Hybrid Zone Dynamics Research Articles

Temporal shifts in the saltmarsh–Nelson’s sparrow hybrid zone revealed by replicated demographic and genetic surveys

Conservation of threatened or endangered species in a hybrid zone requires a comprehensive understanding of interspecific dynamics over time and space. We evaluated changes in location and composition of a hybrid zone over a 15-year period (with replicated sampling in 1997–2000 and 2011–2013) for saltmarsh (Ammodramus caudacutus) and Nelson’s (A. nelsoni) sparrows, two tidal marsh specialists of high conservation priority in the northeastern United States. We combined genetic analyses using microsatellite and mitochondrial markers with species distribution patterns. In both time periods, replicate genetic sampling (n = 85; five sites) and field population surveys (93 sites) were conducted. We compared the distribution of parental species and hybrids and estimates for hybrid zone width and center between the two time periods. An increase in the relative proportion of Nelson’s sparrows in sympatric marshes and an approximate doubling of hybrid zone width provides evidence for expansion. Introgression rates increased over time for neutral loci but declined for a mitochondrial gene and two gene-associated loci under the influence of selection, as expected under a speciation model with barriers to gene flow. On average, the center of the hybrid zone shifted 60 km southward over the 15 years. We placed our findings within a policy framework to evaluate management options for hybrids. We conclude that despite increasing rates of introgression, hybridization poses a substantially lesser threat to parental populations than the imminent consequences of sea-level rise and habitat degradation. Based on our current knowledge of hybrid zone dynamics in this system, we conclude that the conservation of hybrids is warranted along with parental species at this time.

Concordant morphological and molecular clines in a contact zone of the Common and Spined toad (Bufo bufo and B. spinosus) in the northwest of France

BackgroundHybrid zones are regions where individuals of two species meet and produce hybrid progeny, and are often regarded as natural laboratories to understand the process of species formation. Two microevolutionary processes can take place in hybrid zones, with opposing effects on population differentiation. Hybridization tends to produce genetic homogenization, reducing species differences, whereas the presence of mechanisms of reproductive isolation result in barriers to gene flow, maintaining or increasing differences between taxa.ResultsHere we study a contact zone between two hybridizing toad species, Bufo bufo and B. spinosus, through a combination of molecular (12 polymorphic microsatellites, four nuclear and two mitochondrial SNP markers) and morphological data in a transect in the northwest of France. The results show largely concordant clines across markers, defining a narrow hybrid zone of ca. 30 km wide. Most hybrids in the centre of the contact zone are classified as F2 or backcrossed individuals, with no individuals assigned to the F1 hybrid class.ConclusionsWe discuss the implications of these results for our understanding of the evolutionary history of these species. We anticipate that the toad contact zone here described will become an important asset in the study of hybrid zone dynamics and evolutionary biology because of its easy access and the abundance of the species involved.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-016-0184-7) contains supplementary material, which is available to authorized users.

Gene flow among Hancornia speciosa (Apocynaceae) varieties and hybrid fitness

The fate of hybrids and the temporal and spatial dynamics of hybrid zones depend on hybrid fitness in comparison to non-hybrids. We studied cross-pollination among Hancornia speciosa varieties and compared progeny fitness in a nursery to address whether hybrid fitness differed from non-hybrids and whether maternal and paternal taxa contribute differentially to offspring fitness. This species has edible fruit pulp that is used as a raw material for candies, ice cream, and juice by small- and medium-sized enterprises in Central-West and Northeast Brazil. We genotyped 258 adults from a germplasm collection and 320 seeds using seven microsatellite loci to estimate genetic parameters and determine pollen donors. Fitness components, days to shoot, growth rate (mm/day), leaf width (mm), leaf length (mm), stem diameter (mm), and plant height (cm) were analyzed in 200 individuals. Genetic diversity and polymorphism did not differ neither between adults and progeny arrays nor among the four varieties. Genetic differentiation among varieties (F CT = 0.019, p < 0.001) and among populations within varieties (F SC = 0.053, p < 0.001) was significant but low. We detected mating among the four varieties, and no self-pollination was observed in parentage analysis, confirming that H. speciosa is self-incompatible with a high outcrossing rate (t m = 0.990, SE = 0.007). No significant effect of heterosis or exogamic depression was detected for any fitness component, but maternal contribution significantly affected plant height.

Extrinsic and intrinsic factors influence fitness in an avian hybrid zone

The effects of hybridization on evolutionary processes are primarily determined by the differential between hybrid and parental species fitness. Assessing the impacts of hybridization can be challenging, however, as determining the relationship between individual fitness and the extent of introgression in wild populations is difficult. We evaluated the fitness consequences of hybridization for pure and hybrid females in a hybrid zone between two tidal marsh birds, the saltmarsh sparrow (Ammodramus caudacutus), a salt marsh obligate, and Nelson's sparrow (A. nelsoni), which has a broader ecological niche and a much younger evolutionary association with salt marshes. Biotic stressors associated with nesting in tidal environments suggest an important role for differential adaptation in shaping hybrid zone dynamics, with saltmarsh sparrows predicted to be better adapted to nesting in salt marshes. We collected DNA samples from adults (n = 394) and nestlings (n = 431) to determine the extent of introgression using 12 microsatellite loci and tested for the influence of extrinsic (nest placement) and intrinsic (genotype) factors on female reproductive success. We monitored nests (n = 228), collected data on reproductive output, and estimated daily nest survival rates using female genotype and nest characteristics as covariates. To test for reduced survival of hybrid females, we also used capture data to assess the distribution of admixed male and female individuals across age classes. Reproductive success of females varied by genotypic class, but hybrids did not have intermediate success as predicted. Instead, we found that pure Nelson's sparrows had, on average, 33% lower hatching success than any other genotype, whereas F1/F2 hybrids, backcrossed Nelson's sparrows, and backcrossed and pure saltmarsh sparrows all had similar hatching success. We found no effect of genotype or nest placement on daily nest survival probabilities. However, hybrid individuals with a higher proportion of saltmarsh sparrow alleles exhibit nesting behaviours better suited to nesting successfully in tidal marshes. Further, while the proportion of F1/F2 individuals was similar between nestling and adult males, we found that the proportion of F1/F2 individuals was 2.3 times greater in nestling females compared with adult females, indicating reduced survival of F1 females. We conclude that differences in reproductive success among pure and admixed individuals coupled with intrinsic mechanisms (reduced survival in F1 females) shape hybrid zone dynamics in this system.

Heterogeneous genome divergence, differential introgression, and the origin and structure of hybrid zones.

Hybrid zones have been promoted as windows on the evolutionary process and as laboratories for studying divergence and speciation. Patterns of divergence between hybridizing species can now be characterized on a genomewide scale, and recent genome scans have focused on the presence of 'islands' of divergence. Patterns of heterogeneous genomic divergence may reflect differential introgression following secondary contact and provide insights into which genome regions contribute to local adaptation, hybrid unfitness and positive assortative mating. However, heterogeneous genome divergence can also arise in the absence of any gene flow, as a result of variation in selection and recombination across the genome. We suggest that to understand hybrid zone origins and dynamics, it is essential to distinguish between genome regions that are divergent between pure parental populations and regions that show restricted introgression where these populations interact in hybrid zones. The latter, more so than the former, reveal the likely genetic architecture of reproductive isolation. Mosaic hybrid zones, because of their complex structure and multiple contacts, are particularly good subjects for distinguishing primary intergradation from secondary contact. Comparisons among independent hybrid zones or transects that involve the 'same' species pair can also help to distinguish between divergence with gene flow and secondary contact. However, data from replicate hybrid zones or replicate transects do not reveal consistent patterns; in a few cases, patterns of introgression are similar across independent transects, but for many taxa, there is distinct lack of concordance, presumably due to variation in environmental context and/or variation in the genetics of the interacting populations.

Genotype-environment associations support a mosaic hybrid zone between two tidal marsh birds.

Local environmental features can shape hybrid zone dynamics when hybrids are bounded by ecotones or when patchily distributed habitat types lead to a corresponding mosaic of genotypes. We investigated the role of marsh-level characteristics in shaping a hybrid zone between two recently diverged avian taxa - Saltmarsh (Ammodramus caudacutus) and Nelson's (A.nelsoni) sparrows. These species occupy different niches where allopatric, with caudacutus restricted to coastal marshes and nelsoni found in a broader array of wetland and grassland habitats and co-occur in tidal marshes in sympatry. We determined the influence of habitat types on the distribution of pure and hybrid sparrows and assessed the degree of overlap in the ecological niche of each taxon. To do this, we sampled and genotyped 305 sparrows from 34 marshes across the hybrid zone and from adjacent regions. We used linear regression to test for associations between marsh characteristics and the distribution of pure and admixed sparrows. We found a positive correlation between genotype and environmental variables with a patchy distribution of genotypes and habitats across the hybrid zone. Ecological niche models suggest that the hybrid niche was more similar to that of A.nelsoni and habitat suitability was influenced strongly by distance from coastline. Our results support a mosaic model of hybrid zone maintenance, suggesting a role for local environmental features in shaping the distribution and frequency of pure species and hybrids across space.

Ecological differentiation and habitat unsuitability maintaining a ground beetle hybrid zone.

Exogenous selection via interactions between organisms and environments may influence the dynamics of hybrid zones between species in multiple ways. Two major models of a hybrid zone allowed us to hypothesize that environmental conditions influence hybrid zone dynamics in two ways. In the first model, an environmental gradient determines the mosaic distribution at the boundary between ecologically differentiated species (mosaic hybrid zone model). In the second model, a patch of unsuitable habitat traps a hybrid zone between species whose hybrids are unfit (tension zone model). To test these, we examined the environmental factors influencing the spatial structure of a hybrid zone between the ground beetles Carabus maiyasanus and C. iwawakianus using GIS-based quantification of environmental factors and a statistical comparison of species distribution models (SDMs). We determined that both of the hypothetical processes can be important in the hybrid zone. We detected interspecific differences in the environmental factors in presence localities and their relative contribution in SDMs. SDMs were not identical between species even within contact areas, but tended to be similar within the range of each species. These results suggest an association between environments and species, and provide evidence that ecological differentiation between species plays a role in the maintenance of the hybrid zone. Contact areas were characterized by a relatively high temperature, low precipitation, and high topological wetness. Thus, the contact areas were regarded as being located in an unsuitable habitat with a drier climate, where those populations are likely to occur in patches with limited precipitation concentrated. A comparison of spatial scales suggests that exogenous selection via environmental factors may be weaker than endogenous selection via genitalic incompatibility.

Differential effects of climate and species interactions on range limits at a hybrid zone: potential direct and indirect impacts of climate change.

The relative contributions of climate versus interspecific interactions in shaping species distributions have important implications for closely related species at contact zones. When hybridization occurs within a contact zone, these factors regulate hybrid zone location and movement. While a hybrid zone's position may depend on both climate and interactions between the hybridizing species, little is known about how these factors interact to affect hybrid zone dynamics. Here, we utilize SDM (species distribution modeling) both to characterize the factors affecting the current location of a moving North American avian hybrid zone and to predict potential direct and indirect effects of climate change on future distributions. We focus on two passerine species that hybridize where their ranges meet, the Black-capped (Poecile atricapillus) and Carolina (P.carolinensis) chickadee. Our contemporary climate models predict the occurrence of climatically suitable habitat extending beyond the hybrid zone for P.atricapillus only, suggesting that interspecific interactions primarily regulate this range boundary in P.atricapillus, while climatic factors regulate P.carolinensis. Year 2050 climate models predict a drastic northward shift in suitable habitat for P.carolinensis. Because of the greater importance of interspecific interactions for regulating the southern range limit of P.atricapillus, these climate-mediated shifts in the distribution of P.carolinensis may indirectly lead to a range retraction in P.atricapillus. Together, our results highlight the ways climate change can both directly and indirectly affect species distributions and hybrid zone location. In addition, our study lends support to the longstanding hypothesis that abiotic factors regulate species' poleward range limits, while biotic factors shape equatorial range limits.

Integrating phylogeography and paleoecology to investigate the origin and dynamics of hybrid zones: insights from two widespread North American firs.

Secondary contact between closely related taxa routinely occurs during postglacial migrations. After initial contact, the location of hybrid zones may shift geographically or remain spatially stable over time in response to various selective pressures or neutral processes. Studying the extent and direction of introgression using markers having contrasted levels of gene flow can help unravel the historical dynamics of hybrid zones. Thanks to their contrasted maternal and paternal inheritance, resulting in different levels of gene flow for mitochondrial and chloroplast DNA (mtDNA and cpDNA), the Pinaceae stand out as a relevant biological model for this purpose. The objective of the study was to assess whether the hybrid zone between Abies balsamea and Abies lasiocarpa (two largely distributed Pinaceae) has moved or remained stable over time by analysing the distribution of cytoplasmic DNA variation as well as published palaeobotanical data. Interspecific gene flow was higher for cpDNA than mtDNA markers; hence, the geographic distribution of mitotypes was more congruent with species distributions than chlorotypes. This genetic signature was contrary to expectations under a moving hybrid zone scenario, as well as empirical observations in other conifers. Genetic evidence for this rare instance of stable hybrid zone was corroborated by the colonization chronology derived from published fossil data, indicating that the two fir species initially came into contact in the area corresponding to the current sympatric zone 11 kyr ago. While an explanatory analysis suggested the putative influence of various environmental factors on the relative abundance of cytoplasmic genome combinations, further research appears necessary to assess the role of both demographic history and selective factors in driving the dynamics of hybrid zones.

Spatiotemporally consistent genomic signatures of reproductive isolation in a moving hybrid zone.

Studies of hybrid zone dynamics often investigate a single sampling period and draw conclusions from that temporal snapshot. Stochasticity can, however, result in loci with spurious outlier patterns, which is exacerbated by limited temporal or geographic sampling. Comparing admixed populations from different geographic regions is one way to detect repeatedly divergent genomic regions potentially involved in reproductive isolation. Temporal comparisons also allow us to control partially for the role of stochasticity, but the power of temporal sampling has not yet been adequately explored. In North America, black-capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadees hybridize in a contact zone extending from New Jersey to Kansas. The hybrid zone is likely maintained by strong intrinsic selection against hybrids, and it is moving north. We used a reduced representation genomic approach and temporally spaced sampling-two samples of ∼80 individuals separated by a decade-to determine the pattern and consistency of selection and genomic introgression in the chickadee hybrid zone. We report consistently low introgression for highly divergent loci between P. atricapillus and P. carolinensis in this moving hybrid zone. This is strong evidence that these loci may be linked to genomic regions involved in reproductive isolation between chickadees.

Assortative mating and the maintenance of population structure in a natural hybrid zone.

Understanding the factors that give rise to natural hybrid zones and govern their dynamics and structure is important to predicting the evolutionary consequences of hybridization. Here we use a combination of multigenerational population genetic data, mating patterns from a natural population, behavioral assays, and mark-recapture data within clinal hybrid zones of the genus Xiphophorus to test the role of assortative mating in maintaining population structure and the potential for ongoing genetic exchange between heterospecifics. Our data demonstrate that population structure is temporally robust and driven largely by assortative mating stemming from precopulatory isolation between pure species. Furthermore, mark-recapture data revealed that rates of migration within the same stream reach are far below the level needed to support population structure. In contrast to many empirical studies of natural hybrid zones, there appeared to be no hybrid male dysfunction or discrimination against hybrid males by pure parental females, and hybrid females mated and associated with pure species and hybrid males at random. Despite strong isolation between pure parentals, hybrids therefore can act as a conduit for genetic exchange between heterospecifics, which has been shown to increase the tempo of evolutionary change. Additionally, our findings highlight the complexity of natural hybrid zone dynamics, demonstrating that sexual and ecological selection together can give rise to patterns that do not fit classical models of hybrid zone evolution.

Complete mitochondrial genomes of the New World jacanas: Jacana spinosa and Jacana jacana

The New World jacanas, Jacana spinosa (Mexico to Panama and also the West Indies) and Jacana jacana (Panama and South America), are polyandrous freshwater waders that are common throughout the Neotropics. These two species hybridize narrowly at their contact zone in Panama, and as part of a study of the hybrid zone dynamics, we present complete, annotated mitochondrial genomes for both species. The two species have very similar mitochondrial genomes, showing identical gene orders, and differing in size in only two RNA features and the control region, and among protein-coding genes, the two genomes had average uncorrected pairwise divergence of 1.8%, ranging from 0.7% for ND4L and 3.6% for ATP8. However, control region divergence is high (∼16%). These mitochondrial genome sequences may be useful tools for understanding jacana hybridization dynamics, especially regarding potential mitonuclear incompatibilities.

Genetic analysis of admixture and patterns of introgression in foundation cottonwood trees (Salicaceae) in southwestern Colorado, USA

Cottonwoods are well known as foundation riparian trees that support diverse communities and drive ecosystem processes. Although hybridization naturally occurs when the distributions of two or more cottonwood species overlap, few cottonwood hybrid zones have been genetically characterized. We use genetic and genomic analyses to characterize patterns of admixture and introgression for a newly described hybrid zone at the intersection of three species (Populus L. Salicaceae—Populus deltoides, Populus fremontii, and Populus angustifolia) in southwestern Colorado, USA. Analysis of nuclear and chloroplast microsatellite marker data detected substantial genetic variation among individuals, revealing that (1) hybridization is occurring between two, not three, species (P. deltoides and P. angustifolia); (2) gene flow is bidirectional; (3) hybrids are not abundant (admixture detected in only 34 of 270 trees), with most being early-generation F1 hybrids; (4) cytonuclear disequilibria exists and F1 hybrids tend to retain P. deltoides—like chloroplasts; and (5) roughly 30 % of the nuclear markers deviated from a neutral pattern of introgression, suggesting that selection may play a role in shaping the genetic structure of the hybrid zone in this region. Overall, our results show that despite strong selection maintaining species divergence, transfer of allelic variants across species boundaries can occur. Our study assesses the fine-scale genetic structure of hybridization between P. angustifolia and P. deltoides and lays the foundation for examining how geographic differences in hybrid zone dynamics arise and may influence subsequent ecological and evolutionary processes.

Sexual selection against natural hybrids may contribute to reinforcement in a house mouse hybrid zone

Sexual selection may hinder gene flow across contact zones when hybrid recognition signals are discriminated against. We tested this hypothesis in a unimodal hybrid zone between Mus musculus musculus and Mus musculus domesticus where a pattern of reinforcement was described and lower hybrid fitness documented. We presented mice from the border of the hybrid zone with a choice between opposite sex urine from the same subspecies versus hybrids sampled in different locations across the zone. While no preference was evidenced in domesticus mice, musculus males discriminated in favour of musculus signals and against hybrid signals. Remarkably, the pattern of hybrid unattractiveness did not vary across the hybrid zone. Moreover, allopatric populations tested in the same conditions did not discriminate against hybrid signals, indicating character displacement for signal perception or preference. Finally, habituation-discrimination tests assessing similarities between signals pointed out that hybrid signals differed from the parental ones. Overall, our results suggest that perception of hybrids as unattractive has evolved in border populations of musculus after the secondary contact with domesticus. We discuss the mechanisms involved in hybrid unattractiveness, and the potential impact of asymmetric sexual selection on the hybrid zone dynamics and gene flow between the two subspecies.

Structure and dynamics of hybrid zones at different stages of speciation in the common vole (Microtus arvalis)

The genetic structure and dynamics of hybrid zones provide crucial information for understanding the processes and mechanisms of evolutionary divergence and speciation. In general, higher levels of evolutionary divergence between taxa are more likely to be associated with reproductive isolation and may result in suppressed or strongly restricted hybridization. In this study, we examined two secondary contact zones between three deep evolutionary lineages in the common vole (Microtus arvalis). Differences in divergence times between the lineages can shed light on different stages of reproductive isolation and thus provide information on the ongoing speciation process in M. arvalis. We examined more than 800 individuals for mitochondrial (mtDNA), Y-chromosome and autosomal markers and used assignment and cline analysis methods to characterize the extent and direction of gene flow in the contact zones. Introgression of both autosomal and mtDNA markers in a relatively broad area of admixture indicates selectively neutral hybridization between the least-divergent lineages (Central and Eastern) without evidence for partial reproductive isolation. In contrast, a very narrow area of hybridization, shifts in marker clines and the quasi-absence of Y-chromosome introgression support a moving hybrid zone and unidirectional selection against male hybrids between the lineages with older divergence (Central and Western). Data from a replicate transect further support non-neutral processes in this hybrid zone and also suggest a role for landscape history in the movement and shaping of geneflow profiles.

Alternative forms for genomic clines

Understanding factors regulating hybrid fitness and gene exchange is a major research challenge for evolutionary biology. Genomic cline analysis has been used to evaluate alternative patterns of introgression, but only two models have been used widely and the approach has generally lacked a hypothesis testing framework for distinguishing effects of selection and drift. I propose two alternative cline models, implement multivariate outlier detection to identify markers associated with hybrid fitness, and simulate hybrid zone dynamics to evaluate the signatures of different modes of selection. Analysis of simulated data shows that previous approaches are prone to false positives (multinomial regression) or relatively insensitive to outlier loci affected by selection (Barton's concordance). The new, theory-based logit-logistic cline model is generally best at detecting loci affecting hybrid fitness. Although some generalizations can be made about different modes of selection, there is no one-to-one correspondence between pattern and process. These new methods will enhance our ability to extract important information about the genetics of reproductive isolation and hybrid fitness. However, much remains to be done to relate statistical patterns to particular evolutionary processes. The methods described here are implemented in a freely available package “HIest” for the R statistical software (CRAN; http://cran.r-project.org/).

Characterization of a contemporaneous hybrid zone between two darter species (Etheostoma bison and E. caeruleum) in the Buffalo River System

Hybrid zones have long intrigued evolutionary biologists and provide a natural laboratory to explore the evolution of reproductive isolation (speciation). Molecular characterization of hybrid zone dynamics can provide insight into the strength of reproductive isolation as well as the underlying evolutionary processes shaping gene flow. Approximately one-third of darter species naturally hybridize making this species-rich North American freshwater teleost fish clade an ideal system to investigate the extent and direction of hybridization. The objective of this study was to use diagnostic microsatellite markers to calculate genetic hybrid index scores of two syntopic, but distantly related darter species, Etheostoma bison and Etheostoma caeruleum. A combination of hybrid index scores, assignment tests, and mitochondrial haplotype profiles uncovered mixed ancestry in approximately 6% of sampled adult individuals, supporting contemporaneous hybridization that was previously undocumented in E. bison. Moreover, hybrids were not limited to the F1 generation, but encompassed the entire suite of hybrid categories (F1, F2 and backcross hybrids). The low number of hybrids assigned to each hybrid category represents a bimodal hybrid zone, suggesting reproductive isolation is strong (but incomplete) and also advocates for the ability of hybrids to produce second-generation hybrids and backcross into both parental species, mediating introgression across species boundaries. To this end, cytonuclear profiles of the sampled parental species and hybrids were consistent with bidirectional gene flow, although there was an overall trend of asymmetric hybridization between E. caeruleum females and E. bison males. The spatiotemporal variation in hybridization rates and resulting cytonuclear patterns expanded on in this study provide a comparative genetic framework on which future studies can begin to elucidate the underlying processes that not only generate a mosaic hybrid zone, but maintain the distinctness of species in the face of gene flow.

SNP Detection from De Novo Transcriptome Sequencing in the Bivalve Macoma balthica: Marker Development for Evolutionary Studies

Hybrid zones are noteworthy systems for the study of environmental adaptation to fast-changing environments, as they constitute reservoirs of polymorphism and are key to the maintenance of biodiversity. They can move in relation to climate fluctuations, as temperature can affect both selection and migration, or remain trapped by environmental and physical barriers. There is therefore a very strong incentive to study the dynamics of hybrid zones subjected to climate variations. The infaunal bivalve Macoma balthica emerges as a noteworthy model species, as divergent lineages hybridize, and its native NE Atlantic range is currently contracting to the North. To investigate the dynamics and functioning of hybrid zones in M. balthica, we developed new molecular markers by sequencing the collective transcriptome of 30 individuals. Ten individuals were pooled for each of the three populations sampled at the margins of two hybrid zones. A single 454 run generated 277 Mb from which 17K SNPs were detected. SNP density averaged 1 polymorphic site every 14 to 19 bases, for mitochondrial and nuclear loci, respectively. An scan detected high genetic divergence among several hundred SNPs, some of them involved in energetic metabolism, cellular respiration and physiological stress. The high population differentiation, recorded for nuclear-encoded ATP synthase and NADH dehydrogenase as well as most mitochondrial loci, suggests cytonuclear genetic incompatibilities. Results from this study will help pave the way to a high-resolution study of hybrid zone dynamics in M. balthica, and the relative importance of endogenous and exogenous barriers to gene flow in this system.

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 F(1) 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.

Is water really a barrier for the house mouse? A comparative study of two mouse subspecies

House mice are believed to be, unlike rats, only reluctant swimmers. If water is a barrier to mouse dispersal water bodies and streams can have a substantial impact on the genetic structure of populations. Previous studies revealed influence of rivers on the position and structure of the European hybrid zone between two house mouse subspecies, Mus musculus musculus and M. m. domesticus. In this study, we used a simple motivation experiment to test the disposition of both wild and inbred mice representing the subspecies to overcome a water barrier. As the more dispersing sex, males were chosen for the experiment. Mice were tested under two air/water temperature regimes, 20 °C and 10 °C. Contrary to a common belief tested animals entered water rather easily, often even engaging in repeated swimming. We found significant differences in scored behavioural parameters between the subspecies. Under the 20 °C regime, both wild and inbred domesticus males entered and crossed the water earlier and more often swam even when satiated. Strikingly, under the 10 °C regime, the results were rather equivocal but with the opposite tendencies, with musculus males being more willing to swim. We discuss implications of these findings for the hybrid zone dynamics.