Wolbachia-induced unidirectional cytoplasmic incompatibility and speciation: mainland-island model

Bacteria of the genus Wolbachia are among the most common endosymbionts in the world. In many insect species these bacteria induce a sperm-egg incompatibility between the gametes of infected males and uninfected females, commonly called unidirectional cytoplasmic incompatibility (CI). It is generally believed that unidirectional CI cannot promote speciation in hosts because infection differences between populations will be unstable and subsequent gene flow will eliminate genetic differences between diverging populations. In the present study we investigate this question theoretically in a mainland-island model with migration from mainland to island. Our analysis shows that (a) the infection polymorphism is stable below a critical migration rate, (b) an (initially) uninfected "island" can better maintain divergence at a selected locus (e.g. can adapt locally) in the presence of CI, and (c) unidirectional CI selects for premating isolation in (initially) uninfected island populations if they receive migration from a Wolbachia-infected mainland. Interestingly, premating isolation is most likely to evolve if levels of incompatibility are intermediate and if either the infection causes fecundity reductions or Wolbachia transmission is incomplete. This is because under these circumstances an infection pattern with an infected mainland and a mostly uninfected island can persist in the face of comparably high migration. We present analytical results for all three findings: (a) a lower estimation of the critical migration rate in the presence of local adaptation, (b) an analytical approximation for the gene flow reduction caused by unidirectional CI, and (c) a heuristic formula describing the invasion success of mutants at a mate preference locus. These findings generally suggest that Wolbachia-induced unidirectional CI can be a factor in divergence and speciation of hosts.     

Bidirectional cytoplasmic incompatibility and the stable coexistence of two Wolbachia strains in parapatric host populations

Wolbachiaare intracellular bacteria which are very widely distributed among arthropods. In many insect species Wolbachiaare known to induce cytoplasmic mating incompatibility (CI). It has been suggested that Wolbachiacould promote speciation in their hosts if parapatric host populations are infected with two different Wolbachiastrains causing bidirectional mating incompatibilities. A necessary condition for this speciation scenario to work is that the two Wolbachiastrains can stably coexist. The following study investigates this problem analysing a mathematical model with two host populations and migration between them. We show that the stability of bidirectional CI can be fully described in terms of a critical migration rate which is defined as the highest migration below which a stable coexistence of two Wolbachiastrains is possible. For some special cases we could derive analytical solutions for the critical migration rate; for the general case estimations of the critical migration rate are given. Our main finding is that bidirectional CI can stably persist in the face of high migration and can be as high as over 15% per generation for CI levels observed in nature. These results have implications for the potential of Wolbachiato promote genetic divergence and speciation in their hosts.     

Prezygotic isolation,mating preferences,and the evolution of chromosomal inversions

Chromosomal inversions are frequently implicated in isolating species. Models have shown how inversions can evolve in the context of postmating isolation. Inversions are also frequently associated with mating preferences, a topic that has not been studied theoretically. Here, we show how inversions can spread by capturing a mating preference locus and one or more loci involved with epistatic incompatibilities. Inversions can be established under broad conditions ranging from near panmixis to nearly complete speciation. These results provide a hypothesis to explain the growing number of examples of inversions associated with premating isolating mechanisms.     

The evolution of host preference in allopatric vs. parapatric populations of Timema cristinae walking-sticks

Divergent habitat preferences can contribute to speciation, as has been observed for host-plant preferences in phytophagous insects. Geographic variation in host preference can provide insight into the causes of preference evolution. For example, selection against maladaptive host-switching occurs only when multiple hosts are available in the local environment and can result in greater divergence in regions with multiple vs. a single host. Conversely, costs of finding a suitable host can select for preference even in populations using a single host. Some populations of Timema cristinae occur in regions with only one host-plant species present (in allopatry, surrounded by unsuitable hosts) whereas others occur in regions with two host-plant species adjacent to one another (in parapatry). Here, we use host choice and reciprocal-rearing experiments to document genetic divergence in host preference among 33 populations of T. cristinae. Populations feeding on Ceanothus exhibited a stronger preference for Ceanothus than did populations feeding on Adenostoma. Both allopatric and parapatric pairs of populations using the different hosts exhibited divergent host preferences, but the degree of divergence tended to be greater between allopatric pairs. Thus, gene flow between parapatric populations apparently constrains divergence. Host preferences led to levels of premating isolation between populations using alternate hosts that were comparable in magnitude to previously documented premating isolation caused by natural and sexual selection against migrants between hosts. Our findings demonstrate how gene flow and different forms of selection interact to determine the magnitude of reproductive isolation observed in nature.     

ON THE COYNE AND ORR‐IGIN OF SPECIES: EFFECTS OF INTRINSIC POSTZYGOTIC ISOLATION,ECOLOGICAL DIFFERENTIATION,X CHROMOSOME SIZE,AND SYMPATRY ON DROSOPHILA SPECIATION

Coyne and Orr found that mating discrimination (premating isolation) evolves much faster between sympatric than allopatric Drosophila species pairs. Their meta‐analyses established that this pattern, expected under reinforcement, is common and that Haldane's rule is ubiquitous in Drosophila species divergence. We examine three possible contributors to the reinforcement pattern: intrinsic postzygotic isolation, dichotomized as to whether hybrid males show complete inviability/sterility; host‐plant divergence, as a surrogate for extrinsic postzygotic isolation; and X chromosome size, whether roughly 20% or 40% of the genome is X‐linked. We focus on “young” species pairs with overlapping ranges, contrasted with allopatric pairs. Using alternative criteria for “sympatry” and tests that compare either level of prezygotic isolation in sympatry or frequency of sympatry, we find no statistically significant effects associated with X chromosome size or our coarse quantifications of intrinsic postzygotic isolation or ecological differentiation. Although sympatric speciation seems very rare in animals, the pervasiveness of the reinforcement pattern and the commonness of range overlap for close relatives indicate that speciation in Drosophila is often not purely allopatric. It remains to determine whether increased premating isolation with sympatry results from secondary contact versus parapatric speciation and what drives this pattern.     

Comparison of sexual compatibility in crosses between the southern and northern populations of the cabbage beetle Colaphellus bowringi

It is widely accepted that the genetic divergence and reproductive incompat- ibility between closely related species and/or populations is often viewed as an important step toward speciation. In this study, sexual compatibility in crosses between the southern XS population and the northern TA population of the polyandrous cabbage beetle Co- laphellus bowringi was investigated by testing their mating preferences, mating latency, copulation duration, and reproductive performances of post-mating. In choice mating ex- periments, the percentages ofmatings were significantly higher in intra-population crosses than in inter-population crosses. Both isolation index （/） and index of pair sexual isolation （/PSi） indicated partial mating incompatibility or assortative mating in crosses between the two different geographical populations. In single pair mating experiments, XS females in inter-population crosses mated significantly later and copulated significantly shorter than those in intra-population crosses. However, TA females in inter-population crosses mated significantly earlier and copulated longer than those in intra-population crosses, suggesting that larger XS males may enhance heterotypic mating. The lifetime fecundity was highest in XS homotypic matings, lowest in TA homotypic matings, and intermedi- ate in heterotypic rnatings between their parents. The inter-population crosses resulted in significantly lower egg hatching rate and shorter female longevity than intra-population crosses. These results demonstrated that there exist some incompatibilities in premating, postmating-prezygotic, and postzygotic stages between the southern XS population and northern TA population of the cabbage beetle Colaphellus bowringi.     

GENETIC ARCHITECTURE AND POSTZYGOTIC REPRODUCTIVE ISOLATION: EVOLUTION OF BATESON–DOBZHANSKY–MULLER INCOMPATIBILITIES IN A POLYGENIC MODEL

The Bateson–Dobzhansky–Muller model predicts that postzygotic isolation evolves due to the accumulation of incompatible epistatic interactions, but few studies have quantified the relationship between genetic architecture and patterns of reproductive divergence. We examined how the direction and magnitude of epistatic interactions in a polygenic trait under stabilizing selection influenced the evolution of hybrid incompatibilities. We found that populations evolving independently under stabilizing selection experienced suites of compensatory allelic changes that resulted in genetic divergence between populations despite the maintenance of a stable, high‐fitness phenotype. A small number of loci were then incompatible with multiple alleles in the genetic background of the hybrid and the identity of these incompatibility loci changed over the evolution of the populations. For F₁ hybrids, reduced fitness evolved in a window of intermediate strengths of epistatic interactions, but F₂ and backcross hybrids evolved reduced fitness across weak and moderate strengths of epistasis due to segregation variance. Strong epistatic interactions constrained the allelic divergence of parental populations and prevented the development of reproductive isolation. Because many traits with varying genetic architectures must be under stabilizing selection, our results indicate that polygenetic drift is a plausible hypothesis for the evolution of postzygotic reproductive isolation.     

The effect of Wolbachia on genetic divergence between populations: models with two-way migration

Wolbachia are intracellular bacteria that cause various reproduction alterations in their hosts, including cytoplasmic incompatibility (CI), an incompatibility between sperm and egg that typically results in embryonic death. We investigate theoretically the effects of Wolbachia-induced bidirectional CI on levels of divergence between two populations, where there is migration in both directions and differential selection at a single locus. The main findings are as follows: Wolbachia differences in the two populations are maintained up to a threshold migration rate, above which the system collapses to a single Wolbachia type; differential selection at a nuclear locus increases the threshold migration rate below which Wolbachia polymorphisms are maintained; Wolbachia differences between the populations enhance their genetic divergence at the selected locus by reducing the "effective migration rate," and even moderate levels of CI can cause large population differences in allele frequencies; and asymmetric CI can induce strong asymmetries in effective migration rate and dramatically alter the pattern of genetic divergence compared with the No Wolbachia situation. We derive an analytical approximation for the effective migration rate, which matches the simulation results for most parameter values. These results generally support the view that CI Wolbachia can contribute to genetic divergence between populations.     

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.     

Speciation as a positive feedback loop between postzygotic and prezygotic barriers to gene flow

Speciation is intimately associated with the evolution of sex-and-reproduction-related traits, including those affecting hybrid incompatibility (postzygotic isolation) and species recognition (prezygotic isolation). Genes controlling such traits are not randomly distributed in the genome but are particularly abundant on the sex chromosomes. However, the evolutionary consequences of the sex linkage of genes involved in speciation have been little explored. Here, we present simulations of a continent-island diploid model that examines the effects of reduced recombination using both autosomal and sex-linked inheritance. We show first that linkage between genes affecting postzygotic and prezygotic isolation leads to a positive feedback loop in which both are strengthened. As species recognition evolves, genes causing hybrid incompatibility will hitchhike along with those improving premating isolation, leading to stronger hybrid incompatibility and thus increased pressure for further preference divergence. Second, we show that this loop effect is generally enhanced by sex linkage, because recombination is eliminated in the heterogametic sex, leading to tighter effective linkage between the two classes of genes and because natural selection is more efficient at sex-linked loci, as recessive alleles are not masked by dominance in the heterogametic sex. Accordingly, hitchhiking can be important in promoting speciation and can also lead to increased postzygotic isolation through adaptive evolution.     

BEYOND REINFORCEMENT: THE EVOLUTION OF PREMATING ISOLATION BY DIRECT SELECTION ON PREFERENCES AND POSTMATING, PREZYGOTIC INCOMPATIBILITIES

Abstract The evolution of premating isolation after secondary contact is primarily considered in the guise of reinforcement, which relies on low hybrid fitness as the driving force for mating preference divergence. Here I consider two additional forces that may play a substantial role in the adaptive evolution of premating isolation, direct selection on preferences and indirect selection against postmating, prezygotic incompatibilities. First, I argue that a combination of ecological character displacement and sensory bias can cause direct selection on preferences that results in the pattern of reproductive character displacement. Both analytical and numerical methods are then used to demonstrate that, as expected from work in single populations, such direct selection will easily overwhelm indirect selection due to low hybrid fitness as the primary determinant of preference evolution. Second, postmating, prezygotic incompatibilities are presented as a driving force in the evolution of premating isolation. Two classes of these mechanisms, those increasing female mortality after mating but before producing offspring and those reducing female fertility, are shown to be identical in their effects on preference divergence. Analytical and numerical techniques are then used to demonstrate that postmating, prezygotic factors may place strong selection on preference divergence. These selective forces are shown to be comparable if not greater than those produced by the low fitness of hybrids.     

The Allonemobius-Wolbachia host-endosymbiont system: evidence for rapid speciation and against reproductive isolation driven by cytoplasmic incompatibility

Evidence for the evolution of fertilization incompatibilities and rapid speciation can be biased by the occurrence of hybridization and reproductive endosymbionts such as Wolbachia. For example, patterns of mitochondrial DNA (mtDNA) variation can be obscured by mitotypes hitchhiking on extrachromosomal elements like Wolbachia, while such endosymbionts can also induce phenotypes that mirror the operation of intrinsic fertilization incompatibilities between species. Therefore, before strong inferences can be drawn concerning the rates and processes of speciation in arthropod systems, we must first assess whether extrinsic endosymbionts obscure patterns of speciation. Here, I use the Allonemobius fasciatus-socius species complex to determine what role Wolbachia has played in the presumed rapid divergence of this complex by analyzing patterns of mtDNA and nuclear DNA variation in conjunction with sequence and cytoplasmic incompatibility data on Wolbachia. Data on molecular variation suggest that Wolbachia has not induced a strong selective sweep of the mitochondrial genome; nor does Wolbachia appear to induce cytoplasmic incompatibility. Preliminary evidence indicates that a third species identified within this complex, A. sp. nov. Tex, is partially reproductively isolated from A. socius, its closest relative, via conspecific sperm precedence or some form of postzygotic isolation. Moreover, shared mitotypes between A. sp. nov. Tex and A. socius may indicate the occurrence of a hybrid zone between these species near the border of Texas and Louisiana, although they may also represent shared ancestral polymorphisms. Molecular data also indicate that all three species in this complex diverged from a common ancestor as recently as 3000-30,000 years ago. Finally, the radiation of this complex from its ancestral population likely occurred in the presence of one strain of Wolbachia, thus suggesting a minimal role for Wolbachia during this burst of speciation. In total, barriers to gene flow do appear to have evolved very rapidly in this group of crickets.     

Host genotype changes bidirectional to unidirectional cytoplasmic incompatibility in Nasonia longicornis

Wolbachia are the most abundant maternally inherited endosymbionts of insects and cause various reproductive alterations in their hosts. One such manipulation is cytoplasmic incompatibility (CI), which is a sperm-egg incompatibility typically resulting in zygotic death. Nasonia longicornis (Hymenoptera: Pteromalidae) has an A supergroup and two closely related B supergroup Wolbachia infections. The B supergroup bacteria co-diverged in this host genus. Both triple (wNlonAwNlonB1wNlonB2) and double infections (wNlonAwNlonB1, wNlonAwNlonB2) have been obtained from the field. In the present study, CI was determined among the three Wolbachia types in different host genetic backgrounds. Results show that host genetic background determines whether bidirectional CI or unidirectional CI occurs between the two closely related B group Wolbachia. Results show that the wNlonB1-infected males are bidirectionally incompatible with wNlonB2 in their 'native' nuclear genetic background, whereas wNlonB1 males are compatible with wNlonB2 in two other N. longicornis genetic backgrounds, resulting in unidirectional CI. In contrast, wNlonB2-infected males are incompatible with wNlonB1 females in all three host genetic backgrounds. These changes in incompatibility are not due to the loss of the bacteria. We hypothesize that a repressor gene for sperm modification by wNlonB1 is segregating in N. longicornis populations. The relevance of these findings to the potential role of Wolbachia in host-reproductive divergence and speciation is discussed.     

Wolbachia and genetic variability in the birdnest blowfly Protocalliphora sialia

Wolbachia are widespread cytoplasmically inherited bacteria that induce various reproductive alterations in host arthropods, including cytoplasmic incompatibility (CI), an incompatibility between sperm and egg that typically results in embryonic death. CI has been invoked as a possible mechanism for reproductive isolation and speciation in arthropods, by restricting gene flow and promoting maintenance (and evolution) of genetic divergence between populations. Here we investigate patterns of Wolbachia infection and nuclear and mitochondrial differentiation in geographical populations of the birdnest blowfly Protocalliphora sialia. Blowflies in western North America are infected with two A-group Wolbachia, with some individuals singly and others doubly infected. Individuals in eastern North America mostly show single infections with a B-group Wolbachia. Populations in the Midwest are polymorphic for infections and show A- or B-group infection. There is a low level of mitochondrial divergence and perfect concordance of mitochondrial haplotype with infection type, suggesting that two Wolbachia-associated selective sweeps of the mitochondrion have occurred in this species. Amplified fragment length polymorphism analysis of nuclear genetic variation shows genetic differentiation between the eastern-Midwestern and western populations. Both Midwestern and eastern flies infected with A-Wolbachia show eastern nuclear genetic profiles. Current results therefore suggest that Wolbachia has not acted as a major barrier to gene flow between western and eastern-Midwestern populations, although some genetic differentiation between A-Wolbachia infected and B-Wolbachia infected individuals in eastern-Midwestern populations cannot be ruled out.     

Premating, not postmating, barriers drive genetic dynamics in experimental hybrid populations of the endangered Sonoran topminnow

The timing and pattern of reproductive barrier formation in allopatric populations has received much less attention than the accumulation of reproductive barriers in sympatry. The theory of allopatric speciation suggests that reproductive barriers evolve simply as by-products of overall genetic divergence. However, observations of enhanced premating barriers in allopatric populations suggest that sexual selection driven by intraspecific competition for mates may enhance species-specific signals and accelerate the speciation process. In a previous series of laboratory trials, we examined the strength of premating and postmating barriers in an allopatric species pair of the endangered Sonoran topminnow, Poeciliopsis occidentalis and P. sonoriensis. Behavioral observations provided evidence of asymmetrical assortative mating, while reduced brood sizes and male-biased F(1) sex ratios suggest postmating incompatibilities. Here we examine the combined effects of premating and postmating barriers on the genetic makeup of mixed populations, using cytonuclear genotype frequencies of first- and second-generation offspring. Observed genotype frequencies strongly reflect the directional assortative mating observed in behavioral trials, illustrating how isolating barriers that act earlier in the reproductive cycle will have a greater effect on total reproductive isolation and may be more important to speciation than subsequent postmating reproductive barriers.     

Reproductive isolation among allopatric Drosophila montana populations

An outstanding goal in speciation research is to trace the mode and tempo of the evolution of barriers to gene flow. Such research benefits from studying incipient speciation, in which speciation between populations has not yet occurred, but where multiple potential mechanisms of reproductive isolation (RI: i.e., premating, postmating‐prezygotic (PMPZ), and postzygotic barriers) may act. We used such a system to investigate these barriers among allopatric populations of Drosophila montana. In all heteropopulation crosses we found premating (sexual) isolation, which was either symmetric or asymmetric depending on the population pair compared. Postmating isolation was particularly strong in crosses involving males from one of the study populations, and while sperm were successfully transferred, stored, and motile, we experimentally demonstrated that the majority of eggs produced were unfertilized. Thus, we identified the nature of a PMPZ incompatibility. There was no evidence of intrinsic postzygotic effects. Measures of absolute and relative strengths of pre‐ and postmating barriers showed that populations differed in the mode and magnitude of RI barriers. Our results indicate that incipient RI among populations can be driven by different contributions of both premating and PMPZ barriers occurring between different population pairs and without the evolution of postzygotic barriers.     

SPECIATION DUE TO HYBRID NECROSIS IN PLANT–PATHOGEN MODELS

We develop a model for speciation due to postzygotic incompatibility generated by autoimmune reactions. The model is based on frequency‐dependent interactions between host plants and their pathogens, which can generate disruptive selection and give rise to speciation if distant phenotypes become reproductively isolated. Based on recent experimental evidence from Arabidopsis, we assume that at the molecular level, incompatibility between host strains is caused by epistatic interactions between two proteins in the plant immune system—the guard and the guardee. Within each plant strain, immune reactions occur when the guardee protein is modified by a pathogen effector, and the guard subsequently binds to the guardee, thus precipitating an immune response. When guard and guardee proteins come from phenotypically distant parents, a hybrid's immune system can be triggered by erroneous interactions between these proteins even in the absence of pathogen attack, leading to severe autoimmune reactions in hybrids. This generates a Dobzhnasky–Muller incompatibility due to immune reactions. Our model shows how phenotypic variation generated by frequency‐dependent host–pathogen interactions can lead to such postzygotic incompatibilities between extremal types, and hence to speciation.     

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.     

Finding Hybrid Incompatibilities Using Genome Sequences from Hybrid Populations

Natural hybrid zones offer a powerful framework for understanding the genetic basis of speciation in progress because ongoing hybridization continually creates unfavorable gene combinations. Evidence indicates that postzygotic reproductive isolation is often caused by epistatic interactions between mutations in different genes that evolved independently of one another (hybrid incompatibilities). We examined the potential to detect epistatic selection against incompatibilities from genome sequence data using the site frequency spectrum (SFS) of polymorphisms by conducting individual-based simulations in SLiM. We found that the genome-wide SFS in hybrid populations assumes a diagnostic shape, with the continual input of fixed differences between source populations via migration inducing a mass at intermediate allele frequency. Epistatic selection locally distorts the SFS as non-incompatibility alleles rise in frequency in a manner analogous to a selective sweep. Building on these results, we present a statistical method to identify genomic regions containing incompatibility loci that locates departures in the local SFS compared with the genome-wide SFS. Cross-validation studies demonstrate that our method detects recessive and codominant incompatibilities across a range of scenarios varying in the strength of epistatic selection, migration rate, and hybrid zone age. Our approach takes advantage of whole genome sequence data, does not require knowledge of demographic history, and can be applied to any pair of nascent species that forms a hybrid zone.     

A two-locus model of speciation.

Speciation is considered as the evolution of partial or complete cross-incompatibility between the carriers of genes (at a locus called "object locus") that distinguish the prospective species populations. The mating relations at the object locus are modified by the alleles at a second mating modifier locus. Based on a widely applicable concept of fitness and mating preference, it is shown that heterozygote disadvantage in fitness at the object locus is necessary for speciation, which corroborates Wallace's hypothesis. It is pointed out that the difference between sympatric and parapatric speciation essentially lies in the mechanisms stabilizing the polymorphism required at the object locus as a prerequisite for speciation. In the presence of recombination between the object and mating modifier locus speciation may be prevented by forces maintaining gametic phase imbalance between these loci such as can result from unidirectional gene flow between parapatric populations.