Hybrid Zone Research Articles

Aberrant microbiomes are associated with increased antibiotic resistance gene load in hybrid mice.

Antibiotic resistance is a priority public health problem resulting from eco-evolutionary dynamics within microbial communities and their interaction at a mammalian host interface or geographical scale. The links between mammalian host genetics, bacterial gut community, and antimicrobial resistance gene (ARG) content must be better understood in natural populations inhabiting heterogeneous environments. Hybridization, the interbreeding of genetically divergent populations, influences different components of the gut microbial communities. However, its impact on bacterial traits such as antibiotic resistance is unknown. Here, we present that hybridization might shape bacterial communities and ARG occurrence. We used amplicon sequencing to study the gut microbiome and to predict ARG composition in natural populations of house mice (Mus musculus). We compared gastrointestinal bacterial and ARG diversity, composition, and abundance across a gradient of pure and hybrid genotypes in the European House Mouse Hybrid Zone. We observed an increased overall predicted richness of ARG in hybrid mice. We found bacteria-ARG interactions by their co-abundance and detected phenotypes of extreme abundances in hybrid mice at the level of specific bacterial taxa and ARGs, mainly multidrug resistance genes. Our work suggests that mammalian host genetic variation impacts the gut microbiome and chromosomal ARGs. However, it raises further questions on how the mammalian host genetics impact ARGs via microbiome dynamics or environmental covariates.

Genome-wide support for incipient Tula hantavirus species within a single rodent host lineage.

Evolutionary divergence of viruses is most commonly driven by co-divergence with their hosts or through isolation of transmission after host shifts. It remains mostly unknown, however, whether divergent phylogenetic clades within named virus species represent functionally equivalent byproducts of high evolutionary rates or rather incipient virus species. Here, we test these alternatives with genomic data from two widespread phylogenetic clades in Tula orthohantavirus (TULV) within a single evolutionary lineage of their natural rodent host, the common vole Microtus arvalis. We examined voles from forty-two locations in the contact region between clades for TULV infection by reverse transcription (RT)-PCR. Sequencing yielded twenty-three TULV Central North and twenty-one TULV Central South genomes, which differed by 14.9-18.5 per cent at the nucleotide and 2.2-3.7 per cent at the amino acid (AA) level without evidence of recombination or reassortment between clades. Geographic cline analyses demonstrated an abrupt (<1 km wide) transition between the parapatric TULV clades in continuous landscape. This transition was located within the Central mitochondrial lineage of M. arvalis, and genomic single nucleotide polymorphisms showed gradual mixing of host populations across it. Genomic differentiation of hosts was much weaker across the TULV Central North to South transition than across the nearby hybrid zone between two evolutionary lineages in the host. We suggest that these parapatric TULV clades represent functionally distinct, incipient species, which are likely differently affected by genetic polymorphisms in the host. This highlights the potential of natural viral contact zones as systems for investigating the genetic and evolutionary factors enabling or restricting the transmission of RNA viruses.

Gene regulation and speciation in a migratory divide between songbirds

Behavioral variation abounds in nature. This variation is important for adaptation and speciation, but its molecular basis remains elusive. Here, we use a hybrid zone between two subspecies of songbirds that differ in migration – an ecologically important and taxonomically widespread behavior---to gain insight into this topic. We measure gene expression in five brain regions. Differential expression between migratory states was dominated by circadian genes in all brain regions. The remaining patterns were largely brain-region specific. For example, expression differences between the subspecies that interact with migratory state likely help maintain reproductive isolation in this system and were documented in only three brain regions. Contrary to existing work on regulatory mechanisms underlying species-specific traits, two lines of evidence suggest that trans- (vs. cis) regulatory changes underlie these patterns – no evidence for allele-specific expression in hybrids and minimal associations between genomic differentiation and expression differences. Additional work with hybrids shows expression levels were often distinct (transgressive) from parental forms. Behavioral contrasts and functional enrichment analyses allowed us to connect these patterns to mitonuclear incompatibilities and compensatory responses to stress that could exacerbate selection on hybrids and contribute to speciation.

Molecular Markers and Taxonomic Explosion in Herpetology: More or Less Robust Taxonomy? True and False Advantages of DNA Markers

Despite the increasing deficit of taxonomic expertise, the number of newly described species since the early 2010s has grown exponentially. This growth is related to the increased use of DNA markers in taxonomic descriptions. However, routine use of DNA markers in taxonomy did not bring practical taxonomy closer to the theory. Species are unique lineages with irreversible evolutionary pathways, and only the presence of distinct populations within the same geographic range, or at least the presence of narrow hybrid zones between the parapatric ranges is a conclusive evidence of evolutionary irreversibility. In the case of allopatric populations, only very high genetic distances, suggesting several tens of millions of years of independent evolution, can be used for validation of species status. This problem cannot be solved by the broader introduction of genomic phylogenies, which also fail to provide robust criteria for evolutionary irreversibility. We can hardly suppose that robust validation of species status is applicable to all or most of hundreds of thousands of animal species, including 20,000 amphibians and reptiles. Instead, practical taxonomy should concentrate on describing recognizable species, maintaining a trade-off between sufficiently detailed descriptions of world biodiversity and the applicability of these descriptions for practical use and metaanalyses, not pretending that formally described species reflect real lineages with independent and irreversible evolutionary pathways. Simultaneously, the non-critical elevation of the taxonomic status of individual geographic populations, contrary to the declared purpose of better-focusing conservation efforts, often has the opposite effect, leaving many formally described taxa outside the conservation umbrella.

Limited gene flow in adjacent valleys and genetic introgression in a mountain hybrid zone between &lt;i&gt;Aconitum&lt;/i&gt; (Ranunculaceae) species

The number of polyploid plants in angiosperms is estimated at about 30-80%, depending on the study, but hybridization between diploids and tetraploids is not a common phenomenon. In this study, our aim was to determine the extent of hybridization and introgression between &lt;i&gt;Aconitum&lt;/i&gt; species in a mountainous hybrid zone in the Tatra Mountains (Western Carpathians). We hypothesized that topography would affect their populations’ genetic structure. For this purpose, inter simple sequence repeat (ISSR) molecular markers and chloroplast DNA (cpDNA, region &lt;i&gt;trn&lt;/i&gt;L(UAG)–&lt;i&gt;ndh&lt;/i&gt;F) sequencing were used. The results revealed that diploid and tetraploid &lt;i&gt;Aconitum&lt;/i&gt; species had specific cpDNA haplotypes, while the spontaneous hybrid species harbored a cpDNA haplotype typical for tetraploids. Bayesian STRUCTURE analysis showed that introgression occurred between diploid and tetraploid species above the tree line, where a spontaneous hybrid, a putative triploid &lt;i&gt;A. ×berdaui&lt;/i&gt;, originated, probably via the triploid bridge mechanism. Molecular analysis of variance, principal coordinate analysis, and neighbor-net clustering revealed different genetic profiles of the diploids between neighboring mountain valleys. Hence, mountain topography, ridges, and valleys can form an effective geographical barrier, preventing gene flow over longer distances. Introgression happens near or above the tree line, where the hybridization zone is formed. In the introgressants, however, no morphological alterations were discernible.

The effect of inhomogeneous microstructure on the properties of laser-MIG hybrid welded 316L/AH36 dissimilar joints

The present study investigated the inhomogeneity of microstructure and properties of hybrid laser-MIG welded 10-mm-thick 316L/AH36 joints. The study focused on the inhomogeneity of the laser zone, hybrid zone, and transition zone. The temperature field was simulated using ABAQUS and showed a higher cooling rate in the laser zone. As a result, the microstructure in the laser zone was dominated by martensite while that in the hybrid zone was composed of 84 % austenite and 16 % ferrite. The laser zone showed poorer impact toughness and corrosion resistance than the hybrid zone. However, the corrosion resistance of the whole weld was the worst. Because the inhomogeneous microstructure formed a galvanic couple that accelerated the corrosion process. This work indicated that the energy ratio (the ratio of laser to arc energy) should be considered in the welding of dissimilar thick steel to reduce the inhomogeneity in the two zones and obtain a desirable property balance among hardness, toughness, and corrosion resistance.

Drought-induced growth phenotypes are associated with genetic variation across a white pine hybrid zone

Understanding relationships among warming climate, increased drought severity, and the genetic architecture of hybrid drought resilience is necessary for forest conservation and management. We calculated three drought-related tree-ring-growth indices (dendrophenotypes) using tree-ring data from hybrid Pinus strobiformis–Pinus flexilis study trees at nine sites across Colorado, New Mexico, Arizona, and Texas. Along with hybrid index (percentage of P. strobiformis ancestry inherited by a single tree) and climate variables, we used dendrophenotypes to (Q1) examine relationships among climate and hybrid index, (Q2) examine relationships between dendrophenotypes and hybrid index, and (Q3) conduct a genotype–phenotype analysis. We observed a significant correlation between hybrid index and dendrophenotypes resulting from high-temperature drought in addition to a significant correlation between those dendrophenotypes and our dataset of single-nucleotide polymorphisms. We conclude that P. strobiformis–P. flexilis trees exhibiting higher hybrid indices are more resilient to high-temperature drought events and encourage future research that identifies a genetic linkage between relevant loci and their conferred physiological benefits.

Mapping restricted introgression across the genomes of admixed indigenous African cattle breeds

BackgroundThe genomes of indigenous African cattle are composed of components with Middle Eastern (taurine) and South Asian (indicine) origins, providing a valuable model to study hybridization and to identify genetic barriers to gene flow. In this study, we analysed indigenous African cattle breeds as models of hybrid zones, considering taurine and indicine samples as ancestors. In a genomic cline analysis of whole-genome sequence data, we considered over 8 million variants from 144 animals, which allows for fine-mapping of potential genomic incompatibilities at high resolution across the genome.ResultsWe identified several thousand variants that had significantly steep clines (‘SCV’) across the whole genome, indicating restricted introgression. Some of the SCV were clustered into extended regions, with the longest on chromosome 7, spanning 725 kb and including 27 genes. We found that variants with a high phenotypic impact (e.g. indels, intra-genic and missense variants) likely represent greater genetic barriers to gene flow. Furthermore, our findings provide evidence that a large proportion of breed differentiation in African cattle could be linked to genomic incompatibilities and reproductive isolation. Functional evaluation of genes with SCV suggest that mitonuclear incompatibilities and genes associated with fitness (e.g. resistance to paratuberculosis) could account for restricted gene flow in indigenous African cattle.ConclusionsTo our knowledge, this is the first time genomic cline analysis has been applied to identify restricted introgression in the genomes of indigenous African cattle and the results provide extended insights into mechanisms (e.g. genomic incompatibilities) contributing to hybrid differentiation. These results have important implications for our understanding of genetic incompatibilities and reproductive isolation and provide important insights into the impact of cross-breeding cattle with the aim of producing offspring that are both hardy and productive.

The type of Setophaga ruficoronata (Kaup 1851) is a hybrid: implications for the taxonomy of Myioborus warblers (Passeriformes: Parulidae).

Hybridization, rapid diversification, and uncertainties surrounding type specimens add complexity to the already intricate taxonomy of high Andean Myioborus warblers of northern South America. In this study, we propose a reassessment of species boundaries within M. ornatus and M. melanocephalus, drawing on comparisons of name-bearing types. We also consider insights from a recent study of a hybrid zone in southern Colombia and northern Ecuador. We present three species delimitation alternatives that offer improved clarity compared to the current taxonomy, and discuss the rationale behind recognizing chrysops and bairdi as distinct species while redefining the species ornatus and melanocephalus, given the available evidence.

Patterns of admixture and introgression in a mosaic Mytilus galloprovincialis and Mytilus edulis hybrid zone in SW England.

The study of hybrid zones offers important insights into speciation. Earlier studies on hybrid populations of the marine mussel species Mytilus edulis and Mytilus galloprovincialis in SW England provided evidence of admixture but were constrained by the limited number of molecular markers available. We use 57 ancestry-informative SNPs, most of which have been mapped genetically, to provide evidence of distinctive differences between admixed populations in SW England and asymmetrical introgression from M. edulis to M. galloprovincialis. We combine the genetic study with analysis of phenotypic traits of potential ecological and adaptive significance. We demonstrate that hybrid individuals have brown mantle edges unlike the white or purple in the parental species, suggesting allelic or non-allelic genomic interactions. We report differences in gonad development stage between the species consistent with a prezygotic barrier between the species. By incorporating results from publications dating back to 1980, we confirm the long-term stability of the hybrid zone despite higher viability of M. galloprovincialis. This stability coincides with a dramatic change in temperature of UK coastal waters and suggests that these hybrid populations might be resisting the effects of global warming. However, a single SNP locus associated with the Notch transmembrane signalling protein shows a markedly different pattern of variation to the others and might be associated with adaptation of M. galloprovincialis to colder northern temperatures.

Natural male hybrid common shrews with a very long chromosomal multivalent at meiosis appear not to be completely sterile.

Among 36 known chromosomal hybrid zones of the common shrew Sorex araneus, the Moscow-Seliger hybrid zone is of special interest because inter-racial complex heterozygotes (F1 hybrids) produce the longest meiotic configuration, consisting of 11 chromosomes with monobrachial homology (undecavalent or chain-of-eleven: CXI). Different studies suggest that such a multivalent may negatively affect meiotic progression and in general should significantly reduce fertility of hybrids. In this work, by immunocytochemical and electron microscopy methods, we investigated for the first time chromosome synapsis, recombination and meiotic silencing in pachytene spermatocytes of natural inter-racial heterozygous shrew males carrying CXI configurations. Despite some abnormalities detected in spermatocytes, such as associations of chromosomes, stretched centromeres, and the absence of recombination nodules in some arms of the multivalent, a large number of morphologically normal spermatozoa were observed. Possible low stringency of pachytene checkpoints may mean that even very long meiotic configurations do not cause complete sterility of such complex inter-racial heterozygotes.

Are there hybrid zones in Fagus sylvatica L. sensu lato?

The complex of beech taxa in western Eurasia (Fagus sylvatica L. s.l.) represents one of the most widespread broad-leaved trees in the temperate forests of the northern hemisphere. Two subspecies are generally recognized, F. sylvatica ssp. sylvatica in Europe and F. sylvatica ssp. orientalis in southwestern Asia, but several strongly differentiated regional populations were identified within this complex. To reveal geographical and genomic patterns of hybridization and introgression, genetic variation at 6789 single-nucleotide polymorphisms (SNP) resulting from ddRAD sequencing was studied in 288 specimens representing 207 populations distributed across the whole range. Two clusters resulting from a Bayesian analysis of population Structure well coincided with the subspecies, while increasing the number of clusters allowed to distinguish divergent lineages within subspecies. Two zones of potential hybridization between such lineages were analyzed: western Balkans at the contact of both subspecies and the Alpine area at the contact of populations coming from different glacial refugia. Geographical distribution of hybrid indices in the Balkan hybrid zone indicates asymmetric gene flow preferentially directed from F. orientalis to F. sylvatica. Eight SNPs showed a significant shift of genomic cline centers, seven of them toward excess F. orientalis ancestry, suggesting that introgression irregularly affects different regions of the beech genome. On the other hand, no genomic cline outliers were found in the Alpine hybrid zone. Implications for assisted gene flow are shortly discussed.

COMPARATIVE ANALYSIS OF THE EFFECT OF ADHESIVE SYSTEMS ON THE MICROELEMENT COMPOSITION OF THE DENTIN OF THE CROWN PART OF THE TOOTH

Title: Comparative analysis of the effect of adhesive systems on the microelement composition of the dentin of the crown part of the tooth. Purpose: To research the microelement composition of the «filling-tooth» interactive zone, the morphological characteristics of its border, to assess the effect of adhesive systems of the V and VII (self-etching) generations on the ion-exchange composition of the dentin of the crown part of the tooth, and to qualitatively assess the microelement composition of the hybrid dentin zone. Materials and methods: Extracted teeth with preserved crown part (n=20), enamel etching gel (Vladmiva), V generation Singlebond - 2 adhesive system (3M ESPE), self-etching adhesive system Prime-Bond Universal (Dentsply Sirona), light-cured composite material Migrofil (Republic of Belarus). The research of the microstructure of samples with formed and sealed Class I cavities according to Black was carried out on a certified high-resolution scanning electron microscope «Mira» by Tescan (Czech Republic). The research of the microelement composition of the «filling-tooth» border was carried out by X-ray spectral analysis, which consisted of 2 blocks: using the adhesive system of the fifth generation and the seventh generation of the adhesive system. Conclusions: 1. A comparative analysis of the results of using the fifth-generation adhesive system and the selfetching adhesive system showed that when using the total etching technique, trace elements Ca and P migrate from the tooth dentin structure into the hybrid zone. When using the adhesive system VII, the migration of elements was not established, due to the lack of demineralization of the dentin structure and the modification of the smeared layer. 2. Qualitative assessment of planar spectrograms revealed an increase in the carbon element in the modified adhesive layer. which is due to the presence of a smeared layer after odontopreparation of hard tissues of teeth destroyed by a carious process 3. The migration of the Si element indicates the chemical interaction of the adhesive and the filling material, as well as the filling material and the dentin of the tooth through the adhesive layer. 4. The use of the adhesive system of the fifth generation provides a higher quality of the marginal fit of the filling material to the surface of the dentin than when using a self-etching adhesive system.

Seed efficiency of Pinus sibirica and Pinus pumila hybrids on Lake Baikal shore

Two 5-needle pine species, Pinus sibirica and P. pumila, have vast geographic distribution and form wide hybrid zone in the Eastern Siberia. In the interspecies hybridization studies, seed production of hybrids has primary significance for further evolution of the cross-pollinated species. Seed efficiency of the hybrids and their parental species found on the eastern shore of Lake Baikal was studied. It was shown that mature cones of the hybrids were smaller than that in P sibirica but bigger than that in P. pumila. Seed efficiency, i. e., portion of ovules which give rise filled seeds with well-developed embryo, of the hybrids was 34.0 % and was almost 2-fold less than that in P. pumila (66.2 %) and quite comparable with that in P. sibirica (36.8 %). Low seed efficiency in P. sibirica was due to low seed formation frequency and high portion of empty seeds. Main reproductive losses in the hybrids occurred at all stages from the seed development beginning, portions of the aborted and empty seeds were especially high. Thus, seed efficiency of the hybrids appears high enough for further renewal, but, as a rule, only F1 hybrids are found on Lake Baikal shore.

Delimiting phylogeographic diversity in the genomic era: application to an Iberian endemic frog

Abstract The rich genetic and phenotypic diversity of species complexes is best recognized through formal taxonomic naming, but one must first assess the evolutionary history of phylogeographic lineages to identify and delimit candidate taxa. Using genomic markers, mitochondrial DNA barcoding, and morphometric analyses, we examined lineage diversity and distribution in the Iberian endemic frog Rana parvipalmata. We confirmed two deep phylogeographic lineages, one relatively homogenous genetically, found in Asturias and adjacent areas (T2), and one more fragmented and locally genetically impoverished, restricted to Galicia (T1). Analyses of their hybrid zone suggested a shallow transition characterized by far-ranging admixture, which was modelled by a wide geographic cline (~60 km for the genome average) and no obvious barrier loci (i.e. loci showing disproportionally restricted introgression). The relatively young T1 and T2 have thus remained reproductively compatible, which argues against their distinction as biological species, and we accordingly describe T2 as a new subspecies, Rana parvipalmata asturiensis ssp. nov. Remarkably, we highlight striking discordances between mitochondrial and nuclear distributions across their hybrid zones, as well as between their genetic and morphological differentiation. Our study illustrates how genomic-based phylogeographic frameworks can help decipher the high genetic and phenotypic variation of species complexes and substantiate the taxonomic assessment of candidate lineages.

Habitat and climate influence hybridization among three genetically distinct Canada jay (Perisoreus canadensis) morphotypes in an avian hybrid zone complex.

Examining the frequency and distribution of hybrids across contact zones provide insights into the factors mediating hybridization. In this study, we examined the effect of habitat and climate on hybridization patterns for three phenotypically, genetically, and ecologically distinct groups of the Canada jay (Perisoreus canadensis) in a secondary contact zone in western North America. Additionally, we tested whether the frequency of hybridization involving the three groups (referred to as Boreal, Pacific and Rocky Mountain morphotypes) is similar across the hybrid zones or whether some pairs have hybridized more frequently than others. We reanalyzed microsatellite, mtDNA and plumage data, and new microsatellite and plumage data for 526 individuals to identify putative genetic and phenotypic hybrids. The genetically and phenotypically distinct groups are associated with different habitats and occupy distinct climate niches across the contact zone. Most putative genetic hybrids (86%) had Rocky Mountain ancestry. Hybrids were observed most commonly in intermediate climate niches and in habitats where Engelmann spruce (Picea engelmannii) overlaps broadly with boreal and subalpine tree species. Our finding that hybrids occupy intermediate climate niches relative to parental morphotypes matches patterns for other plant and animal species found in this region. This study demonstrates how habitat and climate influence hybridization patterns in areas of secondary contact and adds to the growing body of research on tri-species hybrid zones.

Discerning structure versus speciation in phylogeographic analysis of Seepage Salamanders (Desmognathus aeneus) using demography, environment, geography, and phenotype.

Numerous mechanisms can drive speciation, including isolation by adaptation, distance, and environment. These forces can promote genetic and phenotypic differentiation of local populations, the formation of phylogeographic lineages, and ultimately, completed speciation. However, conceptually similar mechanisms may also result in stabilizing rather than diversifying selection, leading to lineage integration and the long-term persistence of population structure within genetically cohesive species. Processes that drive the formation and maintenance of geographic genetic diversity while facilitating high rates of migration and limiting phenotypic differentiation may thereby result in population genetic structure that is not accompanied by reproductive isolation. We suggest that this framework can be applied more broadly to address the classic dilemma of "structure" versus "species" when evaluating phylogeographic diversity, unifying population genetics, species delimitation, and the underlying study of speciation. We demonstrate one such instance in the Seepage Salamander (Desmognathus aeneus) from the southeastern United States. Recent studies estimated up to 6.3% mitochondrial divergence and four phylogenomic lineages with broad admixture across geographic hybrid zones, which could potentially represent distinct species supported by our species-delimitation analyses. However, while limited dispersal promotes substantial isolation by distance, microhabitat specificity appears to yield stabilizing selection on a single, uniform, ecologically mediated phenotype. As a result, climatic cycles promote recurrent contact between lineages and repeated instances of high migration through time. Subsequent hybridization is apparently not counteracted by adaptive differentiation limiting introgression, leaving a single unified species with deeply divergent phylogeographic lineages that nonetheless do not appear to represent incipient species.

Experimental Hybrids of the Triatoma brasiliensis Species Complex Show Higher Susceptibility to the Trypanosoma cruzi Infection Than Their Parentals.

The Triatoma brasiliensis species complex is a monophyletic group encompassing two subspecies and six species. Recently, a hybrid zone of members of this complex was recorded in the state of Pernambuco. Questions concerning the capability of the hybrids to become infected with Trypanosoma cruzi have been raised. This study aimed to compare the susceptibility of Triatoma b. brasiliensis, Triatoma juazeirensis, and their experimental hybrids to infection with T. cruzi. We infected the parentals and their experimental hybrids (obtained through reciprocal crosses) through artificial feeding with citrated rabbit blood, to which the TcI 0354 strain of T. cruzi had been added. The insects were weighed before and after feeding on the rabbit blood, and then they were dissected on the 10th, 20th, and 30th day after infection. Both the hybrids and the parentals remained infected throughout the experiment. The parasite was mostly found in the epimastigote form. The number of epimastigotes was significantly lower in the stomach and small intestine of T. juazeirensis than in the hybrids or in T. b. brasiliensis. A significantly higher percentage of metacyclic trypomastigotes was detected in the small intestine and rectum of the hybrids. Hybrids demonstrated higher susceptibility to the TcI 0354 strain than their parentals, opening up new avenues to be investigated.

SNP genotyping revealed a hybrid zone between Mytilus chilensis and M. platensis in southern South America (the Strait of Magellan, Isla Grande de Tierra del Fuego and the Falkland Islands).

Hybrid zones among mussel species have been extensively studied in the northern hemisphere. In South America, it has only recently become possible to study the natural hybrid zones, due to the clarification of the taxonomy of native mussels of the Mytilus genus. Analysing 54 SNP markers, we show the genetic species composition and admixture in the hybrid zone between M. chilensis and M. platensis in the southern end of South America. Bayesian, non-Bayesian clustering and re-assignment algorithms showed that the natural hybrid zone between M. chilensis and M. platensis in the Strait of Magellan, Isla Grande de Tierra del Fuego and the Falkland Islands shows clinal architecture. The hybrid zone can be divided into three different areas: the first one is on the Atlantic coast where only pure M. platensis and hybrid were found. In the second one, inside the Strait of Magellan, pure individuals of both species and mussels with variable degrees of hybridisation coexist. In the last area at the Strait in front of Punta Arenas City, fjords on the Isla Grande de Tierra del Fuego, and at the Beagle Channel, only M. chilensis and a low number of hybrids were found. According to the proportion of hybrids, bays with protected conditions away from strong currents would give better conditions for hybridisation. We do not find evidence of any other mussel species such as M. edulis, M. galloprovincialis, M. planulatus or M. trossulus in the zone.

Pollinator and habitat-mediated selection as potential contributors to ecological speciation in two closely related species.

In ecological speciation, incipient species diverge due to natural selection that is ecologically based. In flowering plants, different pollinators could mediate that selection (pollinator-mediated divergent selection) or other features of the environment that differ between habitats of 2 species could do so (environment-mediated divergent selection). Although these mechanisms are well understood, they have received little rigorous testing, as few studies of divergent selection across sites of closely related species include both floral traits that influence pollination and vegetative traits that influence survival. This study employed common gardens in sites of the 2 parental species and a hybrid site, each containing advanced generation hybrids along with the parental species, to test these forms of ecological speciation in plants of the genus Ipomopsis. A total of 3 vegetative traits (specific leaf area, leaf trichomes, and photosynthetic water-use efficiency) and 5 floral traits (corolla length and width, anther insertion, petal color, and nectar production) were analyzed for impacts on fitness components (survival to flowering and seeds per flower, respectively). These traits exhibited strong clines across the elevational gradient in the hybrid zone, with narrower clines in theory reflecting stronger selection or higher genetic variance. Plants with long corollas and inserted anthers had higher seeds per flower at the Ipomopsis tenuituba site, whereas selection favored the reverse condition at the Ipomopsis aggregata site, a signature of divergent selection. In contrast, no divergent selection due to variation in survival was detected on any vegetative trait. Selection within the hybrid zone most closely resembled selection within the I. aggregata site. Across traits, the strength of divergent selection was not significantly correlated with width of the cline, which was better predicted by evolvability (standardized genetic variance). These results support the role of pollinator-mediated divergent selection in ecological speciation and illustrate the importance of genetic variance in determining divergence across hybrid zones.