantly. Thus a reduction in rDNA gene number cannot account for the weak bobbed Xmel, Stk phenotype let alone the Staket phenotype. The rRNA precursor molecules transcribed from the Xmel, Stk rDNA genes seem to be correctly processed in both intraspecific (melanogaster) and interspecific (melanogaster-simulans) conditions. It is therefore suggested that the Xmel, Stk rDNA genes are inefficiently transcribed in the melanogaster-simulans hybrids.

Mating propensity of Indian Drosophila melanogaster populations with D. simulans: a nonadaptive latitudinal cline.

Crosses were investigated between a single strain of Drosophila simulans and 28 natural Indian populations of D. melanogaster. In each case, a mass culture and 10 isofemale lines of D. melanogaster were studied. Crosses were much easier between D. melanogaster females and D. simulans males than in the reciprocal case. Also, hybrid offspring were easier to obtain by using D. melanogaster flies (either males or females) from isofemale lines than from mass cultures. Finally, the crossability, estimated by progeny production, showed a clear-cut latitudinal cline: hybrids were easier to obtain with D. melanogaster populations from higher latitudes. As D. simulans does not occur in India, this cline does not reflect an evolutionary interaction between the two sibling species.

P element-induced mutations in Drosophila melanogaster showing high interspecific crossability between D. melanogaster females and D. simulans males.

Homozygous stocks for the second or the third chromosome of Drosophila melanogaster with a single insertional plwB element were screened for high crossability with D. simulans. Reciprocal crosses between each of these stocks and D. simulans were made, and the insemination rate at two or three days was examined. From two cycles of screening of the original 575 stocks, one stock (# 687) which showed high insemination rate was selected and was backcrossed to a w strain to substitute the background. We obtained a stock which showed 10% insemination rate with D. simulans males (control was 0%). No stocks exhibiting a high crossability with D. simulans females were acquired. Revertant strains, from which the P element had been lost, were obtained from the backcrossed # 687 stock. The insemination rates of 13 revertants to D. simulans males ranged from 1% to 33%. Seven of these 13 were not significantly different from the control line but were significantly different from the backcrossed # 687 stock. It was concluded that the mutation showing high crossability with D. simulans males was caused by the P element transposition.

P element-induced mutations in Drosophila melanogaster showing high interspecific crossability between D. melanogaster females and D. simulans males

P element-induced mutations in Drosophila melanogaster showing high interspecific crossability between D. melanogaster females and D. simulans males

Epistatic interactions among the P element-induced high interspecific crossability strains in Drosophila melanogaster.

Five Drosophila melanogaster strains showing high interspecific crossability with D. simulans males, derived from the previous screening of a set of autosomal plwB transposants, were selected and the effect of the plwB insertion under the same genetic background (white strain) on the interspecific crossability was examined. This phenotype was recessive in the two strains but semidominant in the other three strains. Trans-heterozygotes, however, showed high interspecific crossability compared with the parental homozygotes, suggesting some epistatic interactions between them. In three strains, the effect of the plwB insertion region in different backgrounds (w1118 strain) on the crossability was also tested. Homozygotes of a strain (#687) showed high interspecific crossability comparable to the w background, while homozygotes of both #68 and #783 strains showed lower crossability than w1118. Although #783 heterozygotes showed intermediate values between #783 homozygotes and w1118, #68 heterozygotes showed a significantly higher insemination rate than w1118 and the #68 homozygotes. These results suggest that the region around the plwB insertion sites of #68 and #783 affects the interspecific crossability either positively or negatively depending on the genetic background. In all the stocks, positive correlation between interspecific crossability and the intraspecific mating speed was detected.

The effects of interspecific Y chromosome replacements on hybrid sterility within the Drosophila simulans clade.

We attempted to introgress Y chromosomes between three sibling species of Drosophila: D. simulans, D. sechellia and D. mauritiana. Four D. sechellia Y chromosomes were introgressed into D. simulans without loss of fertility whereas the four reciprocal introgressions (D. simulans Y introgressed into D sechellia) all result in sterility. Both reciprocal Y introgressions of D. simulans and D. mauritiana (four of each) also result in sterility. Compared with D. simulans males, the males with the D. sechellia Y chromosome in D. simulans background had lower productivity but only after multiple matings with virgin females. These males also were inferior compared with pure species males in sperm displacement and/or remating ability. The two different Y genotype males, however, were comparable in viability, longevity and mating success in female choice tests. We also use our results to estimate the effective number of autosomal loci interacting with X-linked genes to produce hybrid male sterility.

Sexual isolation between Drosophila melanogaster females and D. simulans males. Male mating propensities versus success in hybridization.

Genetic variation has been found in males of a D. simulans population for their eagerness to hybridize with D. melanogaster females. In a search for traits involved in this hybridization, males of D. simulans were tested for mating speed and sexual vigour. Between-male differences were detected in both sexual traits, but no relationship was noticed between them, nor with the frequency of hybridization. Thus male mating propensities appear to be unrelated to the breakdown of sexual isolation between these sibling species.

THE GENETICS OF AN ISOLATING MECHANISM BETWEEN TWO SIBLING SPECIES OF DROSOPHILA.

Matings between Drosophila simulans females and males of the sibling species D. mauritiana are of abnormally short duration. These rapid matings interrupt the transfer of sperm, leading to substantial reproductive isolation in interspecific as compared to intraspecific copulations. Genetic analysis of this behavior shows that it is influenced much more by the male than the female genotype, with genes from D. simulans being dominant. In males, shortened copulation is caused by interspecific divergence at a minimum of three loci, with one gene on each of the major chromosomes. This is an underestimate of the true number of loci affecting the trait, which could be much larger. The two autosomes have the largest effect, whereas that of the X chromosome is much smaller. The genetic architecture of copulation duration and the larger effect of male than female genotype suggest that females can detect and discriminate against differences in male genitalia.

Bidirectional selection for female receptivity inDrosophila melanogaster

A wild population of Drosophila melanogaster was subjected to selection for high and low female receptivity. Selection was only applied in females on the basis of the mating times observed in a 30-min period. We applied a selection paradigm that allowed us to conclude that females unmated after 30 min correspond to the truncated upper tail of the distribution underlying mating times. Selection was successful, with mean mating times of about 5 min for the high lines, 35 min for the low lines, and 12 min for the control. Receptivity of the selection lines remained steady after 42 generations of relaxed selection, suggesting a lack of natural selection on receptivity in laboratory conditions. We also found a clear relationship between high or low receptivity and high or low frequency of hybridization with D. simulans males, emphasizing the role of female receptivity in maintaining isolation with its sibling species D. simulans.

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 GENETIC BASIS OF SEXUAL ISOLATION BETWEEN DROSOPHILA MELANOGASTER AND D. SIMULANS.

The genetic analysis of sexual isolation between the closely-related species Drosophila melanogaster and Drosophila simulans involved two experiments with no-choice tests. The efficiency of sexual isolation was measured by the frequency of courtship initiation and interspecific mating. We first surveyed the variation in sexual isolation between D. melanogaster strains and D. simulans strains of different geographic origin. Then, to investigate variation in sexual isolation within strains, we made F1 diallel sets of reciprocal crosses within strains of D. melanogaster and D. simulans. The F1 diallel progeny of one sex were paired with the opposite sex of the other species. The first experiment showed significant differences in the frequency of interspecific mating between geographic strains. There were more matings between D. simulans females and D. melanogaster males than between D. melanogaster females and D. simulans males. The second experiment uncovered that the male genotypes in the D. melanogaster diallel significantly differed in interspecific mating frequency, but not in courtship initiation frequency. The female genotypes in the D. simulans diallel were not significantly different in courtship initiation and interspecific mating frequency. Genetic analysis reveals that in D. melanogaster males sexual isolation was not affected by either maternal cytoplasmic effects, sex-linked effects, or epistatic interaction. The main genetic components were directional dominance and overdominance. The F1 males achieved more matings with D. simulans females than the inbred males. The genetic architecture of sexual isolation in D. melanogaster males argues for a history of weak or no selection for lower interspecific mating propensity. The behavioral causes of variation in sexual isolation between the two species are discussed.

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.

A Temporal Study of Desiccation Resistance of Sibling Drosophila Species from Laguna Verde, Veracruz, Mexico

-The sibling species, Drosophila melanogaster and D. simulans, were collected periodically from Laguna Verde, Veracruz, Mexico, for 3 yr and tested for desiccation resistance. The number of survivors was recorded after the flies had been kept for 4 h, 8 h and 12 h, at 0% RH, and 20 C. Significant temporal changes in relative survival were found as follows: after the 4 h exposure period for D. simulans and after both 8 h and 12 h for D. melanogaster, females from the first three collections were more resistant to desiccation than males but not from the last two. Comparisons of the species by sex, showed that, except for the 4 h exposure period, D. melanogaster was significantly more resistant to desiccation than D. simulans. There was no indication of seasonal variation in desiccation resistance of males or females of either species at any of the three experimental time periods. However, in the collection of July 1987, some of the flies showed significantly less resistance to desiccation than those of the same sex and species from other collections, as follows: at 4 h, D. simulans males; at 8 h, D. simulans males and females, and D. melanogaster males; at 12 h, D. simulans females and D. melanogaster males and females. These findings are discussed with relation to changes in the relative frequencies of the two species, climatological conditions of the area, and the start-up testing period of a nearby nuclear power plant.

Receptivity and sexual maturation of Drosophila melanogaster females in relation to hybridization with D. simulans males: a populational analysis

Females from a wild population of Drosophila melanogaster were examined for the their frequency of hybridization with D. simulans males using no-choice tests, and for receptivity and sexual maturation speed using conspecific males. Genetic variability within the population was revealed for the three traits, and significant positive correlations between them were found. Females that matured earlier had greater receptivity indicating a common physiological basis for both characteristics. Notably, high female receptivity was correlated with high tendency to hybridize with D. simulans males. The involvement of female receptivity in both sexual isolation and sexual selection processes are discussed.

Natural and laboratory hybridization between Drosophila melanogaster and D. simulans.

Interspecific crossability between two sibling species, Drosophila melanogaster and D. simulans, were examined in nature and in the laboratory. About 4.6% of D. melanogaster females were found to be inseminated by D. simulans males in six Japanese natural populations. In the laboratory experiments D. melanogaster females collected and established from non-sympatric or newly sympatric sites with D. simulans were more easily inseminated than those from historically sympatric sites. It may be suggested that the premating isolation does not develop rapidly after the sibling species come sympatry.

Sexual Maturation in Drosophila melanogaster Females and Hybridization with D. simulans Males: A Study of Inheritance Modes

The inheritance mode of Drosophila melanogaster female sexual maturation speed and hybridization with D. simulans males was measured in F1 females from crosses between isofemale lines of D. melanogaster chosen for their high or low hybridization and maturation speed values. The results suggest dominant inheritance for rapid female maturation and intermediate inheritance for hybridization. We discuss results in relation to the evolutionary role of these characters.

The spontaneous mutation rate in Drosophila simulans

The purpose of this investigation was to determine whether there exists a correlation in Drosophila between the spontaneous mutation rate and the amount of dispersed middle repetitive (mobile) DNA sequences. The amount of these sequences is 7 times less in Drosophila simulans as compared to Drosophila melanogaster. Therefore, if a correlation exists, the spontaneous mutation rate in Drosophila simulans should be 7 times lower than that in Drosophila melanogaster. We isolated an X-chromosome inversion after X-irradiation of wild-type Drosophila simulans males, that reduced crossing-over between white and forked, two X-linked visible markers, to less than 1%. This inversion was subsequently used to determine the sex-linked recessive lethal mutation rate in Drosophila simulans males of a laboratory strain marked with white. The frequency of these lethal mutations found is not different from that observed in Drosophila melanogaster males of laboratory strains.

Genetic rescue of inviable hybrids between Drosophila melanogaster and its sibling species.

Post-mating mechanisms are central to the establishment of reproductive isolation between different, but closely related, species. Post-mating isolation mechanisms include hybrid breakdown, hybrid sterility and hybrid lethality and may, in some cases, be reinforced by pre-mating mechanisms such as ethological differentiation. In the Drosophila melanogaster species sub-group post-mating reproductive isolation is ensured by both the inviability and the sterility of hybrids. For example when D. melanogaster females are crossed to D. simulans males the hybrid progeny are normally all female; the hybrid males die as third instar larvae. The viable hybrid females are totally sterile. Little is known of the genetic basis for either hybrid sterility or hybrid inviability, although Coyne and others have begun a genetic analysis of the sterility of hybrids within this species sub-group. We have discovered a single gene difference that rescues the otherwise inviable male hybrids from the cross between D. melanogaster females and males of its three closest relatives. The study of this locus may shed light on the genetic control of both speciation and development.

Sexual isolation between Drosophila melanogaster females and D. simulans males. II. Influence of female receptivity on hybridization

Hybridization between Drosophila melanogaster females and D. simulans males was measured in sympatric and allopatric populations of these sibling species by means of nonchoice tests. Greater sexual isolation was found in allopatric populations over sympatric ones. Notable age differences at which females reached sexual maturity were detected in the D. melanogaster populations. Females with an earlier sexual maturity also showed greater receptivity. Remarkably, female receptivity was the main factor accounting for frequency of hybridization. The role that both male and female mate propensities and discrimination abilities play in natural selection for sexual isolation is discussed. Key words: sexual isolation, Drosophila, female receptivity, hybridization.

VARIATIONS IN CUTICULAR HYDROCARBONS AMONG THE EIGHT SPECIES OF THE DROSOPHILA MELANOGASTER SUBGROUP.

In addition to protecting against desiccation, some of the hydrocarbons of the waxy cuticle have previously been shown to be mating pheromones in Drosophila melanogaster and D. simulans. Therefore, cuticular hydrocarbons were compared among the eight species in the D. melanogaster subgroup. For the two cosmopolitan species and several geographic strains that were studied, all males are quite similar with very abundant monoenes. The major compound in most cases is 7-tricosene. Only three exceptions were found: D. sechellia, and the Afrotropical strains of D. melanogaster and D. simulans. A significant sexual dimorphism exists in three species: D. melanogaster, D. erecta, and D. sechellia. Greater variation was observed in females than in males. D. erecta is singular in the production of long-chain molecules (31-33 carbons). Only three species (D. melanogaster, D. erecta, and D. sechellia) produce diene in significant amounts. Such products, especially 7,11-heptacosadiene, are known to act as aphrodisiacs for D. melanogaster males. In the five other species, females show only quantitative differences from males, generally with 7-tricosene as the most abundant compound. This compound is an aphrodisiac for D. simulans males. Some species such as D. yakuba, D. teissieri, D. orena, D. mauritiana, and the Seychelles strain of D. simulans are almost identical in the chemical composition of cuticular hydrocarbons. In contrast, important variations are observed between geographic populations of D. melanogaster and D. simulans.