Establishment Of Reproductive Isolation Research Articles

Phylogeographic analysis reveals multiple origins of the desert shrub Reaumuria songarica in northern Xinjiang, involving homoploid and tetraploid hybrids.

Hybrid speciation plays an important role in species diversification. The establishment of reproductive isolation is crucial for hybrid speciation, and the identification of diverse types of hybrids, particularly homoploid hybrid species, contributes to a comprehensive understanding of this process. Reaumuria songarica is a constructive shrub widespread in arid Central Asia. Previous studies have inferred that the R. songarica populations in the Gurbantunggut Desert (GuD) originated from homoploid hybridizations between its eastern and western lineages and may have evolved into an incipient species. To further elucidate the genetic composition of different hybrid populations and to determine the species boundary of this hybrid lineage, we investigated the overall phylogeographic structure of R. songarica based on variation patterns of five cpDNA and one nrITS sequences across 32 populations. Phylogenetic analyses demonstrated that within the GuD lineage, the Wuerhe population evolved directly from ancestral lineages, whereas the others originated from hybridizations between the eastern and western lineages. PCoA and genetic barrier analysis supported the subdivision of the GuD lineage into the southern (GuD-S) and northern (GuD-N) groups. Populations in the GuD-S group had a consistent genetic composition and the same ancestral female parent, indicating that they belonged to a homoploid hybrid lineage. However, the GuD-N group experienced genetic admixture of the eastern and western lineages on nrITS and cpDNA, with some populations inferred to be allopolyploid based on ploidy data. Based on cpDNA haplotypes, BEAST analyses showed that the GuD-S and GuD-N groups originated after 0.5 Ma. Our results suggest that multiple expansions and contractions of GuD, driven by Quaternary climatic oscillations and the Kunlun-Yellow River tectonic movement, are important causes of the complex origins of R. songarica populations in northern Xinjiang. This study highlights the complex origins of the Junggar Basin flora and the underappreciated role of hybridization in increasing its species diversity.

Trait Variation and Spatiotemporal Dynamics across Avian Secondary Contact Zones.

A secondary contact zone (SCZ) is an area where incipient species or divergent populations may meet, mate, and hybridize. Due to the diverse patterns of interspecific hybridization, SCZs function as field labs for illuminating the on-going evolutionary processes of speciation and the establishment of reproductive isolation. Interspecific hybridization is widely present in avian populations, making them an ideal system for SCZ studies. This review exhaustively summarizes the variations in unique traits within avian SCZs (vocalization, plumage, beak, and migratory traits) and the various movement patterns of SCZs observed in previous publications. It also highlights several potential future research directions in the genomic era, such as the relationship between phenotypic and genomic differentiation in SCZs, the genomic basis of trait differentiation, SCZs shared by multiple species, and accurate predictive models for forecasting future movements under climate change and human disturbances. This review aims to provide a more comprehensive understanding of speciation processes and offers a theoretical foundation for species conservation.

Incipient speciation in Oncocyclus irises: Eco-geographic isolation and genetic divergence with no reproductive isolation?

The establishment of reproductive isolation is considered to be an ultimate result of ecological speciation, but empirical evidence for the latter is limited. We hypothesized that in the absence of inter-specific gene flow and with subtle environmental differences, local adaptation will not require trade-offs in performance across environments and therefore will not result in reproductive isolation. We tested this hypothesis on four iris species, Iris atrofusca, I. atropurpurea, I. petrana and I. mariae, which have non-overlapping geographical distributions, grow naturally in different environmental conditions with respect to amount of rainfall and soil type, and possess suites of diagnostic traits that previously were assumed to result from local selection. We analyzed their genetic (AFLP) and quantitative trait divergence (by means of a common garden experiment), conducted a habitat suitability analysis, examined experimentally the effects of soil type and water availability on plant performance, and tested for postzygotic reproductive isolation using a crossing experiment. Our results supported the hypothesis that eco-geographical isolation does not necessarily lead to local adaptation or to postzygotic reproductive isolation when environmental differences are subtle. Thus, we obtained some evidence that the desert I. atrofusca was reproductively isolated from coastal I. atropurpurea, but not from desert I. mariae, although genetically the first two are more similar to each other than I. atrofusca is to I. mariae.

Rapid homoploid hybrid speciation in British gardens: The origin of Oxford ragwort (Senecio squalidus).

Hybridisation can lead to homoploid hybrid speciation, i.e., the origin of new species without change in chromosome number between parents and offspring. Central to homoploid hybrid speciation is the role of hybridisation in the establishment of reproductive isolation between the hybrid and the parental species in the early stages of speciation, when typically all species occur at least partly in sympatry. In this work we analyse genome-wide polymorphism data obtained by transcriptome sequencing of the British hybrid species Oxford ragwort (Senecio squalidus, Asteraceae), its two Italian parental species (S. aethnensis and S. chrysanthemifolius) and their naturally occurring hybrids on Mt Etna (Italy). We show that Oxford ragwort most likely originated from de novo hybridisation between its two Italian parental species whilst they were in cultivation in British gardens at the turn of the 18th century. Reproductive isolation between the new hybrid species and its parental species probably resulted from inheritance of genetic incompatibilities between the two parental species and subsequent ecological segregation - both of which have been shown in previous studies. Our results imply that S. squalidus meets the most stringent criteria set forth to identify homoploid hybrid speciation, and call attention to the creative role of hybridisation in responding to novel environmental conditions.

Additive Traits Lead to Feeding Advantage and Reproductive Isolation, Promoting Homoploid Hybrid Speciation.

Speciation through homoploid hybridization (HHS) is considered extremely rare in animals. This is mainly because the establishment of reproductive isolation as a product of hybridization is uncommon. Additionally, many traits are underpinned by polygeny and/or incomplete dominance, where the hybrid phenotype is an additive blend of parental characteristics. Phenotypically intermediate hybrids are usually at a fitness disadvantage compared with parental species and tend to vanish through backcrossing with parental population(s). It is therefore unknown whether the additive nature of hybrid traits in itself could lead successfully to HHS. Using a multi-marker genetic data set and a meta-analysis of diet and morphology, we investigated a potential case of HHS in the prions (Pachyptila spp.), seabirds distinguished by their bills, prey choice, and timing of breeding. Using approximate Bayesian computation, we show that the medium-billed Salvin’s prion (Pachyptila salvini) could be a hybrid between the narrow-billed Antarctic prion (Pachyptila desolata) and broad-billed prion (Pachyptila vittata). Remarkably, P. salvini’s intermediate bill width has given it a feeding advantage with respect to the other Pachyptila species, allowing it to consume a broader range of prey, potentially increasing its fitness. Available metadata showed that P. salvini is also intermediate in breeding phenology and, with no overlap in breeding times, it is effectively reproductively isolated from either parental species through allochrony. These results provide evidence for a case of HHS in nature, and show for the first time that additivity of divergent parental traits alone can lead directly to increased hybrid fitness and reproductive isolation.

The role of hybridization during ecological divergence of southwestern white pine (Pinus strobiformis) and limber pine (P. flexilis).

Interactions between extrinsic factors, such as disruptive selection and intrinsic factors, such as genetic incompatibilities among loci, often contribute to the maintenance of species boundaries. The relative roles of these factors in the establishment of reproductive isolation can be examined using species pairs characterized by gene flow throughout their divergence history. We investigated the process of speciation and the maintenance of species boundaries between Pinus strobiformis and Pinus flexilis. Utilizing ecological niche modelling, demographic modelling and genomic cline analyses, we illustrated a divergence history with continuous gene flow. Our results supported an abundance of advanced generation hybrids and a lack of loci exhibiting steep transition in allele frequency across the hybrid zone. Additionally, we found evidence for climate-associated variation in the hybrid index and niche divergence between parental species and the hybrid zone. These results are consistent with extrinsic factors, such as climate, being an important isolating mechanism. A build-up of intrinsic incompatibilities and of coadapted gene complexes is also apparent, although these appear to be in the earliest stages of development. This supports previous work in coniferous species demonstrating the importance of extrinsic factors in facilitating speciation. Overall, our findings lend support to the hypothesis that varying strength and direction of selection pressures across the long lifespans of conifers, in combination with their other life history traits, delays the evolution of strong intrinsic incompatibilities.

Evidence of Adaptive Evolution and Relaxed Constraints in Sex-Biased Genes of South American and West Indies Fruit Flies (Diptera: Tephritidae).

Several studies have demonstrated that genes differentially expressed between sexes (sex-biased genes) tend to evolve faster than unbiased genes, particularly in males. The reason for this accelerated evolution is not clear, but several explanations have involved adaptive and nonadaptive mechanisms. Furthermore, the differences of sex-biased expression patterns of closely related species are also little explored out of Drosophila. To address the evolutionary processes involved with sex-biased expression in species with incipient differentiation, we analyzed male and female transcriptomes of Anastrepha fraterculus and Anastrepha obliqua, a pair of species that have diverged recently, likely in the presence of gene flow. Using these data, we inferred differentiation indexes and evolutionary rates and tested for signals of selection in thousands of genes expressed in head and reproductive transcriptomes from both species. Our results indicate that sex-biased and reproductive-biased genes evolve faster than unbiased genes in both species, which is due to both adaptive pressure and relaxed constraints. Furthermore, among male-biased genes evolving under positive selection, we identified some related to sexual functions such as courtship behavior and fertility. These findings suggest that sex-biased genes may have played important roles in the establishment of reproductive isolation between these species, due to a combination of selection and drift, and unveil a plethora of genetic markers useful for more studies in these species and their differentiation.

Assortative mating and intrapopulation polymorphism maintenance in natural populations and laboratory lines of insects

Speciation is a microevolutionary process that starts with intraspecies differentiation, followed by the establishment of reproductive isolation. One of the intrinsic isolating factors causing physiological isolation is assortative mating. As a result of many years of observations of the natural populations on the territory of the South Urals (the Bashkortostan Republic) and long-term laboratory experiments, we revealed assortative mating in populations of the potato beetle Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae). The assortativity of mating manifested itself as a statistically significant deviation from the normal distribution of the frequencies of imago pairs belonging to three morphological types, which are defined by the degree of integument pattern melanization, namely, the achromists (A), melanists (M), and intermediate type (I). In copulo, we have not detected mating between achromist males (A) and melanist females (M), with the active choice in this case belonging to males. Under the laboratory conditions, with a sampling of 40 artificially formed pairs, we revealed statistical differences in the longevity and breeding performance of imagoes belonging to different morphological types. Achromists and melanists were characterized by a statistically higher longevity, almost two times higher than that in individuals of the intermediate type. Females of the intermediate type showed statistically higher fecundity in the homonomous crosses than achromists and melanists. It was demonstrated that the viability of the progeny also differed statistically in different pairs. The highest viability was detected for the progeny obtained from the type A and type I females. The observed differences indicated the existence of different reproductive strategies in studied species populations. In the S laboratory line of the common house fly Musca domestica L. (Diptera: Muscidae), we detected individuals with different reproductive strategies associated with longevity. The maximum longevity in two inbred lines, Sh28 (short-living) and L2 line (long-living), isolated from the S line showed a statistical, almost twofold difference. Mass reproduction time is confined to the first or second week of the imago stage in the short-living fly line; it is shifted to the third or fourth week in long-living flies. In reciprocal crosses between the lines, we detected a dramatic decrease in the fecundity of the females and a decrease in the viability of the hybrid females. The results confirm the essential role of assortative mating and frequency-dependent selection in the maintenance of the intrapopulation diversity.

Asymmetric Introgression in a Spotted Salamander Hybrid Zone.

Before the establishment of reproductive isolation, deeply diverged intraspecific lineages can experience complex genetic and behavioral interactions as they come into secondary contact. Divergent selective and demographic processes mediate gene flow among lineages, resulting in hybrid zones with complex biogeographic structure. Discordance in the biogeographic patterns of autosomal and maternally inherited loci provides a useful window to infer the processes mediating admixture and introgression across hybrid zones. Here, we sampled 489 genotypes across a hybrid zone between 2 phylogeographic lineages of the spotted salamander, Ambystoma maculatum, and characterize discordant patterns of nuclear and mitochondrial introgression across the contact boundary. Our results indicate asymmetric introgression of nuclear DNA beyond the contact boundary from the western to eastern lineage, with introgression of eastern mitochondrial DNA into the western lineage. We discuss alternative mechanisms for this pattern and attribute this result to neutral patterns of population expansion of the western lineage into the east in combination with female mate choice for larger-bodied western males. Our results underscore the complexity of interacting mechanisms that give rise to reproductive asymmetries in the earliest stages of the speciation process.

Advances inEcologicalSpeciation: an integrative approach

The role of natural selection in promoting reproductive isolation has received substantial renewed interest within the last two decades. As a consequence, the study of ecological speciation has become an extremely productive research area in modern evolutionary biology. Recent innovations in sequencing technologies offer an unprecedented opportunity to study the mechanisms involved in ecological speciation. Genome scans provide significant insights but have some important limitations; efforts are needed to integrate them with other approaches to make full use of the sequencing data deluge. An international conference 'Advances in Ecological Speciation' organized by the University of Porto (Portugal) aimed to review current progress in ecological speciation. Using some of the examples presented at the conference, we highlight the benefits of integrating ecological and genomic data and discuss different mechanisms of parallel evolution. Finally, future avenues of research are suggested to advance our knowledge concerning the role of natural selection in the establishment of reproductive isolation during ecological speciation.

Male Dimorphism in a New Interstitial Species of the Genus Microloxoconcha (Podocopida: Ostracoda)

Abstract The marine interstitial Microloxoconcha dimorpha n. sp. has two morphotypes in the male, “L type” and “S type”; they occur sympatrically and can be distinguished by the size of their carapaces and the morphology of the male copulatory organs. Conversely, the genetic relationships based on the partial mitochondrial COI gene did not demonstrate an independent clade as belonging to only one type. The L and S types are therefore not reproductively isolated from each other, and they express an intra-sexual dimorphism. We also argue that the morphological features of the male copulatory organ could have changed prior to the establishment of reproductive isolation.

INFLUENCE OF ANTIBIOTIC TREATMENT AND WOLBACHIA CURING ON SEXUAL ISOLATION AMONG DROSOPHILA MELANOGASTER CAGE POPULATIONS

Speciation depends on the establishment of reproductive isolation between populations of the same species. Whether assortative mating evolves as a by-product of adaptation is a major question relevant to the origin of species by reproductive isolation. The long-term selection populations used here were originally established 30 years ago from a single cage population (originating from a maternal one) and subsequently subjected to divergent selection for tolerance of toxins in food (heavy metals versus ethanol) to investigate this question. Those populations now differ in sexual isolation and Wolbachia infection status. Wolbachia are common and widespread bacteria infecting arthropods and nematodes. Attention has recently focused on their potential role in insect speciation, due to post-mating sperm-egg incompatibilities induced by the bacteria. In this paper we examine the potential effect of Wolbachia on the level of sexual isolation. By antibiotic curing, we show that removal of Wolbachia decreases levels of mate discrimination (sexual isolation index) between populations by about 50%. Backcrossing experiments confirm that this effect is due to infection status rather than to genetic changes in the populations resulting from antibiotic treatment. Antibiotic treatment has no effect on mate discrimination level between uninfected populations. Our findings suggest that the presence of Wolbachia (or another undetected bacterial associate) act as an additive factor contributing to the level of pre-mating isolation between these Drosophila melanogaster populations. Given the ubiquity of bacterial associates of insects, such effects could be relevant to some speciation events.

INFLUENCE OF ANTIBIOTIC TREATMENT AND WOLBACHIA CURING ON SEXUAL ISOLATION AMONG DROSOPHILA MELANOGASTER CAGE POPULATIONS

Speciation depends on the establishment of reproductive isolation between populations of the same species. Whether assortative mating evolves as a by-product of adaptation is a major question relevant to the origin of species by reproductive isolation. The long-term selection populations used here were originally established 30 years ago from a single cage population (originating from a maternal one) and subsequently subjected to divergent selection for tolerance of toxins in food (heavy metals versus ethanol) to investigate this question. Those populations now differ in sexual isolation and Wolbachia infection status. Wolbachia are common and widespread bacteria infecting arthropods and nematodes. Attention has recently focused on their potential role in insect speciation, due to post-mating sperm-egg incompatibilities induced by the bacteria. In this paper we examine the potential effect of Wolbachia on the level of sexual isolation. By antibiotic curing, we show that removal of Wolbachia decreases levels of mate discrimination (sexual isolation index) between populations by about 50%. Backcrossing experiments confirm that this effect is due to infection status rather than to genetic changes in the populations resulting from antibiotic treatment. Antibiotic treatment has no effect on mate discrimination level between uninfected populations. Our findings suggest that the presence of Wolbachia (or another undetected bacterial associate) act as an additive factor contributing to the level of pre-mating isolation between these Drosophila melanogaster populations. Given the ubiquity of bacterial associates of insects, such effects could be relevant to some speciation events.

Morphological difference and reproductive isolation: morphometrics in the millipede Parafontaria tonominea and its allied forms

Thin-plate spline and other multivariate statistical methods were used to examine the variation in genital morphology and body size in 74 populations of the millipede Parafontaria tonominea and its allied forms in central Japan. Although the study populations included nine sympatric pairs of two reproductively isolated forms, the variations in genital morphology and body size observed were continuous across the study group. Analyses of the sympatric pairs using bootstrapping revealed that the degree of morphological difference between sympatric populations varied considerably from pair to pair. These results demonstrate that the degree of difference in genital morphology and body size may not correspond with the establishment of reproductive isolation. Significant correlation in the pattern of geographic variation was observed for many combinations of morphological components. However, it was not possible to detect a significant correlation between morphology and geographic distance in many of the morphological components studied. This study suggests that the incorporation of genital characters in classification should be done carefully, particularly for animal groups with low vagility, such as millipedes.

Morphological difference and reproductive isolation: morphometrics in the millipede Parafontaria tonominea and its allied forms

Thin-plate spline and other multivariate statistical methods were used to examine the variation in genital morphology and body size in 74 populations of the millipede Parafontaria tonominea and its allied forms in central Japan. Although the study populations included nine sympatric pairs of two reproductively isolated forms, the variations in genital morphology and body size observed were continuous across the study group. Analyses of the sympatric pairs using bootstrapping revealed that the degree of morphological difference between sympatric populations varied considerably from pair to pair. These results demonstrate that the degree of difference in genital morphology and body size may not correspond with the establishment of reproductive isolation. Significant correlation in the pattern of geographic variation was observed for many combinations of morphological components. However, it was not possible to detect a significant correlation between morphology and geographic distance in many of the morphological components studied. This study suggests that the incorporation of genital characters in classification should be done carefully, particularly for animal groups with low vagility, such as millipedes.

Culex pipiens in London Underground tunnels: differentiation between surface and subterranean populations.

Genetic variation was quantified between surface-dwelling populations of Culex pipiens and the so-called molestus form found in the London Underground (the Underground) railway system. The molestus form is a commercially important biting nuisance and in the southern part of its range is also a disease vector. The surface and subterranean populations were genetically distinct, with no evidence of gene flow between closely adjacent populations of the different forms, whereas there was little differentiation between the different populations of each form. The substantially reduced heterozygosity in the Underground populations and the allelic composition suggest that colonization of the Underground has occurred once or very few times. Breeding experiments show compatibility between the Underground populations but not with those breeding above ground. There is evidence of greater gene flow and a mixing of molestus and pipiens traits in the south of the species range. This paper considers the processes that may allow establishment of reproductive isolation in the north of the species range but not in the south.

What is gradualism? Cryptic speciation in globorotaliid foraminifera

Analysis of the evolution of the Globorotalia (Fohsella) lineage of planktic foraminifera suggests that reproductive ecology and shell shape have evolved independently in this group. The silhouette of fohsellid shells displays a nearly unbroken anagenetic trend, yet isotopic data show that the fohsellids changed their depth of reproduction during the anagenetic evolution of their skeletons. Remarkably, there are no correlations between anagenesis in skeletal shape and the establishment of reproductive isolation. Apparently, anagenesis masks at least one speciation event that is apparent only in the isotopic evidence for a change in reproductive ecology. Although anagenetic trends have been widely cited as evidence for gradual speciation in planktic foraminifera and other microfossil groups, our data suggest that they should not always be considered to record either the tempo or mode of speciation.Speciation was apparently uncoupled from morphological evolution in fohsellids because these evolutionary phenomena occurred in different phases of ontogeny. Gradual morphological changes were associated with the main phase of shell growth of both the ancestor and descendant species in the near-surface ocean. Reproductive isolation occurred when ancestral and descendant populations became established at different depths near the end of the life cycle. Morphological evolution may also be uncoupled from reproductive isolation in other organisms that experience very different selection pressures over the duration of their ontogenies, such as parasites with many hosts, species with multiple phases of metamorphosis, and organisms that broadcast their gametes.

Chromosome Evolution and Speciation in Rodents

Chromosomal differences are frequently associated with taxonomic differences at the species level. It has been assumed that this association in large taxonomic groups, such as the Order Rodentia, means that karyotypic divergence is causally involved both as part of population differentiation (Le. speciation) and as an adaptive component in phyletic evolution (21, 28, 147, 149). There are two proximate effects of chromosomal rearrangements that are generally in· voked to explain these observed associations, which are: (a) the meiotic effects of structural heterozygosity and the establishment of reproductive isolation; or (b) the presumed position·effect alteration of gene function and the creation of cytologically defined, coadapted gene complexes (e.g. 41). These two alternative views look at the effects of different components of fitness. The first focuses on fertility and/or fecundity in relation to the meiotic effects of rearranged chromosomes, and the second emphasizes phenotypic adaptation (i.e. temporal survivorship). Both classes of effects are intuitively appealing, and as a result, both are now commonly assumed in discussions of the role of chromosome change in speciation and adaptive evolution. However, these assumptions need to be critically reexamined.

PLEIOTROPY AND PARAPATRIC SPECIATION.

Parapatric speciation as described by Murray (1972), Bush (1975), and Endler (1977) is the process of species formation from two or more populations that initially have some gene flow among them. This process is different from allopatric speciation, which requires that there first be a complete absence of gene flow, and sympatric speciation, which requires that there initially be random mating (Bush, 1975). Parapatric speciation was discussed in genetical terms by Fisher (1958, p. 142-143), who postulated that if populations were subject to different selection pressures in different locations, then natural selection would reduce the gene flow among them. Fisher proposed two mechanisms for reducing the gene flow, lower dispersal tendencies and greater selectivity of the females in each population for males from their own population. modification of dispersal has not received further discussion as a mechanism of speciation because it does not lead to reproductive isolation, but the evolution of assortative mating-now called the (Murray, 1972)-has been the principal mechanism in existing models of sympatric and parapatric speciation (e.g., Maynard Smith, 1966; Dickinson and Antonovics, 1973; Endler, 1977; Felsenstein, 1981). A consequence of the Wallace effect is that the resultant species are isolated only by pre-zygotic isolating mechanisms. Another way for reproductive isolation to evolve is as a byproduct of genetic changes at selected loci. Muller (1940, 1942) and Thoday and Gibson (1970) used the information on pleiotropy in Drosophila to argue that speciation could result from such pleiotropic effects. Wright (1969 and elsewhere) has long emphasized the importance of pleiotropic effects of most mutations, and Mayr (1963, p. 551) implicitly invokes pleiotropy in the establishment of reproductive isolation in allopatric populations: The ecological shifts in incipient species are bound to have an effect on their isolating mechanisms. thesis of the origin of reproductive isolation as a byproduct of the total genetic reconstitution of the speciating population is consistent with all known facts. In this paper I will investigate the potential importance of pleiotropy in parapatric populations and show, by means of a simple model, that pleiotropic effects at different loci tend to reinforce each other and can lead to speciation in parapatric populations. Reproductive isolation can be a byproduct of genetic changes at a locus under two conditions. Either the locus could control two characters, one of which is subject to natural selection and the other of which affects mating success, or the locus could control a single character which serves two purposes. Traditionally, a pleiotropic locus is one that affects two or more characters, and this is the kind of pleiotropy that Muller and others have had in mind when discussing the role of pleiotropy in speciation. Maynard Smith (1966) argued that this type of pleiotropy was unlikely to be of importance for speciation. However, the second possibility, namely a character that is strongly selected also affecting mating success, may well be important for speciation. A good example of a character that serves two roles is flowering time in plants. McNeilly and Antonovics (1968, Table 2) list 28 examples of known ecotypic differences in flowering time. Differences in flowering time could evolve completely through the Wallace effect as a mechanism causing assortative mating between ecotypes. McNeilly and Antonovics (1968) provide strong evidence

Sex chromosome translocations in the evolution of reproductive isolation.

Haldane's rule states that in organisms with differentiated sex chromosomes, hybrid sterility or inviability is generally expressed more frequently in the heterogametic sex. This observation has been variously explained as due to either genic or chromosomal imbalance. The fixation probabilities and mean times to fixation of sex-chromosome translocations of the type necessary to explain Haldane's rule on the basis of chromosomal imbalance have been estimated in small populations of Drosophila melanogaster. The fixation probability of an X chromosome carrying the long arm of the Y(X.Y(L)) is approximately 30% greater than expected under the assumption of no selection. No fitness differences associated with the attached Y(L) segment were detected. The fixation probability of a deficient Y chromosome is 300% greater than expected when the X chromosome contains the deleted portion of the Y. It is suggested that sex-chromosome translocations may play a role in the establishment of reproductive isolation.