Gametic selection,developmental trajectories,and extrinsic heterogeneity in Haldane's rule

Deciphering the genetic and developmental causes of the disproportionate rarity, inviability, and sterility of hybrid males, Haldane's rule, is important for understanding the evolution of reproductive isolation between species. Moreover, extrinsic and prezygotic factors can contribute to the magnitude of intrinsic isolation experienced between species with partial reproductive compatibility. Here, we use the nematodes Caenorhabditis briggsae and C. nigoni to quantify the sensitivity of hybrid male viability to extrinsic temperature and developmental timing, and test for a role of mito‐nuclear incompatibility as a genetic cause. We demonstrate that hybrid male inviability manifests almost entirely as embryonic, not larval, arrest and is maximal at the lowest rearing temperatures, indicating an intrinsic‐by‐extrinsic interaction to hybrid inviability. Crosses using mitochondrial substitution strains that have reciprocally introgressed mitochondrial and nuclear genomes show that mito‐nuclear incompatibility is not a dominant contributor to postzygotic isolation and does not drive Haldane's rule in this system. Crosses also reveal that competitive superiority of X‐bearing sperm provides a novel means by which postmating prezygotic factors exacerbate the rarity of hybrid males. These findings highlight the important roles of gametic, developmental, and extrinsic factors in modulating the manifestation of Haldane's rule.     

HALDANE'S RULE IS EXTENDED TO PLANTS WITH SEX CHROMOSOMES

Haldane's rule is an empirical phenomenon that has been observed in animals with sex chromosomes. The rule states that the heterogametic sex (XY or ZW) will be “absent, rare, or sterile” following hybridization between two species. Despite the near ubiquity of Haldane's rule in animal hybridizations, it has not been documented in organisms other than animals. Here, we show evidence for both rarity and sterility in hybrid male but not female offspring in crosses between three dioecious plant species from the genus Silene with heteromorphic (XY) sex chromosomes. Our results are consistent with Haldane's rule, extending its applicability to plants with sex chromosomes.     

Differential strength of sex-biased hybrid inferiority in impeding gene flow may be a cause of Haldane's rule

Abstract In animals, if one sex of the F₁ hybrid between two species is sterile or inviable, it is usually the heterogametic (XY or WZ) sex. This phenomenon, known as Haldane's rule, is currently thought to be coincidentally caused by different mechanisms in separate entities. The following questions have never been asked: Are heterogametic and homogametic inferiority (sterility or inviability) equivalent as isolating mechanisms? Could discrepancies between them, if existing, produce Haldane's rule? Here I consider sex‐biased hybrid inferiority strictly as an isolating mechanism, and quantitatively evaluate its strength in impeding gene flow. The comparison reveals that the ability of sex‐biased inferiority to impede gene flow varies according to the sex and chromosome involved. Heterogametic inferiority is a weaker barrier when unidirectional and a much stronger one when in compound reciprocal directions, compared with homogametic inferiority. Such differential strength may affect divergence in speciation and produce Haldane's rule.     

Geographic patterns of postzygotic isolation between two closely related widespread dung fly species (Sepsis cynipsea and Sepsis neocynipsea; Diptera: Sepsidae)

Identifying the contribution of pre‐ and postzygotic barriers to gene flow is a key goal of speciation research. The widespread dung fly species Sepsis cynipsea and Sepsis neocynipsea offer great potential for studying the speciation process over a range of opportunities for gene exchange within and across sister species (cross‐continental allopatry, continental parapatry and sympatry). We examined the role of postcopulatory isolating barriers by comparing female fecundity and egg‐to‐adult viability of F₁ and F₂ hybrids, as well as backcrosses of F₁ hybrids with the parental species, via replicated crosses of sym‐, para‐ and allopatric populations. Egg‐to‐adult viability was strongly but not totally suppressed in hybrids, and offspring production approached nil in the F₂ generation (hybrid breakdown), indicating yet unspecified intrinsic incompatibilities. Viable F₁ hybrid offspring showed almost absolute male (the heterogametic sex) sterility while females remained largely fertile, in accordance with Haldane's rule. Hybridization between the two species in European areas of sympatry (Swiss Alps) indicated only minor reinforcement based on fecundity traits. Crossing geographically isolated European and North American S. neocynipsea showed similar albeit weaker isolating barriers that are most easily explained by random genetic drift. We conclude that in this system with a biogeographic continuum of reproductive barriers, speciation is mediated primarily by genetic drift following dispersal of flies over a wide (allopatric) geographic range, with some role of natural or sexual selection in incidental or direct reinforcement of incompatibility mechanisms in areas of European sympatry. S(ubs)pecies status of continental S. neocynipsea appears warranted.     

X‐AUTOSOME INCOMPATIBILITIES IN DROSOPHILA MELANOGASTER: TESTS OF HALDANE'S RULE AND GEOGRAPHIC PATTERNS WITHIN SPECIES

Substantial genetic variation exists in natural populations of Drosophila melanogaster. This segregating variation includes alleles at different loci that interact to cause lethality or sterility (synthetic incompatibilities). Fitness epistasis in natural populations has important implications for speciation and the rate of adaptive evolution. To assess the prevalence of epistatic fitness interactions, we placed naturally occurring X chromosomes into genetic backgrounds derived from different geographic locations. Considerable amounts of synthetic incompatibilities were observed between X chromosomes and autosomes: greater than 44% of all combinations were either lethal or sterile. Sex‐specific lethality and sterility were also tested to determine whether Haldane's rule holds for within‐species variation. Surprisingly, we observed an excess of female sterility in genotypes that were homozygous, but not heterozygous, for the X chromosome. The recessive nature of these incompatibilities is similar to that predicted for incompatibilities underlying Haldane's rule. Our study also found higher levels of sterility and lethality for genomes that contain chromosomes from different geographical regions. These findings are consistent with the view that genomes are coadapted gene complexes and that geography affects the likelihood of epistatic fitness interactions.     

CYTOLOGICAL CHARACTERIZATION OF PREMEIOTIC VERSUS POSTMEIOTIC DEFECTS PRODUCING HYBRID MALE STERILITY AMONG SIBLING SPECIES OF THE DROSOPHILA MELANOGASTER COMPLEX

In accordance with Haldane's rule, hybridizations between species of the Drosophila simulans clade produce fertile females but sterile males. In this study, a comprehensive characterization was undertaken on the six types of F₁ males that were the result of the crosses between D. simulans, D. sechellia, and D. mauritiana. With the use of light and electron microscopy, it was shown that while each particular hybrid genotype exhibited a specific sterility phenotype, these phenotypes fell into two distinct classes. The two hybrid genotypes that possessed D. mauritiana X-chromosomes contained spermatogenic defects that caused arrests in premeiotic spermatogenic stages. The other four F₁ hybrids possessed postmeiotic spermatogenic defects. Nonsynchronous cell divisions, underdeveloped mitochondrial derivative-axonemal associations, and microtubule abnormalities were common to all of these hybrids. Each particular postmeiotically defective hybrid genotype demonstrated characteristically distinct profiles in sperm bundle number in addition to characteristic spermiogenic arrests in the furthest developed spermatids. These results in species hybrids contrast with the absence of significant differences in spermatogenic characters between species of this clade. In addition, by utilizing an attached-X cross, we investigated the influence of maternal effects and cytoplasmic factors on the sterility of D. simulans F₁ hybrids and found none. However, we discovered a strain of D. simulans (2119) that caused a large shift in sterility from postmeiotic to premeiotic when crossed to D. sechellia. This suggests that D. simulans is polymorphic for genes involving premeiotic and postmeiotic sterility and that the two types of sterilities between species may have a simple genetic basis.     

Gene flow across a hybrid zone maintained by a weak heterogametic incompatibility and positive selection of incompatible alleles

Hybridization between incipient species is more likely to produce sterile or inviable F₁ offspring in the heterogametic (XY or ZW) sex than in the homogametic (XX or ZZ) sex, a phenomenon known as Haldane's rule. Population dynamics associated with Haldane's rule may play an important role in early speciation of sexually reproducing organisms. The dynamics of the hybrid zone maintained by incomplete hybrid inferiority (sterility/inviability) in the heterogametic sex (a ‘weak’ Haldane's rule) caused by a Bateson–Dobzhansky–Muller incompatibility was modelled. The influences and interplays of the strengths of incompatibility, dispersal, density‐dependent regulation (DDR) and local adaptation of incompatible alleles in a scenario of short‐range dispersal (the stepping‐stone model) were examined. It was found that a partial heterogametic hybrid incompatibility could efficiently impede gene flow and maintain characteristic clinal noncoincidence and discordance of alleles. Density‐dependent regulation appears to be an important factor affecting hybrid zone dynamics: it can effectively skew the effects of the partial incompatibility and dispersal as measured by effective dispersal, clinal structures and density depression. Unexpectedly, local adaptation of incompatible alleles in the parental populations, which would be critical for the establishment of the incompatibility, exerts little effect on hybrid zone dynamics. These results strongly support the plausibility of the adaptive origin of hybrid incompatibility and ecological speciation: an adaptive mutation, if it confers a marginal fitness advantage in the local population and happens to cause epistatic inferiority in hybrids, could efficiently drive further genetic divergence that may result in the gene becoming an evolutionary hotspot.     

PATTERNS OF SPECIATION IN DROSOPHILA

To investigate the time course of speciation, we gathered literature data on 119 pairs of closely related Drosophila species with known genetic distances, mating discrimination, strength of hybrid sterility and inviability, and geographic ranges. Because genetic distance is correlated with divergence time, these data provide a cross-section of taxa at different stages of speciation. Mating discrimination and the sterility or inviability of hybrids increase gradually with time. Hybrid sterility and inviability evolve at similar rates. Among allopatric species, mating discrimination and postzygotic isolation evolve at comparable rates, but among sympatric species strong mating discrimination appears well before severe sterility or inviability. This suggests that prezygotic reproductive isolation may be reinforced when allopatric taxa become sympatric. Analysis of the evolution of postzygotic isolation shows that recently diverged taxa usually produce sterile or inviable male but not female hybrids. Moreover, there is a large temporal gap between the evolution of male-limited and female hybrid sterility or inviability. This gap, which is predicted by recent theories about the genetics of speciation, explains the overwhelming preponderance of hybridizations yielding male-limited hybrid sterility or inviability (Haldane's rule).     

Evolution of postzygotic reproductive isolation in galliform birds: analysis of first and second hybrid generations and backcrosses

The process of speciation is a crucial aspect of evolutionary biology. In this study, we analysed the patterns of evolution of postzygotic reproductive isolation in Galliformes using information on hybridization and genetic distance among species. Four main patterns arose: (1) hybrid inviability and sterility in F₁ hybrids increase as species diverge; (2) the presence of geographical overlap does not affect the evolution of postzygotic isolation; (3) the galliforms follow Haldane's rule; (4) hybrid inviability is higher in F₂ than in F₁ hybrids, but does not appear to be increased in the backcrosses. This study contributes to the growing evidence suggesting that the patterns of evolution of postzygotic isolation and the process of speciation are shared among avian groups (and animals in general). In particular, our results support the notion of F₂ hybrid inviability as being key for the maintenance of species genetic integrity when prezygotic isolation barriers are overcome in closely related species, in which postzygotic isolation in the F₁ hybrid might still not be fully developed. To the contrary, hybrids from backcrosses did not show serious inviability problems (at least not more than F₁ hybrids), demonstrating that they could generate gene flow among bird species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 528–542.     

Exogenous selection rather than cytonuclear incompatibilities shapes asymmetrical fitness of reciprocal Arabidopsis hybrids

Reciprocal crosses between species often display an asymmetry in the fitness of F₁ hybrids. This pattern, referred to as isolation asymmetry or Darwin's corollary to Haldane's rule, is a general feature of reproductive isolation in plants, yet factors determining its magnitude and direction remain unclear. We evaluated reciprocal species crosses between two naturally hybridizing diploid species of Arabidopsis to assess the degree of isolation asymmetry at different postmating life stages. We found that pollen from Arabidopsis arenosa will usually fertilize ovules from Arabidopsis lyrata; the reverse receptivity being less complete. Maternal A. lyrata parents set more F₁ hybrid seed, but germinate at lower frequency, reversing the asymmetry. As predicted by theory, A. lyrata (the maternal parent with lower seed viability in crosses) exhibited accelerated chloroplast evolution, indicating that cytonuclear incompatibilities may play a role in reproductive isolation. However, this direction of asymmetrical reproductive isolation is not replicated in natural suture zones, where delayed hybrid breakdown of fertility at later developmental stages, or later‐acting selection against A. arenosa maternal hybrids (unrelated to hybrid fertility, e.g., substrate adaptation) may be responsible for an excess of A. lyrata maternal hybrids. Exogenous selection rather than cytonuclear incompatibilities thus shapes the asymmetrical postmating isolation in nature.     

DISSECTING THE GENETIC ARCHITECTURE OF F1 HYBRID STERILITY IN HOUSE MICE

Hybrid sterility as a postzygotic reproductive isolation mechanism has been studied for over 80 years, yet the first identifications of hybrid sterility genes in Drosophila and mouse are quite recent. To study the genetic architecture of F₁ hybrid sterility between young subspecies of house mouse Mus m. domesticus and M. m. musculus, we conducted QTL analysis of a backcross between inbred strains representing these two subspecies and probed the role of individual chromosomes in hybrid sterility using the intersubspecific chromosome substitution strains. We provide direct evidence that the asymmetry in male infertility between reciprocal crosses is conferred by the middle region of M. m. musculus Chr X, thus excluding other potential candidates such as Y, imprinted genes, and mitochondrial DNA. QTL analysis identified strong hybrid sterility loci on Chr 17 and Chr X and predicted a set of interchangeable autosomal loci, a subset of which is sufficient to activate the Dobzhansky–Muller incompatibility of the strong loci. Overall, our results indicate the oligogenic nature of F₁ hybrid sterility, which should be amenable to reconstruction by proper combination of chromosome substitution strains. Such a prefabricated model system should help to uncover the gene networks and molecular mechanisms underlying hybrid sterility.     

TEMPERATURE EFFECTS AND GENOTYPE-BY-ENVIRONMENT INTERACTIONS IN HYBRIDS: HALDANE'S RULE IN FLOUR BEETLES

When males of the flour beetle, Tribolium castaneum, are crossed to females of its close relative T. freemani, the sex ratio of the hybrids is female biased, owing in part to hybrid male mortality. Morphological abnormalities are also frequent in the surviving hybrid males, but not in the hybrid females. The finding that the heterogametic sex (male) is more adversely affected in interspecific crosses than the homogametic sex is consistent with Haldane's rule, which predicts that hybrid dysfunction should emerge as an indirect byproduct of divergent adaptation to differing environments. If so, environmental effects and genotype-by-environment interactions (GEI) should characterize the expression of Haldane's rule and interspecific hybrid traits in general. We used two wild-collected populations of T. castaneum (from Infantes, Spain, and Madagascar) to investigate the effects of environmental variation on the expression of Haldane's rule. Males from each population were mated to several T. freemani females and the half-sibling hybrid progenies were reared across a series of temperature regimes. For both populations, we found that hybrids raised at higher temperatures exhibited a more extreme expression of Haldane's rule: The hybrid sex ratios were more biased toward females and hybrid males had a much higher incidence of morphological abnormalities. The average response to temperature, the norm of reaction for Haldane's rule, varied between the two populations, and we found considerable and significant GEI for both hybrid traits within both populations. The evolutionary implications of these findings are discussed in the context of speciation arising as an indirect effect of local adaptation.     

THE GENETIC STRUCTURE OF A GYNODIOECIOUS PLANT: NUCLEAR AND CYTOPLASMIC GENES

Sex expression in gynodioecious plants is often determined by an interaction between biparentally and maternally inherited genes. Their relative rates of gene flow should be considered when modeling the evolution of the sex ratio in structured populations. In order to understand patterns of gene flow in Silene vulgaris, a gynodioecious plant, genetic structure was estimated from biparentally inherited genetic markers (allozymes) and a maternally inherited marker (chloroplast DNA) using Wright's F_st. Based on data from 16 local populations, chloroplast DNA showed considerably more genetic structure than did allozymes (F_st values of 0.62 and 0.22, respectively). This suggests that the rate of gene flow is about three times greater for nuclear genes.     

DIVERGENCE OF MEIOTIC DRIVE-SUPPRESSION SYSTEMS AS AN EXPLANATION FOR SEX-BIASED HYBRID STERILITY AND INVIABILITY

Two empirical generalizations about speciation remain unexplained: the tendency of the heterogametic sex to be sterile or inviable in F₁ hybrids (Haldane's rule), and the tendency of the X chromosome to harbor the genetic elements that cause this sex bias in hybrid fitness. I suggest that divergence of meiotic drive systems on the sex chromosomes can explain these observations. The theory follows from two simple facts. First, sex chromosomes are particularly susceptible to the forces of meiotic drive. Second, divergence of meiotic drive systems can cause hybrid sterility and in viability. The main objection to the theory is that meiotic drive is apparently rare, whereas the observed pattern of hybrid fitness is widespread. I answer this objection by showing that divergence of meiotic drive systems can explain the two generalizations even if large departures from Mendelian segregation are rarely observed.     

GENETICS OF STERILITY IN HYBRIDS BETWEEN TWO SUBSPECIES OF DROSOPHILA

Hybrids between D. pseudoobscura bogotana and D. pseudoobscura pseudoobscura are fertile except for males produced in one of the two reciprocal crosses. As there is no premating isolation between these subspecies, nonreciprocal male sterility represents the first step in speciation. Genetic analysis reveals two causes of hybrid F₁ sterility: a maternal effect and incompatibilities between chromosomes within males. The maternal effect appears to play the greatest role in hybrid sterility. The X chromosome has the largest effect on fertility of any chromosome, a ubiquitous result in analyses of hybrid sterility and inviability in Drosophila. This effect is entirely attributable to a region comprising less than 30% of the X chromosome. These results are compared to those from a similar study of D. pseudoobscura-D. persimilis hybrids, an older and more reproductively isolated species pair in the same lineage. Such comparisons may allow one to identify the genetic changes characterizing the early versus late stages of speciation.     

Speciation in two neotropical butterflies: extending Haldane's rule

Anartia fatima and A. amathea form a hybrid zone in Panama where F1 and back-cross hybrids are found. Crosses were carried out to determine the nature of any reproductive isolation between these two butterflies. A novel analysis demonstrated both strong assortative mating among the pure forms and an unusual example of Haldane''s rule: F1 hybrid females (the heterogametic sex) from the cross A. amathea (female) multiplied by A. fatima (male) have a reduced tendency to mate. Historically, Haldane''s rule has been restricted to hybrid mortality or sterility and most studies have concentrated on taxa (predominantly Drosophila) between which strong barriers to gene flow already exist. Our data suggest that Haldane''s rule might be extended to cover any decrease in hybrid fitness and that mating propensity may provide a sensitive and comparable means of assessing such decreases. Other barriers to gene flow were also evident in Anartia: F1 hybrid females have reduced fertility (also a Haldane effect) and larval survivorship was greatly reduced in F2 hybrids of both sexes. These examples of hybrid disruption are expected under the dominance theory of Haldane''s rule but do not exclude other explanations.     

Explaining Darwin’s Corollary to Haldane’s Rule: The Role of Mitonuclear Interactions in Asymmetric Postzygotic Isolation Among Toads

We examine the basis of Darwin’s corollary to Haldane’s rule, which describes viability and fertility differences between F₁ produced from reciprocal crosses. We analyzed asymmetries in hybrid viability from >100 reciprocal crosses involving 36 toad species to test whether relatively high rates of mitochondrial vs. nuclear evolution produce dams with systematically less viable F₁ hybrid progeny. We find no such effect, suggesting a predominant role for stochastic accumulation of asymmetric epistatic incompatibilities.     

CROSSING RELATIONSHIPS AMONG DIPLOID SPECIES OF THE SOLANUM NIGRUM COMPLEX IN NORTH AMERICA

Crosses were made in all combinations of the six diploid species of the Solarium nigrum complex that occur in North America. Some interspecific pollinations failed to yield viable seed; successful crosses gave rise to moderately to highly sterile F₁ hybrids. Results of interspecific crosses suggested phylogenetic relationships that were not completely in accord with those suggested by morphology. Interspecific crosses also gave varied results. All interpopulational crosses within S. interius and S. sarachoides produced fully fertile hybrids. In contrast, hybrids within S. americanum and S. douglasii varied from fully fertile to almost completely sterile. Populations of S. pseudogracile could be divided into two groups which are geographically separated but not morphologically differentiated. Fully fertile hybrids resulted from crosses within a group, whereas crosses between groups gave hybrids with reduced fertility. Four crossing groups were observed within 5. nodiflorum; three of the parental groups are sympatric and are morphologically differentiated. Although hybrid sterility in interspecific crosses is sometimes used to support delimitation of species, the presence of sterility in intraspecific crosses suggests that such an interpretation is unwarranted for the S. nigrum complex. Hybrid sterility, therefore, is not considered to have special taxonomic significance in this complex.