REINFORCEMENT OF STICKLEBACK MATE PREFERENCES: SYMPATRY BREEDS CONTEMPT

Detailed studies of reproductive isolation and how it varies among populations can provide valuable insight into the mechanisms of speciation. Here we investigate how the strength of premating isolation varies between sympatric and allopatric populations of threespine sticklebacks to test a prediction of the hypothesis of reinforcement: that interspecific mate discrimination should be stronger in sympatry than in allopatry. In conducting such tests, it is important to control for ecological character displacement between sympatric species because ecological character divergence may strengthen prezygotic isolation as a by-product. We control for ecological character displacement by comparing mate preferences of females from a sympatric population (benthics) with mate preferences of females from two allopatric populations that most closely resemble the sympatric benthic females in ecology and morphology. No-choice mating trials indicate that sympatric benthic females mate less readily with heterospecific (limnetic) than conspecific (benthic) males, whereas two different populations of allopatric females resembling benthics show no such discrimination. These differences demonstrate reproductive character displacement of benthic female mate choice. Previous studies have established that hybridization between sympatric species occurred in the past in the wild and that hybrid offspring have lower fitness than either parental species, thus providing conditions under which natural selection would favor individuals that do not hybridize. Results are therefore consistent with the hypothesis that female mate preferences have evolved as a response to reduced hybrid fitness (reinforcement), although direct effects of sympatry or a biased extinction process could also produce the pattern. Males of the other sympatric species (limnetics) showed a preference for smaller females, in contrast to the inferred ancestral preference for larger females, suggesting reproductive character displacement of limnetic male mate preferences as well.     

Flies across the water: genetic differentiation and reproductive isolation in allopatric desert Drosophila

Between sister species of Drosophila, both pre- and postzygotic reproductive isolation commonly appear by the time a Nei's genetic distance of 0.5 is observed. The degree of genetic differentiation present when allopatric populations of the same Drosophila species exhibit incipient reproductive isolation has not been systematically investigated. Here we compare the relationship between genetic differentiation and pre- and postzygotic isolation among allopatric populations of three cactophilic desert Drosophila: D. mettleri, D. nigrospiracula, and D. mojavensis. The range of all three is interrupted by the Gulf of California, while two species, D. mettleri and D. mojavensis, have additional allopatric populations residing on distant Santa Catalina Island, off the coast of southern California. Significant population structure exists within all three species, but only for allopatric populations of D. mojavensis is significant isolation at the prezygotic level observed. The genetic distances for the relevant populations of D. mojavensis were in the range of 0.12, similar to that for D. mettleri whose greatest D = 0.11 was unassociated with any form of isolation. These observations suggest further investigations of Drosophila populations with genetic distances in this range be undertaken to identify any potential patterns in the relationship between degree of genetic differentiation and the appearance of pre- and/or postzygotic isolation.     

Multilocus test for introgression between the cactophilic species Drosophila mojavensis and Drosophila arizonae

Information obtained from laboratory studies regarding the efficacy of barriers to gene flow (reproductive isolation) between species is often incomplete or misleading, so detailed genetic analyses are needed to determine whether hybridization and introgression occur in nature. Previous laboratory studies of the cactophilic species Drosophila mojavensis and Drosophila arizonae suggest that reproductive isolation is incomplete and that gene flow may occur in sympatry. We sampled 18 nuclear and one mitochondrial loci from multiple populations of D. arizonae and D. mojavensis to test for the signature of recent or historic gene flow between these two species. We located chromosomal regions that were inverted between these species and analyzed those regions independently of others. Statistical tests for introgression using all loci or only collinear loci failed to reject expectations of an isolation model. Further tests using average nucleotide differences between species and phylogenetic analyses also failed to find support for introgression between D. mojavensis and D. arizonae. Additional ecological and behavioral studies of these species in their natural habitats are required to explain why the signature of gene flow was not detected at the DNA sequence level in populations when laboratory studies suggest such gene flow should be possible.     

SIMULATING RANGE EXPANSION: MALE SPECIES RECOGNITION AND LOSS OF PREMATING ISOLATION IN DAMSELFLIES

Prolonged periods of allopatry might result in loss of the ability to discriminate against other formerly sympatric species, and can lead to heterospecific matings and hybridization upon secondary contact. Loss of premating isolation during prolonged allopatry can operate in the opposite direction of reinforcement, but has until now been little explored. We investigated how premating isolation between two closely related damselfly species, Calopteryx splendens and C. virgo , might be affected by the expected future northward range expansion of C. splendens into the allopatric zone of C. virgo in northern Scandinavia. We simulated the expected secondary contact by presenting C. splendens females to C. virgo males in the northern allopatric populations in Finland. Premating isolation toward C. splendens in northern allopatric populations was compared to sympatric populations in southern Finland and southern Sweden. Male courtship responses of C. virgo toward conspecific females showed limited geographic variation, however, courtship attempts toward heterospecific C. splendens females increased significantly from sympatry to allopatry. Our results suggest that allopatric C. virgo males have partly lost their ability to discriminate against heterospecific females. Reduced premating isolation in allopatry might lead to increased heterospecific matings between taxa that are currently expanding and shifting their ranges in response to climate change.     

Reinforcement shapes clines in female mate discrimination in Drosophila subquinaria

Reinforcement of species boundaries may alter mate recognition in a way that also affects patterns of mate preference among conspecific populations. In the fly Drosophila subquinaria, females sympatric with the closely related species D. recens reject mating with heterospecific males as well as with conspecific males from allopatric populations. Here, we assess geographic variation in behavioral isolation within and among populations of D. subquinaria and use cline theory to understand patterns of selection on reinforced discrimination and its consequences for sexual isolation within species. We find that selection has fixed rejection of D. recens males in sympatry, while significant genetic variation in this behavior occurs within allopatric populations. In conspecific matings sexual isolation is also asymmetric and stronger in populations that are sympatric with D. recens. The clines in behavioral discrimination within and between species are similar in shape and are maintained by strong selection in the face of gene flow, and we show that some of their genetic basis may be either shared or linked. Thus, while reinforcement can drive extremely strong phenotypic divergence, the long‐term consequences for incipient speciation depend on gene flow, genetic linkage of discrimination traits, and the cost of these behaviors in allopatry.     

Mate discrimination and cuticular hydrocarbons in Drosophila elegans and D. gunungcola

In Drosophila elegans, two morphs are known, the brown-morph occurring from southern China to Indonesia and the black-morph occurring in the Ryukyu Islands, Japan, and Taiwan, and brown-morph populations at high altitudes in Indonesia at least sympatrically occur with a sibling species D. gunungcola. Sexual isolation has developed between the two morphs of D. elegans to some extent; females of the black-morph have a higher concentration of pentacosenes on cuticle than those of the brown-morph, and males of these morphs discriminate between the females based on this difference. In this study, it was examined whether sympatry and allopatry with D. gunungcola have resulted in the differentiation of mate recognition system in D. elegans. No significant difference was observed in the degree of mate discrimination between a sympatric pair of D. elegans and D. gunungcola and their allopatric pairs. Thus, no support was obtained for the above notion. Males of the brown- and black-morphs of D. elegans discriminate between females of own morphs and D. gunungcola. However, brown-morph males did not discriminate between females of the black-morph and D. gunungcola, and also black-morph males did not discriminate between females of the brown-morph and D. gunungcola. This may be attributed to that D. gunungcola females retained an intermediate level of pentacosenes between brown- and black-morph females.     

Premating isolation is determined by larval rearing substrates in cactophilic Drosophila mojavensis. VIII. Mating success mediated by epicuticular hydrocarbons within and between isolated populations

We tested the hypothesis that intrademic sexual selection has caused sexual isolation between populations of geographically isolated populations of cactophilic Drosophila mojavensis, and was mediated by epicuticular hydrocarbons (EHCs), contact pheromones in this system. Sexual selection and sexual isolation were estimated using a Baja California and mainland population by comparing the number of mated and unmated males and females in each of four pairwise population mating trials. EHC profiles were significantly different in mated and unmated males in the interdemic (Bajafemale symbol x Mainlandmale symbol and Mainlandfemale symbol x Bajamale symbol), but not the intrademic mating trials. A small number of EHCs was identified that best discriminated among mated and unmated males, mostly alkadienes with 34 and 37 carbons. Females showed population-specific preferences for male EHC profiles. However, EHC profiles between mated and unmated males in the intrademic mating trials were not significantly different, consistent with undetectable sexual selection estimated directly from numbers of copulating pairs vs. unmated adults. Thus, sexual isolation among populations was much stronger than sexual selection within these populations of D. mojavensis.     

SPECIES HYBRIDS IN THE LABORATORY BUT NOT IN NATURE: A REANALYSIS OF PREMATING ISOLATION BETWEEN DROSOPHILA ARIZONAE AND D. MOJAVENSIS

Understanding speciation relies critically on the identification of mechanisms responsible for maintaining species integrity (i.e., reproductive isolation) especially when closely related species are sympatric in nature. Studies of reproductive isolation in Drosophila often involve laboratory mating experiments that assume that patterns of mate choice in the laboratory are similar to those in the wild. Two sibling species, Drosophila arizonae and D. mojavensis , known to exhibit low levels of interspecific hybridization in the laboratory, but not in nature, were used in multiple-choice mating trials using various mating chamber designs as well as synthetic and natural media for developing larvae and courting adults. Sympatric populations were more sexually isolated than allopatric ones, consistent with past studies, and all experimental variables tested (chamber size, host plant presence and rearing substrates) had significant effects on levels of premating isolation between these species. Flies reared on cactus showed increased premating isolation versus those reared on synthetic laboratory food as did providing fermenting host plant tissue during mating trials. Also, surprisingly, smaller mating chambers led to an increase in premating isolation versus larger containers. The design of these types of mating trials is thus critical to understanding how mating behaviors in the laboratory are related to those in natural populations.     

Population differentiation in plethodontid salamanders: divergence of allozymes and sexual compatibility among populations of Desmognathus imitator and D. ochrophaeus (Gaudata: Plethodontidae)

Plethodontid salamanders of the genus Desmognathus exhibit varying levels of genetic differentiation among and within both allopatric and sympatric taxa. This provides excellent opportunities to study population differentiation and speciation. Two morphologically similar species in this genus, D. imitator and D. ochrophaeus, are genetically well-differentiated from one another and occur in sympatry with no evidence of hybridization and introgression. We report that the degree of sexual isolation between these two species is very high, regardless of whether the populations under comparison are allopatric or sympatric with one another. Neither reinforcement nor reproductive character displacement are required to explain the evolution of sexual incompatibility in sympatry. Sexual behaviour apparently diverges while populations are allopatric with one another. Preliminary study indicates that D. imitator consists of populations among which there may be significant sexual isolation in the absence of detectable genetic differentiation.     

REPRODUCTIVE CHARACTER DISPLACEMENT OF MALE STICKLEBACK MATE PREFERENCE: REINFORCEMENT OR DIRECT SELECTION?

Reproductive isolation can evolve between species as a byproduct of adaptation to different niches, through reinforcement, and by direct selection on mating preferences. We investigated the potential role of direct selection in the reproductive isolation between sympatric species of threespine sticklebacks. Each sympatric pair consists of a small-bodied limnetic species and large-bodied benthic species. We compared the mate preferences and courtship behavior of males from one sympatric limnetic population and two allopatric populations. We used limnetic-like allopatric populations to control for the effects of ecological character displacement and adaptation to different niches on mate preferences. The sympatric limnetic males preferred the small limnetic females, whereas the allopatric limnetic-like males preferred the large benthic females, suggesting that adaptation to the limnetic niche does not automatically confer a preference for small limnetic females. This reproductive character displacement of male preference is consistent with the predictions of both reinforcement and direct selection on mate preferences. To test for direct selection, we assessed a prediction of one proposed mechanism: predation by benthic females on eggs guarded by limnetic males. The allopatric males come from populations in which there is no egg predation. Sympatric limnetic males were more aggressive toward benthic females than toward limnetic females, whereas the allopatric limnetic-like males did not treat the two types of females differently. The contrast in male behavior suggests that egg predation has shaped male preferences. Direct selection is potentially more effective than indirect selection via reinforcement, and it is likely that it has been important in building up reproductive isolation between limnetic and benthic sticklebacks.     

Are traits that experience reinforcement also under sexual selection?

Where closely related species occur in sympatry, reinforcement may result in the evolution of traits involved in species recognition that are at the same time used for within-species mate choice. Drosophila serrata lives in forested habitat on the east coast of Australia, and over the northern half of its distribution it coexists with a closely related species, Drosophila birchii. Here we show that the strength of reinforcing selection in natural populations is sufficient to generate reproductive character displacement along a 36-km transect across the contact between sympatric and allopatric populations of D. serrata. The sympatric and allopatric populations display genetically based differences in male cuticular hydrocarbons (CHCs), while female CHCs changed with latitude across the contact. The directional changes observed in male CHCs between sympatric and allopatric regions were the same changes that were generated by experimental sympatry in the laboratory, providing direct evidence that the changes across the contact zone are due to the presence of D. birchii. We show that sympatric and allopatric females differ in preference for male CHCs and that females from allopatric populations prefer allopatric-like male CHCs over sympatric-like CHCs. Male attractiveness within D. serrata may therefore be compromised by reinforcing selection, preventing the spread of sympatric-like blends to the area of allopatry.     

Strength of sexual and postmating prezygotic barriers varies between sympatric populations with different histories and species abundances

The impact of different reproductive barriers on species or population isolation may vary in different stages of speciation depending on evolutionary forces acting within species and through species’ interactions. Genetic incompatibilities between interacting species are expected to reinforce prezygotic barriers in sympatric populations and lead to cascade reinforcement between conspecific populations living within and outside the areas of sympatry. We tested these predictions and studied whether and how the strength and target of reinforcement between Drosophila montana and Drosophila flavomontana vary between sympatric populations with different histories and species abundances. All barriers between D. montana females and D. flavomontana males were nearly complete, while in the reciprocal cross strong postzygotic isolation was accompanied by prezygotic barriers whose strength varied according to population composition. Sexual isolation between D. flavomontana females and D. montana males was increased in long‐established sympatric populations, where D. flavomontana is abundant, while postmating prezygotic (PMPZ) barriers were stronger in populations where this species is a new invader and still rare and where female discrimination against heterospecific males was lower. Strengthening of sexual and PMPZ barriers in this cross also induced cascade reinforcement of respective barriers between D. flavomontana populations, which is a classic signature of reinforcement process.     

Asymmetrical Reinforcement and Wolbachia Infection in Drosophila

Reinforcement refers to the evolution of increased mating discrimination against heterospecific individuals in zones of geographic overlap and can be considered a final stage in the speciation process. One the factors that may affect reinforcement is the degree to which hybrid matings result in the permanent loss of genes from a species' gene pool. Matings between females of Drosophila subquinaria and males of D. recens result in high levels of offspring mortality, due to interspecific cytoplasmic incompatibility caused by Wolbachia infection of D. recens. Such hybrid inviability is not manifested in matings between D. recens females and D. subquinaria males. Here we ask whether the asymmetrical hybrid inviability is associated with a corresponding asymmetry in the level of reinforcement. The geographic ranges of D. recens and D. subquinaria were found to overlap across a broad belt of boreal forest in central Canada. Females of D. subquinaria from the zone of sympatry exhibit much stronger levels of discrimination against males of D. recens than do females from allopatric populations. In contrast, such reproductive character displacement is not evident in D. recens, consistent with the expected effects of unidirectional cytoplasmic incompatibility. Furthermore, there is substantial behavioral isolation within D. subquinaria, because females from populations sympatric with D. recens discriminate against allopatric conspecific males, whereas females from populations allopatric with D. recens show no discrimination against any conspecific males. Patterns of general genetic differentiation among populations are not consistent with patterns of behavioral discrimination, which suggests that the behavioral isolation within D. subquinaria results from selection against mating with Wolbachia-infected D. recens. Interspecific cytoplasmic incompatibility may contribute not only to post-mating isolation, an effect already widely recognized, but also to reinforcement, particularly in the uninfected species. The resulting reproductive character displacement not only increases behavioral isolation from the Wolbachia-infected species, but may also lead to behavioral isolation between populations of the uninfected species. Given the widespread occurrence of Wolbachia among insects, it thus appears that there are multiple ways by which these endosymbionts may directly and indirectly contribute to reproductive isolation and speciation.     

Asymmetrical reinforcement and Wolbachia infection in Drosophila

Reinforcement refers to the evolution of increased mating discrimination against heterospecific individuals in zones of geographic overlap and can be considered a final stage in the speciation process. One the factors that may affect reinforcement is the degree to which hybrid matings result in the permanent loss of genes from a species' gene pool. Matings between females of Drosophila subquinaria and males of D. recens result in high levels of offspring mortality, due to interspecific cytoplasmic incompatibility caused by Wolbachia infection of D. recens. Such hybrid inviability is not manifested in matings between D. recens females and D. subquinaria males. Here we ask whether the asymmetrical hybrid inviability is associated with a corresponding asymmetry in the level of reinforcement. The geographic ranges of D. recens and D. subquinaria were found to overlap across a broad belt of boreal forest in central Canada. Females of D. subquinaria from the zone of sympatry exhibit much stronger levels of discrimination against males of D. recens than do females from allopatric populations. In contrast, such reproductive character displacement is not evident in D. recens, consistent with the expected effects of unidirectional cytoplasmic incompatibility. Furthermore, there is substantial behavioral isolation within D. subquinaria, because females from populations sympatric with D. recens discriminate against allopatric conspecific males, whereas females from populations allopatric with D. recens show no discrimination against any conspecific males. Patterns of general genetic differentiation among populations are not consistent with patterns of behavioral discrimination, which suggests that the behavioral isolation within D. subquinaria results from selection against mating with Wolbachia-infected D. recens. Interspecific cytoplasmic incompatibility may contribute not only to post-mating isolation, an effect already widely recognized, but also to reinforcement, particularly in the uninfected species. The resulting reproductive character displacement not only increases behavioral isolation from the Wolbachia-infected species, but may also lead to behavioral isolation between populations of the uninfected species. Given the widespread occurrence of Wolbachia among insects, it thus appears that there are multiple ways by which these endosymbionts may directly and indirectly contribute to reproductive isolation and speciation.     

Reproductive character displacement of female mate preferences for male cuticular hydrocarbons in Drosophila subquinaria

Several lines of evidence implicate sexual isolation in both initiating and completing the speciation process. Although its existence is straightforward to demonstrate, understanding the evolution of sexual isolation requires identifying the underlying phenotypes responsible so that we can determine how these have diverged. Here, we study geographic variation in female mate preferences for male sexual displays in the fly Drosophila subquinaria. Female D. subquinaria that are sympatric with its sister species D. recens discriminate strongly against both D. recens and allopatric conspecific males, whereas females from allopatric populations do not. Furthermore, female mate preferences target at least in part a suite of cuticular hydrocarbons (CHCs) in males and geographic variation in CHCs mirrors the pattern of mate discrimination. In this study, we quantify female mate preferences for male CHCs from populations that span the geographic range of D. subquinaria. We find that the direction of linear sexual selection varies significantly between populations that are sympatric versus allopatric with D. recens in a pattern of reproductive character displacement. Differences in preference partially align with existing differences in CHCs and patterns of sexual isolation, although discrepancies remain that suggest the involvement of additional traits and/or more complex, nonlinear preference functions.     

Postmating-prezygotic isolation is not an important source of selection for reinforcement within and between species in Drosophila pseudoobscura and D. persimilis

Abstract Most work on adaptive speciation to date has focused on the role of low hybrid fitness as the force driving reinforcement (the evolution of premating isolation after secondary contact that reduces the likelihood of matings between populations). However, recent theoretical work has shown that postmating, prezygotic incompatibilities may also be important in driving premating isolation. We quantified premating, postmating-prezygotic, and early postzygotic fitness effects in crosses among three populations: Drosophila persimilis, D. pseudoobscura USA (sympatric to D. persimilis ), and D. pseudoobscura Bogotá (allopatric to D. persimilis ). Interspecific matings were more likely to fail when they involved the sympatric populations than when they involved the allopatric populations, consistent with reinforcement. We also found that failure rate in sympatric mating trials depended on whether D. persimilis females were paired with D. pseudoobscura males or the reverse. This asymmetry most likely indicates differences in discrimination against heterospecific males by females. By measuring egg laying rate, fertilization success and hatching success, we also compared components of postmating-prezygotic and early postzygotic isolation. Postmating-prezygotic fitness costs were small and not distinguishable between hetero- and conspecific crosses. Early postzygotic fitness effects due to hatching success differences were also small in between-population crosses. There was, however, a postzygotic fitness effect that may have resulted from an X-linked allele found in one of the two strains of D. pseudoobscura USA. We conclude that the postmating-prezygotic fitness costs we measured probably did not drive premating isolation in these species. Premating isolation is most likely driven in sympatric populations by previously known hybrid male sterility.     

Evolutionary relationships of Drosophila mojavensis geographic host races and their sister species Drosophila arizonae

The cactophilic Drosophila mojavensis species group living in the deserts and dry tropical forests of the southwestern United States and Mexico provides a valuable system for studies in diversification and speciation. Rigorous studies of the relationships between host races of D. mojavensis and the relationships among the members of the species group (D. mojavensis, Drosophila arizona, and Drosophila navojoa) are lacking. We used mitochondrial CO1 sequence data to address the phylogenetics and population genetics of this species group. In this study we have found that the sister species D. mojavensis and D. arizonae share no mitochondrial haplotypes and thus show no evidence for recent introgression. We estimate the divergence time between D. mojavensis and D. arizonae to be between 1.91 and 2.97 million years ago. D. arizonae shows little structure in our population genetic analyses but there is phylogenetic differentiation between southeastern and northern populations of D. arizonae. Drosophila mojavensis shows significant population and phylogenetic structure across the four geographic regions of its distribution. The mitochondrial data support an origin of D. mojavensis on the mainland with early differentiation into the populations now found in the Mojave Desert and the Mainland Sonoran Desert and later colonization of the Baja Peninsula, in contrast to previous models. Also, the sister clade to D. mojavensis/D. arizonae includes D. navojoa and Drosophila huaylasi. By defining the genetic relationships among these populations, we provide a foundation for more sophisticated hypothesis testing regarding the timing of early speciation events and host switches in this species group.     

Evolution of the mojavensis cluster of cactophilic Drosophila with descriptions of two new species

The mojavensis cluster of the repleta species group of Drosophila (Drosophilidae: Diptera) consists of three species. One is newly described as D. navojoa. A second species, described here as D. arizonae, replaces D. arizonensis, which has become a junior subjective synonym for D. mojavensis, the third species in the cluster. A phylogeny of the three species is presented, based on chromosomal inversions, morphology, and the ability to produce hybrids. Breakage points are assigned for all inversions, and male genitalia are figured; 186 crosses were made from 225 possible combinations among 15 geographic strains from the southwestern United States, Mexico, and Guatemala. It is confirmed that D. mojavensis and D. arizonae are very closely related and shown that D. navojoa is more distantly related in regard to all criteria. This relationship is supported by the geographical positions of the ancestral gene sequences in each species, which show a sequential northwest movement (D. navojoa----D. arizonae----D. mojavensis) from southern Mexico to southern California and northern Arizona. The relationship is also supported by the fact that D. navojoa breeds in Opuntia cactus, an ancestral behavior, whereas the other two species breed chiefly in Stenocereus cacti, a derived behavior. The possible role of this host plant shift in speciation is discussed.     

Geographical variation in reproductive character displacement in mate choice by male sailfin mollies

Female Amazon mollies, Poecilia formosa, are a unisexual species that reproduce by gynogenesis. They must coexist and mate with males of other species (usually the mollies Poecilia latipinna or Poecilia mexicana) to induce embryogenesis, but inheritance is strictly maternal. We examined the mating preference of the male sailfin molly, P. latipinna, for female sailfin mollies versus Amazon mollies, P. formosa. We compared the mating preferences of sympatric and allopatric populations collected throughout the Gulf Coast of North America. Male P. latipinna from six populations sympatric with Amazon mollies showed a significantly greater strength of preference for conspecific sailfin females than males from five populations that were allopatric with Amazon mollies. These results provide strong evidence for reproductive character displacement of male mate choice in sympatry. Furthermore, the large geographical range of populations that we tested revealed variation among populations within sympatry and allopatry, indicating that it is important to evaluate a large number of populations when examining reproductive character displacement.     

An Autosomal Factor from Drosophila Arizonae Restores Normal Spermatogenesis in Drosophila Mojavensis Males Carrying the D. Arizonae Y Chromosome

Males of Drosophila mojavensis whose Y chromosome is replaced by the Y chromosome of the sibling species Drosophila arizonae are sterile. It is shown that genetic material from the fourth chromosome of D. arizonae is necessary and sufficient, in single dose, to restore fertility in these males. In introgression and mapping experiments this material segregates as a single Mendelian factor (sperm motility factor, SMF). Light and electron microscopy studies of spermatogenesis in D. mojavensis males whose Y chromosome is replaced by introgression with the Y chromosome of D. arizonae (these males are symbolized as mojY(a)) revealed postmeiotic abnormalities all of which are restored when the SMF of D. arizonae is co-introgressed (these males are symbolized as mojY(a)SMF(a)). The number of mature sperm per bundle in mojY(a)SMF(a) is slightly less than in pure D. mojavensis and is even smaller in males whose fertility is rescued by introgression of the entire fourth chromosome of D. arizonae. These observations establish an interspecific incompatibility between the Y chromosome and an autosomal factor (or more than one tightly linked factors) that can be useful for the study of the evolution of male hybrid sterility in Drosophila and the genetic control of spermatogenesis.