DISENTANGLING THE EFFECTS OF MATING PROPENSITY AND MATING CHOICE IN DROSOPHILA

Incipient sexual isolation between genotypes, lines, or populations of the same species is commonly measured in Drosophila by choice tests. Results of these tests are known to be influenced, in an undetermined manner, by the mating propensity of competitors and by discriminatory factors during courtship. We have approached the problem by measuring male and female propensities in separate, independent tests, and by examining whether these estimates could explain the results of the choice tests. First, male and female choice tests were used to measure sexual isolation between populations of Drosophila melanogaster and between populations of D. simulans. Significant deviations from random mating occurred in 31 out of 48 tests, in agreement with the propensity values of the tested genotypes. We conclude that mating propensity instead of discrimination is directly involved in the estimation of sexual isolation in our populations, and advise against the application of male and female choice tests to assess intraspecific isolation without a proper knowledge of the mating propensities of competing individuals. Second, multiple choice tests were used to assess isolation between D. melanogaster populations. In examining the dynamics of matings throughout the test, we show that if competing individuals differ in mating propensities and tests are long enough to allow most matings to happen, a spurious sexual isolation can appear. We recommend that multiple choice tests be terminated once 50 percent of matings had been observed.     

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.     

A comparison of behavioural change in Drosophila during exposure to thermal stress

In order to understand how adaptive tolerance to stress has evolved, we compared related species and populations of Drosophila for a variety of fitness relevant traits while flies directly experienced the stress. Two main questions were addressed. First, how much variation exists in the frequency of both courtship and mating among D. melanogaster, D. simulans, and D. mojavensis when each are exposed to a range of temperatures? Second, how does variation in these same behaviours compare among four geographically isolated populations of D. mojavensis, a desert species with a well defined ecology? Our hierarchical study demonstrated that mating success under stress can vary as much between related species, such as D. melanogaster and D. simulans, as between the ecologically disparate pair, D. melanogaster and D. mojavensis. Strains of this latter desert species likewise varied in tolerance, with differences approaching the levels observed among species. The consequences of stress on male courtship differed markedly from those on female receptivity to courtship, as mating behaviours among species and among strains of D. mojavensis varied in subtle but significant ways. Finally, a comparison of variation in thermotolerance of F₁ hybrids between the two most extreme D. mojavensis populations confirmed that genetic variation underlying traits such as survival or the ability to fly after heat stress is completely different. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 83 , 197–205.     

GENETICS OF SEXUAL ISOLATION AND COURTSHIP DYSFUNCTION IN MALE HYBRIDS OF DROSOPHILA PSEUDOOBSCURA AND DROSOPHILA PERSIMILIS

Despite the importance of sexual isolation to speciation, few studies have analyzed the genetic basis of interspecific mating discrimination, particularly using hybrid males. In this study, I investigated the genetic basis of sexual isolation using male hybrids of Drosophila pseudoobscura and D. persimilis. Hybrid male mating success was caused by interactions between the X-chromosome and autosomes (or Y-chromosome), and different arms of the X-chromosome contributed to mating success with females of each species. Further, although there was an X-chromosome component to mating success, its magnitude was not disproportionately large when compared with the proportion of the genome contained on this chromosome. Some hybrid males courted with an anomalously low intensity, so I simultaneously mapped the genetic basis of this “courtship dysfunction.” The courtship dysfunction was caused by an interaction between the left arm of the X-chromosome in D. persimilis with the autosomes or Y-chromosome from D. pseudoobscura. Anomalous courtship behavior in interspecific hybrids can obscure the conclusions of studies of the genetics of sexual isolation, so courtship intensity should be evaluated in all such investigations.     

Impact of experimental design on Drosophila sexual isolation studies: direct effects and comparison to field hybridization data

Abstract Many studies of speciation rely critically on estimates of sexual isolation obtained in the laboratory. Here we examine the sensitivity of sexual isolation to alterations in experimental design and mating environment in two sister species of Drosophila, D. santomea and D. yakuba. We use a newly devised measure of mating frequencies that is able to disentangle sexual isolation from species differences in mating propensity. Variation in fly density, presence or absence of a quasi‐natural environment, degree of starvation, and relative frequency of species had little or no effect on sexual isolation, but one factor did have a significant effect: the possibility of choice. Designs that allowed flies to choose between conspecific and heterospecific mates showed significantly more sexual isolation than other designs that did not allow choice. These experiments suggest that sexual isolation between these species (whose ranges overlap on the island of STo Tomé) is due largely to discrimination against D. yakuba males by D. santomea females. This suggestion was confirmed by direct observations of mating behavior. Drosophila santomea males also court D. yakuba females less ardently than conspecific females, whereas neither males nor females of D. yakuba show strong mate discrimination. Thus, sexual isolation appears to be a result of evolutionary changes in the derived island endemic D. santomea. Surprisingly, as reported in a companion paper (Llopart et al. 2005), the genotypes of hybrids found in nature do not accord with expectations from these laboratory studies: all F₁ hybrids in nature come from matings between D. santomea females and D. yakuba males, matings that occur only rarely in the laboratory.     

The pattern of polytene chromosome synapsis in Drosophila species and interspecific hybrids

The pairing of polytene chromosomes was investigated in Drosophila melanogaster, Drosophila simulans and their hybrids as well as in species of the D. virilis group and in F₁ hybrids between the species of this group. The study of frequency and extent of asynapsis revealed non-random distribution along chromosome arms both in interspecific hybrids and pure Drosophila species. It is suggested that definite chromosome regions exhibiting high pairing frequency serve as initiation sites of synapsis in salivary gland chromosomes.     

Dynamics of mating success in experimental groups ofDrosophila melanogaster (Diptera: Drosophilidae)

Determinants of male courtship success in Drosophila melanogasterwere examined in groups of five males sequentially presented with five individual females. Thirty-three percent of males never mated, while approximately half of the males mated two or three times. Rapid courtship initiation was associated with male success in early matings only. Male size was important for courtship outcome, but the size distributions of mating and nonmating males and their progeny numbers indicate balancing rather than directional selection on size- dependent courtship success.     

Sexual Isolation Between Drosophila Melanogaster, D. Simulans and D. Mauritiana: Sex and Species Specific Discrimination

The sexual isolation among the related species Drosophila melanogaster, D. simulans and D. mauritiana is asymmetrical. While D. mauritiana males mate well with both D. melanogaster and D. simulans females, females of D. mauritiana discriminate strongly against males of these two species. Similarly, D. simulans males mate with D. melanogaster females but the reciprocal cross is difficult. Interspecific crosses between several populations of the three species were performed to determine if (i) males and females of the same species share a common sexual isolation genetic system, and (ii) males (or females) use the same genetic system to discriminate against females (or males) of the other two species. Results indicate that although differences in male and female isolation depend on the populations tested, the isolation behaviour between a pair of species is highly correlated despite the variations. However, the rank order of the isolation level along the populations was not correlated in both sexes, which suggests that different genes act in male and female sexual isolation. Neither for males nor for females, the isolation behaviour of one species was paralleled in the other two species, which indicates that the genetic systems involved in this trait are species-pair specific. The implications of these results are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Male fruit flies learn to avoid interspecific courtship

Experimental data suggest, and theoretical models typicallyassume, that males of many fruit flies (Drosophila spp) areat least partially indiscriminate while searching for mates,and that it is mostly the females who exert selective mate choice,which can lead to incipient speciation. Evidence on learningby male D. melanogaster in the context of courtship, however,raises the possibility that the initially indiscriminate malesbecome more selective with experience. I tested this possibilityby comparing the courtship behavior of male D. melanogasterexperienced at courting females of the closely related species,D. simulans, and inexperienced males. I found that comparedwith the inexperienced males, the males experienced with courtingD. simulans females showed significantly lower courtship towardfemale D. simulans. Both male treatments, however, showed virtuallyidentical courtship durations with female D. melanogaster. Theseresults indicate that male fruit flies adaptively refine theircourtship behavior with experience and suggest that the malescontribute more to assortative mating and incipient speciationthan is commonly assumed.     

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.     

THE REPRODUCTIVE RELATIONSHIPS OF DROSOPHILA SECHELLIA WITH D. MAURITIANA,D. SIMULANS,AND D. MELANOGASTER FROM THE AFROTROPICAL REGION

Hybridization tests among the four sibling species of the Drosophila melanogaster complex were made to determine the reproductive status of the recently discovered D. sechellia (which is endemic to a few islands and islets of the Seychelles archipelago) with regard to its three close relatives, D. mauritiana (endemic to Mauritius) and Afrotropical strains of the two cosmopolitan species D. melanogaster and D. simulans. Interstrain variation in the ability to hybridize with other species was also analyzed for D. melanogaster and D. simulans. D. mauritiana and D. simulans appear to be more weakly isolated from each other than either species is from D. sechellia. A striking unilateral mating success is observed in the cross of D. sechellia with D. simulans. The most extreme isolation is between D. melanogaster and its three siblings. Variation in the ability of strains to hybridize is observed in heterospecific crosses between D. simulans and either D. melanogaster or D. mauritiana.     

Sexual Experience Enhances Drosophila melanogaster Male Mating Behavior and Success

Competition for mates is a wide-spread phenomenon affecting individual reproductive success. The ability of animals to adjust their behaviors in response to changing social environment is important and well documented. Drosophila melanogaster males compete with one another for matings with females and modify their reproductive behaviors based on prior social interactions. However, it remains to be determined how male social experience that culminates in mating with a female impacts subsequent male reproductive behaviors and mating success. Here we show that sexual experience enhances future mating success. Previously mated D. melanogaster males adjust their courtship behaviors and out-compete sexually inexperienced males for copulations. Interestingly, courtship experience alone is not sufficient in providing this competitive advantage, indicating that copulation plays a role in reinforcing this social learning. We also show that females use their sense of hearing to preferentially mate with experienced males when given a choice. Our results demonstrate the ability of previously mated males to learn from their positive sexual experiences and adjust their behaviors to gain a mating advantage. These experienced-based changes in behavior reveal strategies that animals likely use to increase their fecundity in natural competitive environments.     

Frequency dependent selection: I. Rare male phenomenon in D. subobscura dependent on the proportion of Amy genotypes and substrate composition

Analysis of the rare male mating advantage in D. subobscura, as a type of frequency dependent selection on maltose and starch media, was done by applying different statistical approaches (χ², cross-product ratio, variance and regression analysis). They reveal that mating occurs at random when proportions of prospective mates are equal, and that mating success of the males homozygous for Amy-locus genotypes (S/S and F/F) depends on their proportion. Regression analysis showed that the F/F males are sexually more active (have higher vigour) than S/S males. Rare male effect is one-sided and appears in F/F males that partake in more heterogamic matings. Comparison of the number of observed and expected homo- and heterogamic matings shows that homogamic matings are more frequent. Multifactorial analysis of variance shows that the number of matings are different for nine pairs of lines and four possible mating types (S_fS_m, S_fF_m, F_fS_m, F_fF_m). The rare male phenomenon is not dependent on different food composition, but is associated with variations in individual genotypes.     

Genetics of sexual isolation based on courtship song between two sympatric species: Drosophila ananassae and D. pallidosa

Sexual isolation has been considered one of the primary causes of speciation and its genetic study has the potential to reveal the genetics of speciation. In Drosophila, the importance of courtship songs in sexual isolation between closely related species has been well investigated, but studies analysing the genetic basis of the difference in the courtship songs associated with sexual isolation are less well documented. Drosophila ananassae and Drosophila pallidosa are useful for studies of sexual isolation, because of their sympatric distribution and absence of postmating isolation. Courtship songs are known to play a crucial role in sexual isolation between these two species, and the female discrimination behaviour against the courting male has been revealed to be controlled by a very narrow region on the second chromosome. In this study we investigated the genetic basis controlling the song differences associated with their sexual isolation, using intact and wingless males with chromosomes substituted between species. The results obtained from F₁ hybrid males between these species indicate the dominance of the song characters favoured by D. pallidosa females. In addition, the results obtained from backcross F₂ males indicate that chromosome 2 had a major effect on the control of the song characters associated with sexual isolation.     

Ecological differences and competitive interaction between Drosophila melanogaster and Drosophila simulans in small laboratory populations

Summary In interspecific competition studies, some cases of apparent change in competitive ability have been reported. But the change in competitive outcome could equally well be due to character displacement. As a preliminary to studies of the effects of association of D. melanogaster (yellow white mutant strain) and D. simulans (vermilion mutant strain), the nature and extent of ecological differences between them, and the nature of their competitive interaction was studied. Differences between the strains were shown for oviposition site preferences, and for larval and pupal distribution. In pure species cultures, simulans showed a greater preference than melanogaster for oviposition in the center of the medium surface. In mixed populations, simulans had an increased preference for this oviposition site, where melanogaster was at low frequency. D. simulans larvae utilized the lower half of the medium to a significantly greater extent than did melanogaster. At low density (5 pairs of parents) in pure species cultures, 68.7% of simulans pupae were on the medium surface. As parental numbers increased, this proportion decreased. The distribution of melanogaster pupae was quite different, with only 8 to 12% on the medium at all densities. But the remaining pupae tended to occur higher on the cylinder wall as parental numbers increased. The competitive interaction changed during the developmental period. At four and eight days after culture initiation, simulans appeared superior, while for total adult progeny production, melanogaster was slightly superior. These strans of the two species were not ecologically equivalent.     

The magnitude of behavioral isolation is affected by characteristics of the mating community

Gene exchange between species occurs in areas of secondary contact, where two species have the opportunity to hybridize. If heterospecific males are more common than conspecific males, females will experience more encounters with males of other species. These encounters might increase the likelihood of heterospecific matings, and lead to the production of hybrid progeny. I studied the mating behavior of two pairs of sibling species endemic to Africa: Drosophila yakuba/Drosophila santomea and Drosophila simulans/Drosophila sechellia. Drosophila yakuba and D. simulans are cosmopolitan species widely distributed in the African continent, while D. santomea and D. sechellia are island endemics. These pairs of species hybridize in nature and have the potential to exchange genes in natural conditions. I used these two pairs of Drosophila species, and constructed mating communities of different size and different heterospecific:conspecific composition. I found that both the total number of potential mates and the relative frequency of conspecific versus heterospecific males affect female mating decisions in the cosmopolitan species but not in the island endemics. These results suggest that the population characteristics, in which mating occurs, may affect the magnitude of premating isolation. Community composition might thus facilitate, or impair, gene flow between species.     

Patterns of reproductive isolation within and between two Lygaeus species characterized by sexual conflicts over mating

Theory suggests that, under some circumstances, sexual conflict over mating can lead to divergent sexually antagonistic coevolution among populations for traits associated with mating, and that this can promote reproductive isolation and hence speciation. However, sexual conflict over mating may also select for traits (e.g. male willingness to mate) that enhance gene flow between populations, limiting population divergence. In the present study, we compare pre‐ and post‐mating isolation within and between two species characterized by male–female conflict over mating rate. We quantify sexual isolation among five populations of the seed bug Lygaeus equestris collected from Italy and Sweden, and two replicates of a population of the sister‐species Lygaeus simulans, also collected from Italy. We find no evidence of reproductive isolation amongst populations of L. equestris, suggesting that sexual conflict over mating has not led to population divergence in relevant mating traits in L. equestris. However, there was strong asymmetric pre‐mating isolation between L. equestris and L. simulans: male L. simulans were able to mate successfully with female L. equestris, whereas male L. equestris were largely unable to mate with female L. simulans. We found little evidence for strong post‐mating isolation between the two species, however, with hybrid F₂ offspring being produced. Our results suggest that sexual conflict over mating has not led to population divergence, and indeed perhaps supports the contrary theoretical prediction that male willingness to mate may retard speciation by promoting gene flow.     

Sexual isolation between two sibling species with overlapping ranges: Drosophila santomea and Drosophila yakuba

Abstract.— .Drosophila yakuba is widespread in Africa, whereas D. santomea, its newly discovered sister species, is endemic to the volcanic island of São Tomé in the Gulf of Guinea. Drosophila santomea probably formed after colonization of the island by a D. yakuba‐like ancestor. The species presently have overlapping ranges on the mountain Pico do São Tome, with some hybridization occurring in this region. Sexual isolation between the species is uniformly high regardless of the source of the populations, and, as in many pairs of Drosophila species, is asymmetrical, so that hybridizations occur much more readily in one direction than the other. Despite the fact that these species meet many of the conditions required for the evolution of reinforcement (the elevation of sexual isolation by natural selection to avoid maladaptive interspecific hybridization), there is no evidence that sexual isolation between the species is highest in the zone of overlap. Sexual isolation is due to evolutionary changes in both female preference for heterospecific males and in the vigor with which males court heterospecific females. Heterospecific matings are also slower to take place than are homospecific matings, constituting another possible form of reproductive isolation. Genetic studies show that, when tested with females of either species, male hybrids having a D. santomea X chromosome mate much less frequently with females of either species than do males having a D. yakuba X chromosome, suggesting that the interaction between the D. santomea X chromosome and the D. yakuba genome causes behavioral sterility. Hybrid F₁ females mate readily with males of either species, so that sexual isolation in this sex is completely recessive, a phenomenon seen in other Drosophila species. There has also been significant evolutionary change in the duration of copulation between these species; this difference involves genetic changes in both sexes, with at least two genes responsible in males and at least one in females.     

No evidence for learned mating discrimination in male Drosophila pseudoobscura

Background  

Since females often pay a higher cost for heterospecific matings, mate discrimination and species recognition are driven primarily by female choice. In contrast, frequent indiscriminate matings are hypothesized to maximize male fitness. However, recent studies show that previously indiscriminate males (e.g., Drosophila melanogaster and Poecilia reticulata) can learn to avoid heterospecific courtship. This ability of males to discriminate against heterospecific courtship may be advantageous in populations where two species co-occur if courtship or mating is costly.