The genetic basis of prezygotic reproductive isolation between Drosophila santomea and D. yakuba due to mating preference

Sexual isolating mechanisms that act before fertilization are often considered the most important genetic barriers leading to speciation in animals. While progress has been made toward understanding the genetic basis of the postzygotic isolating mechanisms of hybrid sterility and inviability, little is known about the genetic basis of prezygotic sexual isolation. Here, we map quantitative trait loci (QTL) contributing to prezygotic reproductive isolation between the sibling species Drosophila santomea and D. yakuba. We mapped at least three QTL affecting discrimination of D. santomea females against D. yakuba males: one X-linked and one autosomal QTL affected the likelihood of copulation, and a second X chromosome QTL affected copulation latency. Three autosomal QTL also affected mating success of D. yakuba males with D. santomea. No epistasis was detected between QTL affecting sexual isolation. The QTL do not overlap between males and females and are not disproportionately concentrated on the X chromosome. There was some overlap in map locations of QTL affecting sexual isolation between D. santomea and D. yakuba with QTL affecting sexual isolation between D. simulans and D. mauritiana and with QTL affecting differences in pigmentation between D. santomea and D. yakuba. Future high-resolution mapping and, ultimately, positional cloning, will reveal whether these traits do indeed have a common genetic basis.     

The genetic basis of behavioral isolation between Drosophila mauritiana and D. sechellia

Understanding how species form is a fundamental question in evolutionary biology. Identifying the genetic bases of barriers that prevent gene flow between species provides insight into how speciation occurs. Here, I analyze a poorly understood reproductive isolating barrier, prezygotic reproductive isolation. I perform a genetic analysis of prezygotic isolation between two closely related species of Drosophila, D. mauritiana and D. sechellia. I first confirm the existence of strong behavioral isolation between D. mauritiana females and D. sechellia males. Next, I examine the genetic basis of behavioral isolation by (1) scanning an existing set of introgression lines for chromosomal regions that have a large effect on isolation; and (2) mapping quantitative trait loci (QTL) that underlie behavioral isolation via backcross analysis. In particular, I map QTL that determine whether a hybrid backcross female and a D. sechellia male will mate. I identify a single significant QTL, on the X chromosome, suggesting that few major-effect loci contribute to behavioral isolation between these species. In further work, I refine the map position of the QTL to a small region of the X chromosome.     

Genetics of sexual isolation in females of the Drosophila simulans species complex.

Genetic analysis of hybrids between Drosophila simulans and D. sechellia shows that sexual isolation in females is caused by at least two genes, one on each major autosome, while the X chromosome has no effect. These results are similar to those of a previous study of hybrids between D. simulans and another sibling species, D. mauritiana. In this latter hybridization, each arm of the second chromosome carries genes causing sexual isolation in females, implying a total divergence of at least three loci. The genetic similarity between the D. simulans/D. mauritiana and D. simulans/D. sechellia hybridizations probably results from independent evolution and not phylogenetic artifacts, because the dominance relationships and behavioural interactions differ between the two hybridizations. The lack of an X-chromosome effect on sexual isolation contrasts with genetic studies of post-zygotic reproductive isolation, which invariably show strong effects of this chromosome.     

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.     

Cryptic reproductive isolation in the Drosophila simulans species complex

Forms of reproductive isolation that act after copulation but before fertilization are potentially important components of speciation, but are studied only infrequently. We examined postmating, prezygotic reproductive isolation in three hybridizations within the Drosophila simulans species complex. We allowed females to mate only once, observed and timed all copulations, dissected a subset of the females to track the storage and retention of sperm, examined the number and hatchability of eggs laid after insemination, counted all progeny produced, and measured the longevity of mated females. Each of the three hybridizations is characterized by a different set of cryptic barriers to heterospecific fertilization. When D. simulans females mate with D. sechellia males, few heterospecific sperm are transferred, even during long copulations. In contrast, copulations of D. simulans females with D. mauritiana males are often too short to allow sperm transfer. Those that are long enough to allow insemination, however, involve the transfer of many sperm, but only a fraction of these heterospecific sperm are stored by females, who also lay fewer eggs than do D. simulans females mated with conspecific males. Finally, when D. mauritiana females mate with D. simulans males, sperm are transferred and stored in abundance, but are lost rapidly from the reproductive tract and are therefore used inefficiently. These results add considerably to the list of reproductive isolating mechanisms in this well-studied clade and possibly to the list of evolutionary processes that could contribute to their reproductive isolation.     

Genetic dissection of hybrid incompatibilities between Drosophila simulans and D. mauritiana. II. Mapping hybrid male sterility loci on the third chromosome

Hybrid male sterility (HMS) is a rapidly evolving mechanism of reproductive isolation in Drosophila. Here we report a genetic analysis of HMS in third-chromosome segments of Drosophila mauritiana that were introgressed into a D. simulans background. Qualitative genetic mapping was used to localize 10 loci on 3R and a quantitative trait locus (QTL) procedure (multiple-interval mapping) was used to identify 19 loci on the entire chromosome. These genetic incompatibilities often show dominance and complex patterns of epistasis. Most of the HMS loci have relatively small effects and generally at least two or three of them are required to produce complete sterility. Only one small region of the third chromosome of D. mauritiana by itself causes a high level of infertility when introgressed into D. simulans. By comparison with previous studies of the X chromosome, we infer that HMS loci are only approximately 40% as dense on this autosome as they are on the X chromosome. These results are consistent with the gradual evolution of hybrid incompatibilities as a by-product of genetic divergence in allopatric populations.     

Comparative genetics of potential prezygotic and postzygotic isolating barriers in a Lycopersicon species cross

I compare the genetic basis of quantitative traits that potentially contribute to pre- and postzygotic isolation between the plant species Solanum lycopersicum (formerly Lycopersicon esculentum) and Solanum habrochaites (formerly Lycopersicon hirsutum), using quantitative trait loci (QTL) mapping in a set of near-isogenic lines. Putative prezygotic isolating traits include flower size, flower shape, stigma exertion, and inflorescence length, that can influence pollinator preferences and/or selfing rates, and therefore gene flow between divergent types. Postzygotic isolating traits are hybrid pollen and seed sterility. Three substantive results emerge from these analyses. First, the genetic basis of floral differentiation appears to be somewhat less complex than the genetic basis of postzygotic hybrid sterility, although these differences are very modest. Second, there is little evidence that traits for floral differentiation are causally or mechanistically associated with hybrid sterility traits in this species cross. Third, there is little evidence that hybrid sterility QTL are more frequently associated with chromosomal centromeric regions, in comparison to floral trait QTL, a prediction of centromeric drive models of hybrid sterility. Although genome-wide associations are not evident in this analysis, several individual chromosomal regions that contain clusters of QTL for both floral and sterility traits, or that indicate hybrid sterility effects at centromere locations, warrant further fine-scale investigation.     

WHEN MORPHOLOGY MISLEADS: INTERPOPULATION UNIFORMITY IN SEXUAL SELECTION MASKS GENETIC DIVERGENCE IN HARLEQUIN BEETLE-RIDING PSEUDOSCORPION POPULATIONS

Differences in secondary sexual characteristics of males often provide the most conspicuous means of distinguishing between closely related species. Does this therefore imply that the absence of differentiation in exaggerated male traits between allopatric populations provides evidence of a single, genetically cohesive species? We addressed this question with a comprehensive investigation of two populations (French Guiana and Panama) of the harlequin beetle-riding pseudoscorpion, Cordylochernes scorpioides. This highly sexually dimorphic pseudoscorpion is currently described as a single species, ranging throughout the Neotropics. Our morphometric analyses detected minimal differentiation between the two populations in all nine external morphological characters measured, including sexually dimorphic traits in males. Only in traits of the spermatophore was there any appreciable level of differentiation. Behavior differentiation and prezygotic reproductive isolation were also limited: 78.3% of males successfully transferred sperm to “foreign” females, and in 63.9% of these cases, females' eggs were successfully fertilized. By contrast, extensive divergence existed in two of nine electrophoretic loci, including an essentially fixed-allele difference at the Ldh locus. Most significantly, postzygotic reproductive isolation was complete, with heteropopulation zygotes invariably aborting early in development. These results strongly suggest that the two populations are, in fact, sibling species, a conclusion supported by our recently published findings on their marked divergence in minisatellite DNA. How can such interpopulation homogeneity in male sexually dimorphic traits exist in the face of strong genetic divergence? We propose that sexual selection, oscillating between favoring small and then large males, maintains such high levels of male variability within each population that it has obscured a speciation event in which genetic divergence and postzygotic incompatibility have clearly outpaced the evolution of prezygotic reproductive isolation.     

Evolution of reproductive isolation as a by‐product of divergent life‐history evolution in laboratory populations of Drosophila melanogaster

We show that two complementary asymmetric isolating mechanisms, likely mediated by divergence in body size, underlie the evolution of incipient reproductive isolation between a set of Drosophila melanogaster populations selected for rapid development and their ancestral controls. Selection has led to great reduction in body size in the fast developing lines. Small males belonging to fast developing lines obtain few matings with large control females, both in presence and absence of large control line males, giving rise to unidirectional, premating isolation caused by sexual selection. Conversely, small selected line females suffer greatly increased mortality following mating with large control males, causing unidirectional postcopulatory prezygotic isolation. We discuss preliminary evidence for evolution of reduced male harm caused to females upon mating in the fast developing lines, and speculate that the females from these lines have coevolved reduced resistance to male harm such that they can no longer resist the harm caused by males from control lines. This potentially implicates differing levels of sexual conflict in creating reproductive barrier between the selected line females and the control males. We also show that a large difference in development time is not sufficient to cause postzygotic incompatibilities in the two sets of populations reaffirming the belief that prezygotic isolation can evolve much earlier than postzygotic isolation.     

Courtship song recognition in the Drosophila melanogaster complex: heterospecific songs make females receptive in D. melanogaster, but not in D. sechellia

Abstract. The courtship song emitted by male wing vibration has been regarded as one of the most important signals in sexual isolation in the species of the Drosophila melanogaster complex. Inter- and intraspecific crosses were observed using males whose wings were removed (mute) or females whose aristae were removed (deaf). Females of D. melanogaster, D. simulans , and D. mauritiana mated with heterospecific males in the song-present condition (cross between normal females and winged males) more often than in the no-song condition (cross between normal females and wingless males or between aristaless females and winged males) or they showed no preference between the two conditions. It is possible that in these females heterospecific courtship songs play a role as if they were conspecific. In contrast, the females of D. sechellia mated with D. melanogaster or D. simulans males in the no-song condition more often than in the song-present condition, suggesting that they reject males with heterospecific song. Female mate recognition depending on the courtship song in D. melanogaster, D. simulans , and D. mauritiana is considered to be relatively broader and that in D. sechellia narrower.     

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.     

FEMALE HETEROGAMETY AND SPECIATION: REDUCED INTROGRESSION OF THE Z CHROMOSOME BETWEEN TWO SPECIES OF NIGHTINGALES

Several lines of evidence suggest that the X chromosome plays a large role in intrinsic postzygotic isolation. The role of the Z chromosome in speciation is much less understood. To explore the role of the Z chromosome in reproductive isolation, we studied nucleotide variation in two closely related bird species, the Thrush Nightingale ( Luscinia luscinia ) and the Common Nightingale ( L. megarhynchos ). These species are isolated by incomplete prezygotic isolation and female hybrid sterility. We sequenced introns of four Z-linked and eight autosomal loci and analyzed patterns of polymorphism and divergence using a divergence-with-gene flow framework. Our results suggest that the nightingale species diverged approximately 1.8 Mya. We found strong evidence of gene flow after divergence in both directions, although more introgression occurred from L. megarhynchos into L. luscinia . Gene flow was significantly higher on the autosomes than on the Z chromosome. Our results support the idea that the Z chromosome plays an important role in intrinsic postzygotic isolation in birds, although it may also contribute to the evolution of prezygotic isolation through sexual selection. This highlights the similarities in the genetic basis of reproductive isolation between organisms with heterogametic males and organisms with heterogametic females during the early stages of speciation.     

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.     

Genetics of a Pheromonal Difference Affecting Sexual Isolation between Drosophila Mauritiana and D. Sechellia

Females of the sibling species Drosophila sechellia and D. mauritiana differ in their cuticular hydrocarbons: the predominant compound in D. sechellia is 7,11-heptacosadiene (7,11-HD), while that in D. mauritiana is 7-tricosene (7-T). We investigate the genetic basis of this difference and its involvement in reproductive isolation between the species. Behavioral studies involving hydrocarbon transfer suggest that these compounds play a large role in the sexual isolation between D. mauritiana males and D. sechellia females, while sexual isolation in the reciprocal hybridization results more from differences in female behavior than hydrocarbons. This interspecific difference in hydrocarbon profile is due to evolutionary change at a minimum of six loci, all on the third chromosome. The localization of evolutionary change to the third chromosome has been seen in every other genetic analysis of female hydrocarbon differences in the D. melanogaster group. We suggest that the high 7,11-HD phenotype seen in two species evolved twice independently from ancestors having the high 7-T phenotype, and that the recurrent third-chromosome effects are evolutionary convergences that may be due to a concentration of ``hydrocarbon genes' on that chromosome.     

Species divergence in field crickets: genetics,song, ecomorphology,and pre‐ and postzygotic isolation

Studies that simultaneously estimate levels of species divergence in genetics, reproductive and ecological traits, and pre‐ and postzygotic isolation are relatively rare. Here we compare levels of divergence in three allopatric sister species of field crickets. We compare divergence in both nuclear and mitochondrial DNA, male song, female ovipositor length, levels of pre‐ and postzygotic isolation, and male versus female contributions to prezygotic isolation. Taken together, our data show the accumulation of a multitude of potential reproductive isolating barriers if secondary contact were to become established. Furthermore, ecological and behavioural prezygotic isolation appear significantly more advanced than postzygotic isolation, with prezygotic isolation due to female behaviour exceeding that due to male behaviour.     

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.     

Do quantitative trait loci (QTL) for a courtship song difference between Drosophila simulans and D. sechellia coincide with candidate genes and intraspecific QTL?

The genetic architecture of traits influencing sexual isolation can give insight into the evolution of reproductive isolation and hence speciation. Here we report a quantitative trait loci (QTL) analysis of the difference in mean interpulse interval (IPI), an important component of the male courtship song, between Drosophila simulans and D. sechellia. Using a backcross analysis, we find six QTL that explain a total of 40.7% of the phenotypic variance. Three candidate genes are located in the intervals bounded by two of the QTL and there are no significant QTL on the X chromosome. The values of mean IPI for hybrid individuals imply the presence of dominant alleles or epistasis. Because unisexual hybrid sterility prevents an F(2) analysis, we cannot distinguish dominant from additive genetic effects at the scale of QTL. A comparison with a study of QTL for intraspecific variation in D. melanogaster shows that, for these strains, the QTL we have identified for interspecific variation cannot be those that contribute to intraspecific variation. We find that the QTL have bidirectional effects, which indicates that the genetic architecture is compatible with divergence due to genetic drift, although other possibilities are discussed.     

Genetic architecture of variation in sex-comb tooth number in Drosophila simulans

The sex comb on the forelegs of Drosophila males is a secondary sexual trait, and the number of teeth on these combs varies greatly within and between species. To understand the relationship between the intra- and interspecific variation, we performed quantitative trait locus (QTL) analyses of the intraspecific variation in sex-comb tooth number. We used five mapping populations derived from two inbred Drosophila simulans strains that were divergent in the number of sex-comb teeth. Although no QTLs were detected on the X chromosome, we identified four QTLs on the second chromosome and three QTLs on the third chromosome. While identification and estimated effects of the second-chromosome QTLs depend on genetic backgrounds, significant and consistent effects of the two third-chromosome QTLs were found in two genetic backgrounds. There were significant epistatic interactions between a second-chromosome QTL and a third-chromosome QTL, as well as between two second-chromosome QTLs. The third-chromosome QTLs are concordant with the locations of the QTLs responsible for the previously observed differences in sex-comb tooth number between D. simulans and D. mauritiana.     

An Introgression Analysis of Quantitative Trait Loci That Contribute to a Morphological Difference between Drosophila Simulans and D. Mauritiana

Drosophila simulans and D. mauritiana differ markedly in morphology of the posterior lobe, a male-specific genitalic structure. Both size and shape of the lobe can be quantified by a morphometric variable, PC1, derived from principal components and Fourier analyses. The genetic architecture of the species difference in PC1 was investigated previously by composite interval mapping, which revealed largely additive inheritance, with a minimum of eight quantitative trait loci (QTL) affecting the trait. This analysis was extended by introgression of marked segments of the mauritiana third chromosome into a simulans background by repeated backcrossing. The two types of experiment are consistent in suggesting that several QTL on the third chromosome may have effects in the range of 10-15% of the parental difference and that all or nearly all QTL have effects in the same direction. Since the parental difference is large (30.4 environmental standard deviations), effects of this magnitude can produce alternative homozygotes with little overlap in phenotype. However, these estimates may not reflect the effects of individual loci, since each interval or introgressed segment may contain multiple QTL. The consistent direction of allelic effects suggests a history of directional selection on the posterior lobe.     

Genetic architecture of hybrid male sterility in Drosophila: analysis of intraspecies variation for interspecies isolation

Background

The genetic basis of postzygotic isolation is a central puzzle in evolutionary biology. Evolutionary forces causing hybrid sterility or inviability act on the responsible genes while they still are polymorphic, thus we have to study these traits as they arise, before isolation is complete.

Methodology/Principal Findings

Isofemale strains of D. mojavensis vary significantly in their production of sterile F₁ sons when females are crossed to D. arizonae males. We took advantage of the intraspecific polymorphism, in a novel design, to perform quantitative trait locus (QTL) mapping analyses directly on F₁ hybrid male sterility itself. We found that the genetic architecture of the polymorphism for hybrid male sterility (HMS) in the F₁ is complex, involving multiple QTL, epistasis, and cytoplasmic effects.

Conclusions/Significance

The role of extensive intraspecific polymorphism, multiple QTL, and epistatic interactions in HMS in this young species pair shows that HMS is arising as a complex trait in this system. Directional selection alone would be unlikely to maintain polymorphism at multiple loci, thus we hypothesize that directional selection is unlikely to be the only evolutionary force influencing postzygotic isolation.