Hybrids In Natural Populations Research Articles

Molecular and morphological evidence of hybridization between two dimorphic sympatric species of Fuchsia (Onagraceae).

Hybridization is commonly reported in angiosperms, generally based on morphology, and in few cases confirmed by molecular markers. Fuchsia has a long tradition of ornamental cultivars with different hybrids produced by artificial crosses, so natural hybridization between sympatric Fuchsia species could be common. Natural hybridization between F. microphylla and F. thymifolia was tested using six newly developed microsatellites for F. microphylla in addition to other molecular markers with codominant and maternal inheritance. Geometric morphometrics of leaves and floral structures were also used to identify putative hybrids. Hybrids showed a different degree of genetic admixture between both parental species. Chloroplast DNA (cpDNA) sequences indicated that hybridization occurs in both directions, in fact, some of the hybrids showed new haplotypes for cpDNA and ITS (internal transcriber spacer of nuclear ribosomal RNA genes) sequences. The morphology of hybrid individuals varied between the two parental species, but they could be better identified by their leaves and floral tubes. Our study is the first to confirm the hybridization in natural populations of Fuchsia species and suggests that hybridization has probably occurred repeatedly throughout the entire distribution of the species. Phylogeographic analysis of both species will be essential to understanding the impact of hybridization throughout their complete distribution.

Juvenile Resistance of Barley Cultivars and Accessions to Net-, Spot-, and Hybrid (Net × Spot) Forms of <i>Pyrenophora teres</i>

Barley net blotch is an economically important disease. The causative agent is an ascomycete, Pyrenophora teres, which exists in two forms: P. teres f. teres (Ptt) and P. teres f. maculata (Ptm), which differ in their symptoms on barley plants. These two forms are easily crossed in laboratory conditions with the formation of fertile offspring, however, it is extremely difficult to prove the hybrid nature of fungal isolates, sometimes found in natural populations of the pathogen and bearing signs of both forms. In 2020, we first identified Ptt × Ptm hybrids in natural populations of P. teres in Krasnodar Region in isolates collected in 2016. The aim of the studies was to compare the virulence of two Ptt isolates, two Ptm isolates of different origin and a hybrid isolate Ptt × Ptm to a wide set of barley genotypes from the VIR collection pre-selected for Ptt resistance, to determine variability of the virulence trait in the hybrid isolate and characterize resistance to both forms of the fungus and hybrid. Depending on the barley genotype, 3 types of disease symptoms were manifested upon inoculation with the Ptt × Ptm hybrid isolate: (1) similar to Ptt, (2) similar to Ptm, and (3) a mixed type. Apparently, the manifestation of symptoms after inoculation with the hybrid isolate depends on the barley genotype influencing the expression of certain pathogen effector genes. It was shown that in most cases Ptm isolates differ in virulence from Ptt isolates to the same barley genotypes, and the Ptt × Ptm hybrid isolate from both Ptt and Ptm. On average, the Ptt × Ptm hybrid isolate was less aggressive than the Ptt and Ptm isolates. A comparison of the types of responses of barley genotypes to all studied isolates of Ptt and Ptm revealed 8.8% of genotypes resistant to both forms of P. teres and 5.6% to Ptt, Ptm and the hybrid isolate Ptt × Ptm. The virulence of natural hybrid between two forms of P. teres Ptt × Ptm was studied for the first time. The barley genotypes resistant to the two forms of the net blotch are valuable source of resistance for barley breeding.

Hybridization between Pyrenophora teres Forms in Natural Populations of Russia and the Republic of Belarus

Two forms of the pathogenic fungus Pyrenophora teres, P. teres f. teres (a net form) and P. teres f. maculata (a spot form), cause different disease signs, net or spot blotch, on barley leaves. The net form of P. teres is widespread wherever barley is cultivated, while the spot form was first identified in Krasnodar krai of Russia in 2011 and Brest oblast of Belarus in 2016. The two forms of the pathogen easily mate each other in laboratory conditions, but their hybrids either do not form or are difficult to detect in nature. The question as to whether hybrids between the net and spot forms are produced and maintained in natural populations is pressing and bears applied significance because different genes determine resistance to the different P. teres forms in barley. Hybrid forms may be virulent to resistance donors used in breeding. The objective of this work was to search Russian and Belarussian natural populations for hybrids between P. teres f. teres and P. teres f. maculata with the use of new form-specific markers, Ptt and Ptm. The study included 138 single-conidium isolates from four P. teres f. maculatа and four P. teres f. teres populations. The isolates were collected from commercial barley plantations of Leningrad oblast, Krasnodar krai (Russia), and Brest oblast (Belarus) from 2013 to 2016. A genotyping with 10 form-specific markers was performed in all isolates. Several isolates were found to combine markers of both of the P.teres forms and were conventionally identified as hybrids between the forms. Hypotheses were advanced to explain the occurrence of hybrids in natural populations. The most plausible hypothesis suggests that sexual or somatic hybridization between the two forms coexisting in barley plantations accounts for the origin of the P. teres isolates that combine markers specific to P. teres f. teres and markers specific to P. teres f. maculata in their genomes. It is also possible that a third, possibly ancestral, intermediate form was preserved during divergence in the species P. teres.

Diagnostic locus for identification of silverside hybrids (odontesthes humensis and odontesthes bonariensis) in a natural population

Hybridization is a natural phenomenon that occurs more often in fish than in other vertebrates. The use of nuclear and mitochondrial molecular markers provides valuable results in the detection of these events. The aim of this study was to investigate the occurrence of interspecific hybrids in natural populations of silverside. The samples of Odontesthes humensis, Odontesthes bonariensis, and individuals that were morphologically different from pure species were collected in the Mangueira lagoon, located in southern Brazil. Result: Six tetranucleotide microsatellite loci were synthesized and tested. The UFPEL_OH3 locus proved to be diagnostic for the detection of silverside hybrids, and it was possible to distinguish between pure and hybrid species. The mitochondrial marker gene cytb synthesized from conserved Odontesthes sequences in the GenBank genetic database showed no differences in the genetic sequence of the samples, needing further studies to confirm the hypothesis.

Historical Differentiation and Recent Hybridization in Natural Populations of the Nematode-Trapping Fungus Arthrobotrys oligospora in China.

Maintaining the effects of nematode-trapping fungi (NTF) agents in order to control plant-parasitic nematodes (PPNs) in different ecological environments has been a major challenge in biological control applications. To achieve such an objective, it is important to understand how populations of the biocontrol agent NTF are geographically and ecologically structured. A previous study reported evidence for ecological adaptation in the model NTF species Arthrobotrys oligospora. However, their large-scale geographic structure, patterns of gene flow, their potential phenotypic diversification, and host specialization remain largely unknown. In this study, we developed a new panel of 20 polymorphic short tandem repeat (STR) markers and analyzed 239 isolates of A. oligospora from 19 geographic populations in China. In addition, DNA sequences at six nuclear gene loci and strain mating types (MAT) were obtained for these strains. Our analyses suggest historical divergence within the A. oligospora population in China. The genetically differentiated populations also showed phenotypic differences that may be related to their ecological adaptations. Interestingly, our analyses identified evidence for recent dispersion and hybridization among the historically subdivided geographic populations in nature. Together, our results indicate a changing population structure of A. oligospora in China and that care must be taken in selecting the appropriate strains as biocontrol agents that can effectively reproduce in agriculture soil while maintaining their nematode-trapping ability.

Ecological Observations on Hybrid Populations of European Plethodontid Salamanders, Genus Speleomantes

Speleomantes are the only plethodontid salamanders present in Europe. Multiple studies have been performed to investigate the trophic niche of the eight Speleomantes species, but none of these studies included hybrid populations. For the first time, we studied the trophic niche of five Speleomantes hybrid populations. Each population was surveyed twice in 2020, and stomach flushing was performed on each captured salamander; stomach flushing is a harmless technique that allows stomach contents to be inspected. We also assessed the potential divergence in size and body condition between natural and introduced hybrids, and their parental species. Previously collected data on Speleomantes were included to increase the robustness of these analyses. In only 33 out of 134 sampled hybrid Speleomantes we recognized 81 items belonging to 11 prey categories. The frequency of empty stomachs was higher in females and individuals from natural hybrid populations, whereas the largest number of prey was consumed by males. We compared the total length and body condition of 685 adult salamanders belonging to three types of hybrids and three parental (sub)species. Three group of salamanders (one hybrid and two parental species) showed significantly larger size, whereas no difference in body condition was observed. This study provided novel ecological information on Speleomantes hybrid populations. We also provided insights into the potential divergence between hybrids and parental species in terms of size and body condition. We discuss our findings, and formulate several hypotheses that should be tested in the future.

The Genetic Architecture of Variation in the Sexually Selected Sword Ornament and Its Evolution in Hybrid Populations

Biologists since Darwin have been fascinated by the evolution of sexually selected ornaments, particularly those that reduce viability. Uncovering the genetic architecture of these traits is key to understanding how they evolve and are maintained. Here, we investigate the genetic architecture and evolutionary loss of a sexually selected ornament, the "sword" fin extension that characterizes many species of swordtail fish (Xiphophorus). Using sworded and swordless sister species of Xiphophorus, we generated a mapping population and show that the sword ornament is polygenic-with ancestry across the genome explaining substantial variation in the trait. After accounting for the impacts of genome-wide ancestry, we identify one major-effect quantitative trait locus (QTL) that explains ~5% of the overall variation in the trait. Using a series of approaches, we narrow this large QTL interval to several likely candidate genes, including genes involved in fin regeneration and growth. Furthermore, we find evidence of selection on ancestry at one of these candidates in four natural hybrid populations, consistent with selection against the sword in these populations.

Integrative genomic phylogeography reveals signs of mitonuclear incompatibility in a natural hybrid goby population.

Hybridization between divergent lineages generates new allelic combinations. One mechanism that can hinder the formation of hybrid populations is mitonuclear incompatibility, that is, dysfunctional interactions between proteins encoded in the nuclear and mitochondrial genomes (mitogenomes) of diverged lineages. Theoretically, selective pressure due to mitonuclear incompatibility can affect genotypes in a hybrid population in which nuclear genomes and mitogenomes from divergent lineages admix. To directly and thoroughly observe this key process, we de novo sequenced the 747‐Mb genome of the coastal goby, Chaenogobius annularis, and investigated its integrative genomic phylogeographics using RNA‐sequencing, RAD‐sequencing, genome resequencing, whole mitogenome sequencing, amplicon sequencing, and small RNA‐sequencing. Chaenogobius annularis populations have been geographically separated into Pacific Ocean (PO) and Sea of Japan (SJ) lineages by past isolation events around the Japanese archipelago. Despite the divergence history and potential mitonuclear incompatibility between these lineages, the mitogenomes of the PO and SJ lineages have coexisted for generations in a hybrid population on the Sanriku Coast. Our analyses revealed accumulation of nonsynonymous substitutions in the PO‐lineage mitogenomes, including two convergent substitutions, as well as signals of mitochondrial lineage‐specific selection on mitochondria‐related nuclear genes. Finally, our data implied that a microRNA gene was involved in resolving mitonuclear incompatibility. Our integrative genomic phylogeographic approach revealed that mitonuclear incompatibility can affect genome evolution in a natural hybrid population.

The global epidemiology of emerging Histoplasma species in recent years.

Histoplasmosis is a serious infectious disease in humans caused by Histoplasma spp. (Onygenales), whose natural reservoirs are thought to be soil enriched with bird and bat guano. The true global burden of histoplasmosis is underestimated and frequently the pulmonary manifestations are misdiagnosed as tuberculosis. Molecular data on epidemiology of Histoplasma are still scarce, even though there is increasing recognition of histoplasmosis in recent years in areas distant from the traditional endemic regions in the Americas. We used multi-locus sequence data from protein coding loci (ADP-ribosylation factor, H antigen precursor, and delta-9 fatty acid desaturase), DNA barcoding (ITS1/2+5.8s), AFLP markers and mating type analysis to determine the genetic diversity, population structure and recognise the existence of different phylogenetic species among 436 isolates of Histoplasma obtained globally. Our study describes new phylogenetic species and the molecular characteristics of Histoplasma lineages causing outbreaks with a high number of severe outcomes in Northeast Brazil between 2011 and 2015. Genetic diversity levels provide evidence for recombination, common ancestry and clustering of Brazilian isolates at different geographic scales with the emergence of LAm C, a new genotype assigned to a separate population cluster in Northeast Brazil that exhibited low diversity indicative of isolation. The global survey revealed that the high genetic variability among Brazilian isolates along with the presence of divergent cryptic species and/or genotypes may support the hypothesis of Brazil being the center of dispersion of Histoplasma in South America, possibly with the contribution of migratory hosts such as birds and bats. Outside Brazil, the predominant species depends on the region. We confirm that histoplasmosis has significantly broadened its area of occurrence, an important feature of emerging pathogens. From a practical point of view, our data point to the emergence of histoplasmosis caused by a plethora of genotypes, and will enable epidemiological analysis focused on understanding the processes that lead to histoplasmosis. Further, the description of this diversity opens avenues for comparative genomic studies, which will allow progress toward a consensus taxonomy, improve understanding of the presence of hybrids in natural populations of medically relevant fungi, test reproductive barriers and to explore the significance of this variation.

Species boundary and extensive hybridization and introgression in Petunia

ABSTRACT Studying the role of hybridization in the speciation of plants is one of the most thrilling areas of evolutionary biology. Hybridization in natural populations can act in opposition to divergence, contribute to adaptation through introgression or foster the emergence of new lineages via hybrid speciation. Species of the plant genus Petunia grow in open areas in southern South America. Some natural interspecific hybrid events have been described for the genus, such as between the endemic P. exserta and the widespread P. axillaris. Both species occur in sympatry in Serra do Sudeste (Brazil), where they occur in diverse habitats and exhibit floral divergence, which has been related to the attraction of different primary pollinators. The present study evaluates the maintenance of the species boundaries front of hybridization and introgression. Direct and indirect methods of estimating gene exchange employed genotyping 720 reproductive plants and 611 progenies of both species with eight microsatellite loci. Gene exchange was found to be frequent and bidirectional between the species, indicating that introgression changes their genetic constitution in areas of sympatry. Limits of the studied species are being maintained because of the high level of inbreeding and backcrosses that are habitat-dependent.

Fitness of reciprocal F1 hybrids between Rhinanthus minor and Rhinanthus major under controlled conditions and in the field.

The performance of first-generation hybrids determines to a large extent the long-term outcome of hybridization in natural populations. F1 hybrids can facilitate further gene flow between the two parental species, especially in animal-pollinated flowering plants. We studied the performance of reciprocal F1 hybrids between Rhinanthus minor and R.major, two hemiparasitic, annual, self-compatible plant species, from seed germination to seed production under controlled conditions and in the field. We sowed seeds with known ancestry outdoors before winter and followed the complete life cycle until plant death in July the following season. Germination under laboratory conditions was much lower for the F1 hybrid formed on R.major compared with the reciprocal hybrid formed on R.minor, and this confirmed previous results from similar experiments. However, this difference was not found under field conditions, which seems to indicate that the experimental conditions used for germination in the laboratory are not representative for the germination behaviour of the hybrids under more natural conditions. The earlier interpretation that F1 hybrid seeds formed on R.major face intrinsic genetic incompatibilities therefore appears to be incorrect. Both F1 hybrids performed at leastas well as andsometimes better than R.minor, which had a higher fitness than R.major in one of the two years in the greenhouse and in the field transplant experiment. The high fitness of the F1 hybrids confirms findings from naturally mixed populations, where F1 hybrids appear in the first year after the two species meet, which leads to extensive advanced-hybrid formation and introgression in subsequent generations.

Paternity analysis reveals constraints on hybridization potential between native and introduced bluebells (Hyacinthoides)

The native UK bluebell Hyacinthoides non-scripta is considered to be at risk from hybridization with naturalised non-native bluebells. The non-natives, likely to be hybrid themselves (H. x massartiana) between H. non-scripta and H. hispanica, occur in relatively small numbers throughout the UK range of natives. Full interfertility between taxa has been repeatedly asserted in reporting spread of non-natives and predicting genetic erosion or assimilation of the native. However, there have been no published data to support suppositions that non-natives arose from in-situ hybridization, or that natural hybridization represents an ongoing threat to the native bluebell. Here we first investigated hybridization potential via reciprocal hand-crosses and observed overlap in flowering periods of native and non-native bluebells, finding that flowering was largely synchronous and that seed set and early seedling survival were equivalent for between-taxon crosses. We then established an experimental array allowing natural pollination to occur among flowering plants, and determined the paternity of offspring using microsatellite markers. We found that natives were more likely to produce seeds than non-natives, and that paternities approximated three native to one non-native, regardless of the identity of the maternal parent. Our results demonstrate that hybridization in natural populations and introgression between natives and non-natives are possible. However, lower reproductive success of non-natives coupled with the massive numerical advantage of natives represents a substantial constraint against ‘extinction-by-hybridization’ of H. non-scripta in the UK.

Selection, recombination, and hybrid evolution

Evolutionary Genomics Hybridization is an important force in evolution. The effects of hybridization across the whole genome are not understood. Using a fine-scale genetic map, Schumer et al. examined local ancestry in replicate natural hybrid populations of swordtail fish. Each parental species

Isolation and Characterization of Key Genes that Promote Flavonoid Accumulation in Purple-leaf Tea (Camellia sinensis L.)

There were several high concentrations of flavonoid components in tea leaves that present health benefits. A novel purple-leaf tea variety, ‘Mooma1’, was obtained from the natural hybrid population of Longjing 43 variety. The buds and young leaves of ‘Mooma1’ were displayed in bright red. HPLC and LC-MS analysis showed that anthocyanins and O-Glycosylated flavonols were remarkably accumulated in the leaves of ‘Mooma1’, while the total amount of catechins in purple-leaf leaves was slightly decreased compared with the control. A R2R3-MYB transcription factor (CsMYB6A) and a novel UGT gene (CsUGT72AM1), that were highly expressed in purple leaf were isolated and identified by transcriptome sequencing. The over-expression of transgenic tobacco confirmed that CsMYB6A can activate the expression of flavonoid-related structural genes, especially CHS and 3GT, controlling the accumulation of anthocyanins in the leaf of transgenic tobacco. Enzymatic assays in vitro confirmed that CsUGT72AM1 has catalytic activity as a flavonol 3-O-glucosyltransferase, and displayed broad substrate specificity. The results were useful for further elucidating the molecular mechanisms of the flavonoid metabolic fluxes in the tea plant.

Lineages of Silene nutans developed rapid, strong, asymmetric postzygotic reproductive isolation in allopatry.

Reproductive isolation can rise either as a consequence of genomic divergence in allopatry or as a byproduct of divergent selection in parapatry. To determine whether reproductive isolation in gynodioecious Silene nutans results from allopatric divergence or from ecological adaptation following secondary contact, we investigated the pattern of postzygotic reproductive isolation and hybridization in natural populations using two phylogeographic lineages, western (W1) and eastern (E1). Experimental crosses between the lineages identified strong, asymmetric postzygotic isolation between the W1 and the E1 lineages, independent of geographic overlap. The proportion of ovules fertilized, seeds aborted, and seeds germinated revealed relatively little effect on the fitness of hybrids. In contrast, hybrid mortality was high and asymmetric: while half of the hybrid seedlings with western lineage mothers died, nearly all hybrid seedlings with E1 mothers died. This asymmetric mortality mirrored the proportion of chlorotic seedlings, and is congruent with cytonuclear incompatibility. We found no evidence of hybridization between the lineages in regions of co-occurrence using nuclear and plastid markers. Together, our results are consistent with the hypothesis that strong postzygotic reproductive isolation involving cytonuclear incompatibilities arose in allopatry. We argue that the dynamics of cytonuclear gynodioecy could facilitate the evolution of reproductive isolation.

Polymorphism of RAPD and ISSR markers in grain amaranth species

Different molecular genetic markers are effectively used in agricultural genetic selection programs. Genetic markers can be used in commercial breeds certification performance, fast and reliable genotype identification, genetic maps creation, genetics, phylogeny and plant systematic studies, which can accelerate selection, provide effective study and culture genofond maintenance. Amaranth, in this regard, has not been studied well, there is not enough data to effectively perform amaranth marker selection or for the certification of new and existing varieties; there are inaccuracies in the systematization of the crop. Important are the questions about the origin of grain amaranth species and the processes of their evolutionary formation. Amaranth is a pseudo-cereal with a millennia-long history, it has been actively cultivated in many countries around the world in recent decades. A high level of alterability and the formation of a huge number of spontaneous hybrids in natural populations of amaranth significantly complicate the identification of individual genotypes and entire taxonomic units of Amaranthus L. Due to the lack of research and depending on the environmental conditions, the morphological markers of amaranth are not able to provide sufficient genomic information to the breeder; thus it is necessary to search for reliable genetic markers that allow the genetic diversity of the Amaranthus L. species to be studied and effectively maintained. This research includes grain amaranth species DNA polymorphism analysis. Using RAPD and ISSR technologies, 203 loci have been identified, of which 173 appear to be polymorphic, 30 monomorphic (found in all genotypes analyzed) and 13 unique (found only in one genotype). Unique and monomorphic DNA loci can be used as specific genetic markers, in particular, for the certification of breeds, which is especially important for the identification of plant material and plant genetic variability monitoring. A high level of DNA polymorphism was revealed (about 85 %), a genetic relationship between grain amaranth species established, their monophyletic origin theory verified. A. mantegazzianus Passer. was proved to be an A. caudatus L. subspecies.

Introgression and Habitat Segregation in a Pair of Ladybird Beetle Species in the Genus Propylea (Coccinellidae, Coccinellinae) in Northern Japan.

The ladybird beetles Propylea quatuordecimpunctata and P. japonica have largely overlapping distributions in northern Japan, and in the laboratory produce fertile hybrids. In this study, we surveyed the distribution and morphological differentiation of these species and the hybrids in natural populations, with a focus on western Hokkaido, northern Japan. Phenotypic analyses were conducted for 987 individuals collected at 90 localities. In addition, the nuclear internal transcribed spacer-II (ITS2) region (549 bp) and part of the mitochondrial cytochrome c oxidase subunit I (COI) gene (700 bp) were sequenced for 620 individuals from 53 localities. Analyses of both phenotypic and genotypic features discriminated two distinct entities assignable to P. quatuordecimpunctata and P. japonica. However, individuals with intermediate phenotypes and/or genotypes also occurred extensively, indicating natural hybridization. Putative P. quatuordecimpunctata individuals were collected across a wide range of altitudes (30-600 m), whereas those of P. japonica were found mostly lower than 300 m alt. In addition, P. quatuordecimpunctata was dominant in semi-open habitats shaded by canopy foliage, whereas P. japonica was frequent in more open habitats. The perceived altitudinal difference in the distributions may thus in part be a consequence of this different habitat preference, as open habitats are more common at lower altitudes in the study area.

Displacement of native Patagonian freshwater silverside populations (Odontesthes hatcheri, Atherinopsidae) by introgressive hybridization with introduced O. bonariensis

The Patagonian silverside Odontesthes hatcheri is a native fish restricted to streams and lakes of Patagonia (Argentina and Chile). Stocking programs to enhance recreational fisheries in man-made reservoirs have introduced a nonnative, closely-related species (the pejerrey O. bonariensis) in Patagonia almost a century ago, and yet little is known about the invasiveness of this species. To evaluate the impact of these introductions we analyze genetic data (microsatellite markers and mitochondrial DNA) to quantify the incidence of hybridization between these two species and assess potential effects on native population structure. Phylogeographic analyses reveal weak geographic differentiation among populations of O. hatcheri, in agreement with previous studies for other freshwater fishes in Patagonia strongly influenced by Quaternary glaciations and hydrographic basin changes since the last glaciation. However, many populations have unique genetic pools. In several areas, introductions resulted in extensive hybridization, with high frequencies of F2 and backcrossed hybrids in natural populations, and in some cases O. bonariensis has completely displaced the native populations. The negative impact of these introductions on native populations is correlated to temperature, a critical parameter in the face of global warming, suggesting that invasiveness of O. bonariensis may increase in the future. Our results advise against continuing stocking programs to preserve the integrity of natural populations of the Patagonian silverside.

A Natural Population Derived from Species Hybridizationin the Drosophila ananassae Species Complexon Penang Island, Malaysia.

We surveyed natural population of the Drosophila ananassae species complex on Penang Island, Malaysia. Analyses of phenotypic traits, chromosome arrangements, molecular markers, and reproductive isolation suggest the existence of two species: D. ananassae and D. cf. parapallidosa. Molecular marker analysis indicates that D. cf. parapallidosa carries chromosome Y and 4 introgressions from D. ananassae. Thus, D. cf. parapallidosa seems to be a hybrid descendant that recently originated from a natural D. parapallidosa♀× D. ananassae♂ cross. Furthermore, D. cf. parapallidosa behaves differently from authentic D. parapallidosa with respect to its reproductive isolation from D. ananassae. Premating isolation is usually seen in only the D. ananassae♀× D. parapallidosa♂ cross, but we observed it in crosses of both directions between D. ananassae and D. cf. parapallidosa. In addition, hybrid males from the D. ananassae♀× D. parapallidosa♂ cross are usually sterile, but they were fertile when D. ananassae♀ were mated with D. cf. parapallidosa ♂. We attempted an artificial reconstruction of the hybrid species to simulate the evolutionary process(es) that produced D. cf. parapallidosa. This is a rare case of natural hybrid population in Drosophila and may be a useful system for elucidating speciation with gene flow.

Differential introgression and reorganization of retrotransposons in hybrid zones between wild wheats.

The maintenance of species integrity despite pervasive hybridization is ruled by the interplay between reproductive barriers. Endogenous postzygotic isolation will shape the patterns of introgression in hybrid zones, leading to variable outcomes depending on the genetic mechanism involved. Here, we analysed experimental and natural hybrid populations of Aegilops geniculata and Aegilops triuncialis to examine the genetics of species boundaries in the face of gene flow. Because long-terminal repeat retrotransposons (LTR-RTs) showing differential evolutionary trajectories are probably to affect hybrid dysgenesis and reproductive isolation between these wild wheat species, we addressed the impact of LTR-RTs in shaping introgression between them. Experimental settings involving artificial sympatry and enforced crossings quantified strong, but incomplete reproductive isolation, and highlighted asymmetrical endogenous postzygotic isolation between the two species. Natural hybrid zones located in the northern Golan Heights were analysed using plastid DNA, amplified fragment length polymorphisms (AFLP) marking random sequences, and sequence-specific amplified polymorphisms (SSAP) tracking insertions from six LTR-RT families. This analysis demonstrated asymmetrical introgression and genome reorganization. In comparison with random sequences and quiescent LTR-RTs, those LTR-RTs predicted to be activated following conflicting interactions in hybrids revealed differential introgression across the hybrid zones. As also reported for synthetic F1 hybrids, such LTR-RTs were specifically reorganized in the genomes of viable hybrids, confirming that conflicts between selfish LTR-RTs may represent key incompatibilities shaping species boundaries and fostering long-term species integrity in the face of gene flow.