Homoploid Hybrid Research Articles

Oh, what a tangled web we weave

Oh, what a tangled web we weave

Molecular and morphological evidence for hybrid origin and matroclinal inheritance of an endangered wild rose, Rosa × pseudobanksiae (Rosaceae) from China

The distinction between hybrids and species of hybrid origin is significant for plant diversity conservation. Interspecific hybridization within the genus Rosa is a common phenomenon. Based on field observation, we hypothesized that one previously published species, Rosa pseudobanksiae (endangered category: CR) might be a hybrid. In this study, we utilized molecular approaches involving three nuclear markers and four plastid markers to examine the origin of R. pseudobanksiae. Our evidence shows that it is indeed a natural homoploid hybrid between R. banksiae var. normalis and R. multiflora var. cathayensis, and that gene flows bidirectionally. Our multivariate morphometric analyses provide strong evidence for a matrocliny in R. × pseudobanksiae individuals. R. × pseudobanksiae has diverse morphological variations among individuals andshares the overlapped distributions with its parental progenitors. This taxon has not developed into a stabilized and self-evolving lineage with the high sterility in seed sets.

Fifteen shades of green: The evolution of Bufotes toads revisited

The radiation of Palearctic green toads (Bufotes) holds great potential to evaluate the role of hybridization in phylogeography at multiple stages along the speciation continuum. With fifteen species representing three ploidy levels, this model system is particularly attractive to examine the causes and consequences of allopolyploidization, a prevalent yet enigmatic pathway towards hybrid speciation. Despite substantial efforts, the evolutionary history of this species complex remains largely blurred by the lack of consistency among the corresponding literature. To get a fresh, comprehensive view on Bufotes phylogeography, here we combined genome-wide multilocus analyses (RAD-seq) with an extensive compilation of mitochondrial, genome size, niche modelling, distribution and phenotypic (bioacoustics, morphometrics, toxin composition) datasets, representing hundreds of populations throughout Eurasia. We provide a fully resolved nuclear phylogeny for Bufotes and highlight exceptional cyto-nuclear discordances characteristic of complete mtDNA replacement (in 20% of species), mitochondrial surfing during post-glacial expansions, and the formation of homoploid hybrid populations. Moreover, we traced the origin of several allopolyploids down to species level, showing that all were exclusively fathered by the West Himalayan B. latastii but mothered by several diploid forms inhabiting Central Asian lowlands, an asymmetry consistent with hypotheses on mate choice and Dobzhansky-Muller incompatibilities. Their intermediate call phenotypes potentially allowed for rapid reproductive isolation, while toxin compositions converged towards the ecologically-closest parent. Across the radiation, we pinpoint a stepwise progression of reproductive isolation through time, with a threshold below which hybridizability is irrespective of divergence (<6My), above which species barely admix and eventually evolve different mating calls (6–10My), or can successfully cross-breed through allopolyploidization (>15My). Finally, we clarified the taxonomy of Bufotes (including genetic analyses of type series) and formally described two new species, B. cypriensis sp. nov. (endemic to Cyprus) and B. perrini sp. nov. (endemic to Central Asia). Embracing the genomic age, our framework marks the advent of a new exciting era for evolutionary research in these iconic amphibians.

Hybridization and a loss of sex shape genome-wide diversity and the origin of species in the evening primroses (Oenothera, Onagraceae).

Hybridization is thought to promote speciation in at least two ways - by fixation of heterozygosity from diploid progenitors in allopolyploids, and by generation of transgressive phenotypes and shifting fitness optima during homoploid hybrid speciation. While recent studies support a hybrid origin for a growing number of species, the extent to which hybrid origins shape patterns of diversity in asexual species remains underexplored. Here we employed transcriptome sequencing and population genomic analysis to describe patterns of genomic variation in the 13 species belonging to Oenothera subsection Oenothera. Eight of these species are functionally asexual and arose by hybrid speciation from parents spanning a range of phylogenetic divergence. We showed that genomic divergence between parents has been retained as heterozygosity in functionally asexual species, and that genome-wide levels of heterozygosity in these asexuals largely reflects the divergence of parental haplotypes coupled with a breakdown in recombination and segregation across the genome. These results show that divergence between parental species and loss of sex in hybrids shape patterns of whole-genome diversity and the origin of asexual species.

Hybridization preceded radiation in diploid wheats

Evolutionary relationships among the Aegilops-Triticum relatives of cultivated wheats have been difficult to resolve owing to incomplete lineage sorting and reticulate evolution. Recent studies have suggested that the wheat D-genome lineage (progenitor of Ae. tauschii) originated through homoploid hybridization between the A-genome lineage (progenitor of Triticum s.str.) and the B-genome lineage (progenitor of Ae. speltoides). This scenario of reticulation has been debated, calling for adequate phylogenetic analyses based on comprehensive sampling. To reconstruct the evolution of Aegilops-Triticum diploids, we here combined high-throughput sequencing of 38 nuclear low-copy loci of multiple accessions of all 13 species with inferences of the species phylogeny using the full-parameterized MCMC_SEQ method. Phylogenies recovered a monophyletic Aegilops-Triticum lineage that began diversifying ~6.6 Ma ago and gave rise to four sublineages, i.e. the A- (2 species), B- (1 species), D- (9 species) and T- (Ae. mutica) genome lineage. Full-parameterized phylogenies as well as patterns of tree dilation and tree compression supported a hybrid origin of the D-genome lineage from A and B ~3.0–4.0 Ma ago, and did not indicate additional hybridization events. Conflicting ABBA-BABA tests suggestive of further reticulation were shown here to result from ancestral population structure rather than hybridization. This comprehensive and dated phylogeny of wheat relatives indicates that the origin of the hybrid D-genome was followed by intense diversification into the majority of extant diploid as well as allopolyploid wild wheats.

Mitochondrial heteroplasmy in an avian hybrid form (Passer italiae: Aves, Passeriformes)

Mitochondrial heteroplasmy is the result from biparental transmission of mitochondrial DNA (mtDNA) to the offspring. In such rare cases, maternal and paternal mtDNA is present in the same individual. Though recent studies suggested that mtDNA heteroplasmy might be more common than previously anticipated, that phenomenon is still poorly documented and was mostly detected in case studies on hybrid populations. The Italian sparrow, Passer italiae is a homoploid hybrid form that occurs all across the Italian Peninsula mostly under strict absence of either of its parent species, the house sparrow (P. domesticus) and the Spanish sparrow (P. hispaniolensis). In this study, we document a new case of mitochondrial heteroplasmy from two island populations of P. italiae (Ustica and Lipari). Our analysis was based on the mitochondrial NADH dehydrogenase subunit 2 (ND2) that allows for a clear distinction between mitochondrial lineages of the two parental species. We amplified and sequenced the mitochondrial ND2 gene with specifically designed primer combinations for each of the two parental species. In two of our study populations, a single individual carried two different ND2 haplotypes from each of the two parental lineages. These findings contribute to current knowledge on the still poorly documented phenomenon of paternal leakage in vertebrates.

Plant speciation in the age of climate change.

Species diversity is likely to undergo a sharp decline in the next century. Perhaps as many as 33 % of all plant species may expire as a result of climate change. All parts of the globe will be impacted, and all groups of organisms will be affected. Hundreds of species throughout the world have already experienced local extinction. While thousands of species may become extinct in the next century and beyond, species formation will still occur. I consider which modes of plant species formation are likely to prevail in the next 500 years. I argue that speciation primarily will involve mechanisms that produce reproductively isolated lineages within less (often much less) than 100 generations. I will not especially consider the human element in promoting species formation, because it will continue and because the conclusions presented here are unaffected by it. The impact of climate change may be much more severe and widespread. The most common modes of speciation likely to be operative in the next 500 years ostensibly will be auto- and allopolyploidy. Polyploid species or the antecedents thereof can arise within two generations. Moreover, polyploids often have broader ecological tolerances, and are likely to be more invasive than are their diploid relatives. Polyploid species may themselves spawn additional higher level polyploids either through crosses with diploid species or between pre-existing polyploids. The percentage of polyploid species is likely to exceed 50 % within the next 500 years vs. 35 % today. The stabilized hybrid derivatives (homoploid hybrid speciation) could emerge within a hundred generations after species contact, as could speciation involving chromosomal rearrangements (and perhaps number), but the number of such events is likely to be low. Speciation involving lineage splitting will be infrequent because the formation of substantive pre- and post-zygotic barriers typically takes many thousands of years.

Homoploid hybridization of plants in the Hengduan mountains region.

The Hengduan Mountains Region (HMR) is a major global biodiversity hotspot. Complex tectonic and historical climatic conditions created opportunities for natural interspecific hybridization. Likewise, anthropogenic disturbance potentially raises the frequency of hybridization. Among species studies to date, the frequency of homoploid hybridization appears in the HMR. Of nine taxa in which natural hybridization has been detected, three groups are involved in homoploid hybrid speciation, and species pairs from the remaining six genera suggest that continuous gene flow occurs in hybrid zones. Reproductive isolation may greatly affect the dynamic and architecture of hybrid zones in the HMR. Asymmetrical hybridization and introgression can primarily be attributed to both prezygotic and postzygotic barriers. The frequent observation of such asymmetry may imply that reproductive barrier contributes to maintaining species boundaries in the alpine region. Ecological isolations with environmental disturbance may promote breeding barriers between parental species and hybrids. Hybrid zones may be an important phase for homoploid hybrid speciation. Hybrid zones potentially provided abundant genetic resources for the diversification of the HMR flora. The ecological and molecular mechanisms of control and mediation for natural hybridization will help biologists to understand the formation of biodiversity in the HMR. More researches from ecological and molecular aspects were required in future studies.

Molecular cytogenetic characterisation of Elytrigia \xd7mucronata, a natural hybrid of E. intermedia and E. repens (Triticeae, Poaceae)

BackgroundInterspecific hybridisation resulting in polyploidy is one of the major driving forces in plant evolution. Here, we present data from the molecular cytogenetic analysis of three cytotypes of Elytrigia ×mucronata using sequential fluorescence (5S rDNA, 18S rDNA and pSc119.2 probes) and genomic in situ hybridisation (four genomic probes of diploid taxa, i.e., Aegilops, Dasypyrum, Hordeum and Pseudoroegneria).ResultsThe concurrent presence of Hordeum (descended from E. repens) and Dasypyrum + Aegilops (descended from E. intermedia) chromosome sets in all cytotypes of E. ×mucronata confirmed the assumed hybrid origin of the analysed plants. The following different genomic constitutions were observed for E. ×mucronata. Hexaploid plants exhibited three chromosome sets from Pseudoroegneria and one chromosome set each from Aegilops, Hordeum and Dasypyrum. Heptaploid plants harboured the six chromosome sets of the hexaploid plants and an additional Pseudoroegneria chromosome set. Nonaploid cytotypes differed in their genomic constitutions, reflecting different origins through the fusion of reduced and unreduced gametes. The hybridisation patterns of repetitive sequences (5S rDNA, 18S rDNA, and pSc119.2) in E. ×mucronata varied between and within cytotypes. Chromosome alterations that were not identified in the parental species were found in both heptaploid and some nonaploid plants.ConclusionsThe results confirmed that both homoploid hybridisation and heteroploid hybridisation that lead to the coexistence of four different haplomes within single hybrid genomes occur in Elytrigia allopolyploids. The chromosomal alterations observed in both heptaploid and some nonaploid plants indicated that genome restructuring occurs during and/or after the hybrids arose. Moreover, a specific chromosomal translocation detected in one of the nonaploids indicated that it was not a primary hybrid. Therefore, at least some of the hybrids are fertile. Hybridisation in Triticeae allopolyploids clearly and significantly contributes to genomic diversity. Different combinations of parental haplomes coupled with chromosomal alterations may result in the establishment of unique lineages, thus providing raw material for selection.

A Quasi-Domesticate Relic Hybrid Population of Saccharomyces cerevisiae × S. paradoxus Adapted to Olive Brine.

The adaptation of the yeast Saccharomyces cerevisiae to man-made environments for the fermentation of foodstuffs and beverages illustrates the scientific, social, and economic relevance of microbe domestication. Here we address a yet unexplored aspect of S. cerevisiae domestication, that of the emergence of lineages harboring some domestication signatures but that do not fit completely in the archetype of a domesticated yeast, by studying S. cerevisiae strains associated with processed olives, namely table olives, olive brine, olive oil, and alpechin. We confirmed earlier observations that reported that the Olives population results from a hybridization between S. cerevisiae and S. paradoxus. We concluded that the olive hybrids form a monophyletic lineage and that the S. cerevisiae progenitor belonged to the wine population of this species. We propose that homoploid hybridization gave rise to a diploid hybrid genome, which subsequently underwent the loss of most of the S. paradoxus sub-genome. Such a massive loss of heterozygosity was probably driven by adaptation to the new niche. We observed that olive strains are more fit to grow and survive in olive brine than control S. cerevisiae wine strains and that they appear to be adapted to cope with the presence of NaCl in olive brine through expansion of copy number of ENA genes. We also investigated whether the S. paradoxus HXT alleles retained by the Olives population were likely to contribute to the observed superior ability of these strains to consume sugars in brine. Our experiments indicate that sugar consumption profiles in the presence of NaCl are different between members of the Olives and Wine populations and only when cells are cultivated in nutritional conditions that support adaptation of their proteome to the high salt environment, which suggests that the observed differences are due to a better overall fitness of olives strains in the presence of high NaCl concentrations. Although relic olive hybrids exhibit several characteristics of a domesticated lineage, tangible benefits to humans cannot be associated with their phenotypes. These strains can be seen as a case of adaptation without positive or negative consequences to humans, that we define as a quasi-domestication.

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.

'At the crossroads towards polyploidy': genomic divergence and extent of homoploid hybridization are drivers for the formation of the ox-eye daisy polyploid complex (Leucanthemum, Compositae-Anthemideae).

Polyploidy plays a paramount role in phytodiversity, but the causes of this evolutionary pathway require further study. Here, we use phylogenetic methods to examine possible polyploidy-promoting factors by comparing diploid representatives of the comprehensive European polyploid complex Leucanthemum with members of its strictly diploid North African counterpart Rhodanthemum. We investigate genetic divergence and gene flow among all diploid lineages of both genera to evaluate the role of genomic differentiation and hybridization for polyploid speciation. To test whether hybridization in Leucanthemum has been triggered by the geological conditions during its diversification, we additionally generate a time-calibrated phylogeny of 46 species of the subtribe Leucantheminae. Leucanthemum shows a significantly higher genetic divergence and hybridization signal among diploid lineages compared with Rhodanthemum, in spite of a similar crown age and diversification pattern during the Quaternary. Our study demonstrates the importance of genetic differentiation among diploid progenitors and their concurrent affinity for natural hybridization for the formation of a polyploid complex. Furthermore, the role of climate-induced range overlaps on hybridization and polyploid speciation during the Quaternary is discussed.

Revisiting the Homoploid Hybrid Speciation Process of the Triatoma brasiliensis macromelasoma Galvão, 1956 (Hemiptera, Triatominae) Using Cytogenetic and Molecular Markers

Triatomines are vectors of Trypanosoma cruzi, the etiologic agent of Chagas disease. Although the evolutionary process in triatomine is considered as disruptive, cryptic speciation and homoploid hybridization also are possible modes of speciation. Several analyses suggested Triatoma brasiliensis macromelasoma as a product of hybridization between T. brasiliensis and Triatoma juazeirensis. Thus, we analyzed genetic characteristics (chromosomal analysis, genetic distance for the mitochondrial ND1 gene, and the pattern of bands of internal transcribed spacer [ITS]-1) of these species, with emphasis on the phenomenon of homoploid hybridization. All species showed the same cytogenetic characteristics, low genetic distance for ND1 gene, and the same pattern of ITS-1 bands. We consider that these genetic characteristics, together with the large chromatic polymorphism and the viability of experimental crosses possibly are due to the processes of introgression that these species suffered during the process of homoploid hybridization.

A rearranged homoploid hybrid species of Origanum (Lamiaceae): O. ×munzurense Kit Tan & Sorger

ABSTRACTOriganum munzurense Kit Tan & Sorger (Sect. Brevifilamentum Ietsw.) was described by Kit Tan & Sorger in 1984, and it is an accepted species related to O. acutidens (Hand.-Mazz.) Ietsw. belonging to sect. Brevifilamentum. It is morphologically intermediate between O. acutidens and O. vulgare L. subsp. gracile (K.Koch) Ietsw., and its type locality lies in the overlap zone between the geographic distributions of these two taxa. We hypothesized that O. munzurense is in fact a hybrid between the two aforementioned taxa and we tested this hypothesis with the general morphology, chromosome features, and nuclear (ITS1–ITS2) and chloroplast (rpl32) DNA sequences of specimens of O. munzurense and its putative parental species, O. vulgare subsp. gracile and O. acutidens. The somatic chromosome number of O. munzurense, O. vulgare subsp. gracile and O. acutidens were counted as 2n = 30. Origanum munzurense presented polymorphic sites in the nuclear ITS region, but the chloroplast markers did not allow us to identify the putative maternal of O. munzurense. Our results suggest that O. munzurense is indeed a hybrid, and its rearrangement as a new homoploid hybrid (O. ×munzurense) would be more accurate. These present results support previous reports for the speciation of Origanum members via hybridization.

A natural hybrid of Impatiens, in the introduced range, demonstrated by sequence analysis of the nuclear ribosomal DNA-gene repeat

ABSTRACTIn a nature reserve, in Ticino canton in Switzerland amidst a mixed population of non-native Impatiens parviflora and I. glandulifera aberrant plants of I. parviflora were observed. A herbarium voucher and seeds were collected and seedlings subsequently raised in a greenhouse. Molecular analysis for ITS region and chloroplast markers revealed the plants to be a hybrid between I. parviflora and I. balfourii, another non-native Impatiens species occurring in the Ticino canton. No crossing out nor reduced fertility was observed in F2 and F3 generations, raised in a greenhouse. Sizeable populations of the hybrid have been observed in 2014 in the Ticino canton in the absence of the parental species. Nuclear genome size as measured by flow cytometry with propidium iodide did not indicate polyploidy and the ITS region remained 100% uniform. Therefore a homoploid hybrid speciation event is assumed.

Morphological, genetic and epigenetic aspects of homoploid hybridization between Salvia officinalis L. and Salvia fruticosa Mill.

The inheritance of phenotypic, genetic and epigenetic traits in hybridization events is difficult to predict, as numerous evolutionary, ecological, and genetic factors can play a crucial role in the process of hybridization. In the middle Adriatic island of Vis, we investigated hybridization between Salvia officinalis and S. fruticosa at morphological, genetic and epigenetic levels. SSR results revealed that hybrid individuals were characterized by diploid set of chromosomes suggesting homoploid hybridization. A well-defined group that mostly comprised of F1 generation individuals was detected. For the majority of analysed morphological characteristics, hybrids were placed in-between parental taxa, while at the same time, values of different genetic parameters were mostly higher in hybrids than in parental species. The results revealed a high contrast in the levels of phenotypic variability and epigenetic excitation between parental taxa. Environmental niche modelling confirmed that in the studied location S. officinalis experiences optimal climatological conditions, while S. fruticosa struggles with unsuitable conditions. Very low levels of gene flow between the parental species were detected. In addition, contrasting levels of epigenetic excitation in the studied groups clearly demonstrated the importance of an epigenetic response to an altered environment and confirmed the trans-generational nature of the epigenetic changes.

Codweb: Whole-genome sequencing uncovers extensive reticulations fueling adaptation among Atlantic, Arctic, and Pacific gadids.

Introgressive hybridization creates networks of genetic relationships across species. Among marine fish of the Gadidae family, Pacific cod and walleye pollock are separate invasions of an Atlantic cod ancestor into the Pacific. Cods are ecological success stories, and their ecologies allow them to support the largest fisheries of the world. The enigmatic walleye pollock differs morphologically, behaviorally, and ecologically from its relatives, representing a niche shift. Here, we apply whole-genome sequencing to Pacific, Arctic, and Atlantic gadids and reveal extensive introgression among them with the ABBA-BABA test and pseudolikelihood phylogenetic network analysis. We propose that walleye pollock resulted from extensive adaptive introgression or homoploid hybrid speciation. The path of evolution of these taxa is more web than a tree. Their ability to invade and expand into new habitats and become ecologically successful may depend on genes acquired through adaptive introgression or hybrid speciation.

Insights from genomes into the evolutionary importance and prevalence of hybridization in nature.

Hybridization is an evolutionary phenomenon that has fascinated biologists for centuries. Prior to the advent of whole-genome sequencing, it was clear that hybridization had played a role in the evolutionary history of many extant taxa, particularly plants. The extent to which hybridization has contributed to the evolution of Earth's biodiversity has, however, been the topic of much debate. Analyses of whole genomes are providing further insight into this evolutionary problem. Recent studies have documented ancient hybridization in a diverse array of taxa including mammals, birds, fish, fungi, and insects. Evidence for adaptive introgression is being documented in an increasing number of systems, though demonstrating the adaptive function of introgressed genomic regions remains difficult. And finally, several new homoploid hybrid speciation events have been reported. Here we review the current state of the field and specifically evaluate the additional insights gained from having access to whole-genome data and the challenges that remain with respect to understanding the evolutionary relevance and frequency of ancient hybridization, adaptive introgression, and hybrid speciation in nature.

The Timing and Direction of Introgression Under the Multispecies Network Coalescent.

Introgression is a pervasive biological process, and many statistical methods have been developed to infer its presence from genomic data. However, many of the consequences and genomic signatures of introgression remain unexplored from a methodological standpoint. Here, we develop a model for the timing and direction of introgression based on the multispecies network coalescent, and from it suggest new approaches for testing introgression hypotheses. We suggest two new statistics, D1 and D2, which can be used in conjunction with other information to test hypotheses relating to the timing and direction of introgression, respectively. D1 may find use in evaluating cases of homoploid hybrid speciation (HHS), while D2 provides a four-taxon test for polarizing introgression. Although analytical expectations for our statistics require a number of assumptions to be met, we show how simulations can be used to test hypotheses about introgression when these assumptions are violated. We apply the D1 statistic to genomic data from the wild yeast Saccharomyces paradoxus-a proposed example of HHS-demonstrating its use as a test of this model. These methods provide new and powerful ways to address questions relating to the timing and direction of introgression.

Completing the hybridization triangle: the inheritance of genetic incompatibilities during homoploid hybrid speciation in ragworts (Senecio).

A new homoploid hybrid lineage needs to establish a degree of reproductive isolation from its parent species if it is to persist as an independent entity, but the role hybridization plays in this process is known in only a handful of cases. The homoploid hybrid ragwort species, Senecio squalidus (Oxford ragwort), originated following the introduction of hybrid plants to the UK approximately 320 years ago. The source of the hybrid plants was from a naturally occurring hybrid zone between S. aethnensis and S. chrysanthemifolius on Mount Etna, Sicily. Previous studies of the parent species found evidence for multiple incompatibility loci causing transmission ratio distortion of genetic markers in their hybrid progeny. This study closes the hybridization triangle by reporting a genetic mapping analysis of the remaining two paired cross combinations between S. squalidus and its parents. Genetic maps produced from F2 mapping families were generally collinear but with half of the linkage groups showing evidence of genomic reorganization between genetic maps. The new maps produced from crosses between S. squalidus and each parent showed multiple incompatibility loci distributed across the genome, some of which co-locate with previously reported incompatibility loci between the parents. These findings suggest that this young homoploid hybrid species has inherited a unique combination of genomic rearrangements and incompatibilities from its parents that contribute to its reproductive isolation.