Pelayo Casares

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.

Male mating speed inDrosophila melanogaster: Differences in genetic architecture and in relative performance according to female genotype

From a critical review of the literature on mating speed inDrosophila the importance of fast mating in male fitness is questioned. The genetic architecture of male mating speed (MMS) has been evaluated inD. melanogaster through a populational analysis and a full 5×5 diallel cross between inbred lines. The results emphasize the fundamental role of the female genotype in both the absolute and the relative MMS performances. Somewhat different genetic architectures for MMS are revealed according to the female used in the tests. It is suggested that different parts of the complex genetic system involved in the males “behavioral sexual phenotype” are relevant depending on the females characteristics, thus causing the heterogeneity in the MMS genetic architecture. An overall picture reveals a genetic system characterized by additivity, dominance for fast mating, and no influence of the X chromosome. There results do not support strong natural selection favoring fast mating inDrosophila males.

Bidirectional selection for female receptivity in Drosophila melanogaster

A wild population ofDrosophila 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 withD. simulans males, emphasizing the role of female receptivity in maintaining isolation with its sibling speciesD. simulans.

Genetic basis for female receptivity in Drosophila melanogaster : a diallel study

Receptivity of sexually mature, virgin females, measured as the time to copulation, has been investigated. Two independent 5 × 5 diallel crosses were carried out, each one with a different male tester genotype. Interactions between male and female genotypes were detected because some lines appeared as dominant in one diallel and recessive in the other. The overall picture emerging from the analyses shows a genetic system for female receptivity characterized by additive genes, with dominance for high receptivity in one of the diallel crosses. This remarkable additive component does not support the action of natural selection favouring females with extreme values of receptivity, rather, the lack of intrasexual female competition for mates is suggested.

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

Abstract 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.

GENETICS OF HYBRIDIZATION BETWEEN DROSOPHILA SIMULANS FEMALES AND D. MELANOGASTER MALES

Drosophila simulans and D. melanogaster are sibling cosmopolitan species with imperfect ethological isolation. Hybridization is easy between D. melanogaster females and D. simulans males, but the reciprocal cross has been traditionally considered as very scarce and little is known about the environmental and genetic factors that affect it. We used classical genetic analyses to determine the influence of each major chromosome on the breakdown of sexual isolation between females of D. simulans and D. melanogaster males. In addition, we have made a first attempt to locate the genetic systems involved in this process. At least two genes, or two groups of genes, are responsible for hybridization, located in the X chromosome and in the left arm of chromosome II. The inheritance mode of both genetic systems is different. The genes in the X chromosome show dominance for high levels of hybridization, whereas those in chromosome II show dominance for low levels. These results contrast with other investigations on the melanogaster subgroup, suggesting independent evolutionary events during the speciation process in each species.

Pupation height in Drosophila: sex differences and influence of larval developmental time.

Several lines ofDrosophila simulans andD. melanogaster of different origin were examined for pupation height. In all lines male larvae pupated, on average, higher than females. The pupation heights of early-, intermediate-, and late-pupating larvae were also recorded. As pupation progressed in the vials, larvae tended to pupate lower and lower, possibly as a response to diminishing levels of humidity inside the vials, which suggests a strong negative correlation between larval developmental time and pupation height. Thus, selection experiments for pupation height may also select for developmental rate. Since females generally pupate later than males, larval sex differences in pupation height may reflect sex differences in duration of development. The joint effects of sex and duration of development upon pupation height are discussed in relation to the lack of response previously reported in some experiments selecting for pupation height.

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.

Chromosomal substitution analysis of receptivity and sexual isolation in Drosophila melanogaster females.

A chromosome substitution analysis was carried out in Drosophila melanogaster to elucidate the contribution of the X, II and III chromosomes to female receptivity, and to sexual isolation with its sibling species D. simulans. Two lines from the same base population with extreme phenotypes, the one showing high receptivity and weak isolation and the other showing low receptivity and strong isolation, were used. Genes involved in receptivity were located in chromosome III whereas those affecting sexual isolation were located in all the three chromosomes. These results indicate that the direct relationship between female receptivity and sexual isolation previously detected in D. melanogaster could be because of the occurrence of genes common to the two traits in the chromosome III.

Location of Shfr, a new gene that rescues hybrid female viability in crosses between Drosophila simulans females and D. melanogaster males

As a rule, progeny of crosses between Drosophila simulans females and D. melanogaster males are formed by sterile males, because females die as embryos. However, populations of these species have been found that produce a certain frequency of viable hybrid females. We have found that 94% of the females of a D. simulans population from Tel Aviv gave hybrid progenies with both sexes. The segregation of phenotypes with different rescue success adjusts to the action of a single, dominant, zygotic-acting gene involved in hybrid female viability. This gene, which we named ‘Simulans hybrid females rescue’ (Shfr), is temperature-sensitive, showing a much smaller effect as temperature increases. Reciprocal crosses between Tel Aviv and a nonrescue population indicate some influence of cytoplasm or maternal effect in rescue. Using a chromosome substitution analysis we have located Shfr on the second chromosome. Using synthetic lines with this chromosome having different segments from Tel Aviv and from a multimarker strain we have mapped Shfr between black (2 L-43.0) and pearly (2 R-74.0).