Origin(s) of the diploid hybrid species Helianthus deserticola (Asteraceae)

Homoploid hybrid speciation has traditionally been considered a rare event, dependent on the establishment of both a novel, balanced genotype and reproductive isolating barriers between the new species and its progenitors. However, more recent studies have shown that synthetic hybrids converge toward the chromosomal structure of natural hybrids after only a few generations, suggesting that this phenomenon may be more frequent than previously assumed. Here, the possibility that the diploid hybrid species Helianthus deserticola arose from more than one hybrid speciation event was investigated using patterns of variation from cpDNA, 18 nuclear microsatellite loci, and population interfertility. Helianthus deserticola contains cpDNA haplotypes characteristic of both parental species, is polyphyletic with one parental species based on nine microsatellite loci, and has a high degree of interfertility among populations. The data are consistent with either a single origin followed by introgression with the parental species or multiple origins. Analysis of microsatellite variation places the origin of H. deserticola between 170?000 and 63?000 years before present, making it unlikely that anthropogenic disturbances influenced its origin. Finally, the hybrid species generally has lower levels of genetic diversity but higher levels of differentiation among populations than either parental species.     

IRIS NELSONII (IRIDACEAE): ORIGIN AND GENETIC COMPOSITION OF A HOMOPLOID HYBRID SPECIES

Nuclear and chloroplast DNA variation was assayed for two populations of Louisiana irises (Bayou Teche and Young's Coulee) that demonstrated extreme morphological variation and for a sample of the putative hybrid species, Iris nelsonii. The genetic markers examined in this analysis were diagnostic for either Iris fulva. Iris hexagona, or Iris brevicaulis. These data demonstrated that the two morphologically variable populations were hybrid associations involving all three of these species and that all three of these species were involved in the origin of I. nelsonii. The distribution of genetic variation in I. nelsonii was significantly different from that present in either of the two hybrid populations. I. nelsonii demonstrated significantly fewer foreign markers than the two hybrid populations. This finding is in accord with the prediction that I. nelsonii is a hybrid species that has undergone stabilization with regard to genetic recombination and segregation. Although the genotypic makeup of I. nelsonii was significantly different from other parental and hybrid populations, individual plants from this species cannot be unequivocally differentiated from either I. fulva or certain hybrid genotypes. This reflects the paradoxical nature of genotypic variation in hybrid species. Thus, a hybrid species may include genotypes that overlap with both parental and contemporary hybrid populations. In the case of I. nelsonii it is necessary to utilize additional information (morphological, chromosomal, ecological) to identify plants belonging to this taxon. One hybrid population (Young's Coulee) is suggested as a paradigm for the progenitor population that gave rise to I. nelsonii.     

Speciation in Progress? A Continuum of Reproductive Isolation in <Emphasis Type="Italic">Drosophila Bipectinata</Emphasis>

Incipient species in the early stages of divergence can provide crucial information about the genetic basis of reproductive isolation and the evolutionary forces that promote speciation. In this report, we describe two subspecies of Drosophila bipectinata that show a continuum of reproductive isolation. Crosses between strains of the same subspecies produce fully fertile offspring. At the same time, each subspecies harbors extensive variation for the degree of reproductive isolation from the other subspecies. The percentage of fertile hybrid males varies from 0 to 90%, depending on the origin of parental strains, indicating that the genes responsible for hybrid sterility are not fixed within either subspecies, or even within local populations. Reproductive isolation is non-transitive, so that the extent of hybrid sterility depends on the particular combination of strains. The two subspecies show little or no evidence of genetic differentiation at three nuclear loci, suggesting that they diverged very recently or continue to experience significant levels of gene flow. A hybrid zone between the two subspecies may exist in New Guinea and Northeastern Australia.     

The evolution of Spartina anglica C.E. Hubbard (Gramineae): origin and genetic variability

An extensive survey of isozyme phenotypes in British populations of the amphidiploid salt marsh grass Spartina anglica and its putative parents has confirmed that the species arose by chromosome doubling in S. × townsendii , a sterile hybrid between S. maritima and S. alterniflora. Isozyme phenotypes and seed protein profiles indicate that S. anglica is almost totally lacking in genetic variation. Isozyme evidence also indicates that the parental species are characterized by low levels of genetic variation. The lack of variation in S. anglica is proposed as being due to a narrow genetic base resulting from a single origin, or a multiple origin from uniform parents; the fact that many populations are derived from very small founder populations; and because preferential pairing between identical homologous chromosomes prevents recombination between the divergent component genomes of the species. The low levels of isozyme variation that occur appear to be due to chromosome loss.
The consequences for the future evolution of S. anglica , given its lack of genetic variation, are discussed.     

Genetic diversity and distribution patterns of diploid and polyploid hybrid water frog populations (Pelophylax esculentus complex) across Europe

Polyploidization is a rare yet sometimes successful way for animals to rapidly create geno‐ and phenotypes that may colonize new habitats and quickly adapt to environmental changes. In this study, we use water frogs of the Pelophylax esculentus complex, comprising two species (Pelophylax lessonae, genotype LL; Pelophylax ridibundus, RR) and various diploid (LR) and triploid (LLR, LRR) hybrid forms, summarized as P. esculentus, as a model for studying recent hybridization and polyploidization in the context of speciation. Specifically, we compared the geographic distribution and genetic diversity of diploid and triploid hybrids across Europe to understand their origin, maintenance and potential role in hybrid speciation. We found that different hybrid and parental genotypes are not evenly distributed across Europe. Rather, their genetic diversity is structured by latitude and longitude and the presence/absence of parental species but not of triploids. Highest genetic diversity was observed in central and eastern Europe, the lowest in the northwestern parts of Europe. This gradient can be explained by the decrease in genetic diversity during postglacial expansion from southeastern glacial refuge areas. Genealogical relationships calculated on the basis of microsatellite data clearly indicate that hybrids are of multiple origin and include a huge variety of parental genomes. Water frogs in mixed‐ploidy populations without any parental species (i.e. all‐hybrid populations) can be viewed as evolutionary units that may be on their way towards hybrid speciation. Maintenance of such all‐hybrid populations requires a continuous exchange of genomes between diploids and triploids, but scenarios for alternative evolutionary trajectories are discussed.     

Natural and experimental hybridization inArmeria (Plumbaginaceae):A. villosa subsp.carratracensis

An experimental crossing program was carried out in order to test the hypothesis thatArmeria villosa subsp.carratracensis, a serpentine endemic from Southern Spain, is of hybrid origin. Fruit-set and mainly pollen stainability data in artificial hybrids demonstrate that internal interspecific barriers are weak. Two generations of backcrossing with the two putative parents —A. colorata andA. villosa subsp.longiaristata— can restore pollen stainability to an average of 83.2 and 68.7, respectively. A morphometric analysis of 148 specimens belonging to parental species, different artificial hybrid generations and wild putative hybrid race shows that (1) morphological characters have a strong genetic basis and thus are good markers in identifying hybrids and introgression inArmeria; (2) artificial backcrossing produces in morphometrical characters a variation which conforms in both trend and magnitude to what is commonly attributed in the wild to introgression; (3) the position of the putative hybrid race is intermediate between the two parental species used in the program. These results are in accordance with the hypothesis of hybrid origin ofA. villosa subsp.carratracensis. Natural and experimental hybridization inArmeria, I     

Genetic variation and self-incompatibility within and outside a Rorippa hybrid zone (Brassicaceae)

Rorippa amphibia and R. sylvestris are both self-incompatible, perennial species. In northern Germany they show a geographic pattern of interspecific gene flow: R. amphibia and R. sylvestris form a hybrid zone at the river Elbe but do not hybridise elsewhere in northern Germany. In the present paper we analyse the relationship between molecular diversity, seed set and seed germination within and outside the hybrid zone. In both species we observed high genetic variation and high seed set in the hybrid zone, and low genetic variation and low seed set outside the hybrid zone. In R. amphibia, high genetic variation is correlated with high seed set, but with low germination rates of the seeds. The results of an isolation experiment show that self-incompatibility is maintained in the parental species and their hybrids. The impact of genetic self-incompatibility on hybrid zone formation and hybrid fitness is discussed.     

Variable post‐zygotic isolation in Drosophila melanogaster/D. simulans hybrids

The study of hybrid inviability reveals cryptic divergence between the genetic interactions that maintain stable phenotypes in the pure species . We characterized the effects of natural variation on the penetrance of hybrid inviability phenotypes in crosses between Drosophila melanogaster and two species of the D. simulans subcomplex, D. simulans and D. sechellia. Using a panel of wild‐caught lines, we studied the levels of genetic variance present in D. simulans and D. sechellia affecting prezygotic and post‐zygotic isolation in hybridizations with D. melanogaster females. We observed extensive variability in the viability of hybrid individuals, dependent on the genotype of the parents, suggesting that intraspecific natural variation manifests directly in hybrid phenotypes. Furthermore, we found that genetic background significantly affects the penetrance of a well‐studied determinant of hybrid inviability: the interaction between Hmr^mel–Lhr^sim. Our results suggest that hybrid inviability – and reproductive isolation generally – can be modified by polymorphisms at multiple loci segregating within the parental species. Just as the penetrance of most mutant phenotypes can be modified by the genetic background within the pure species, the penetrance of hybrid inviability phenotypes is highly influenced by the parental genotypes.     

Copy number increases of transposable elements and protein‐coding genes in an invasive fish of hybrid origin

Evolutionary dynamics of structural genetic variation in lineages of hybrid origin is not well explored, although structural mutations may increase in controlled hybrid crosses. We therefore tested whether structural variants accumulate in a fish of recent hybrid origin, invasive Cottus, relative to both parental species Cottus rhenanus and Cottus perifretum. Copy‐number variation in exons of 10,979 genes was assessed using comparative genome hybridization arrays. Twelve genes showed significantly higher copy numbers in invasive Cottus compared to both parents. This coincided with increased expression for three genes related to vision, detoxification and muscle development, suggesting possible gene dosage effects. Copy number increases of putative transposons were assessed by comparative mapping of genomic DNA reads against a de novo assembly of 1,005 repetitive elements. In contrast to exons, copy number increases of repetitive elements were common (20.7%) in invasive Cottus, whereas decrease was very rare (0.01%). Among the increased repetitive elements, 53.8% occurred at higher numbers in C. perifretum compared to C. rhenanus, while only 1.4% were more abundant in C. rhenanus. This implies a biased mutational process that amplifies genetic material from one ancestor. To assess the frequency of de novo mutations through hybridization, we screened 64 laboratory‐bred F₂ offspring between the parental species for copy‐number changes at five candidate loci. We found no evidence for new structural variants, indicating that they are too rare to be detected given our sampling scheme. Instead, they must have accumulated over more generations than we observed in a controlled cross.     

Studies on phenotypic variation between Chinese hamster Kupffer cell lines : II. Correlation relationships and coordinate control of enzyme activities

This study examines somatic cell hybrids between parental cells of identical origin which exhibited quantitative differences in enzyme activities. Nine enzymes in cultured Chinese hamster Kupffer cell hybrids were studied. The parental Kupffer cell clones of identical genetic origin differed several-fold in the specific activities of catalase, arginase, microsomal heme oxygenase, peroxidase, β-glucuronidase, alcohol dehydrogenase, lactate dehydrogenase, isocitrate dehydrogenase and glucose-6-phosphate dehydrogenase. A selective system based on fusion of ouabain and vinblastine resistant clones of the parental cells was used to isolate hybrids. In hybrid clones, the specific activities of catalase, microsomal heme oxygenase, peroxidase and the dehydrogenases were expressed at the level characteristic of the parental clone with high activities of these enzymes. This result implied interaction between the parental genomes and suggested mechanisms regulating the quantitative levels of several enzyme activities. In contrast, the specific activities of arginase and β-glucuronidase in hybrid clones were intermediate between those possessed by the parental cells and indicated that for each parental genome in the hybrid there was autonomous regulation of the levels of these two enzyme activities.     

Plant genetic differences influence herbivore community structure: evidence from a hybrid willow system

To determine the influence of plant genetic variation on community structure of insect herbivores, we examined the abundances of 14 herbivore species among six genetic classes of willow: Salix eriocephala, S. sericea, their F₁ and F₂ interspecific hybrids, and backcross hybrids to each parental species. We placed 1-year-old plants, grown from seeds generated from controlled crosses, in a common garden. During the growing season, we censused gall-inducing flies and sawflies, leaf-mining insects, and leaf-folding Lepidoptera to determine the community structure of herbivorous insects on the six genetic classes. Our results provided convincing evidence that the community structure of insect herbivores in this hybrid willow system was shaped by genetic differences among the parental species and the hybrid genetic classes. Using MANOVA, we detected significant differences among genetic classes for both absolute and relative abundance of herbivores. Using canonical discriminant analysis, we found that centroid locations describing community structure of the insect herbivores differed for each genetic class. Moreover, the centroids for the four hybrid classes were located well outside of the range between the centroids for the parental species, suggesting that more than additive genetic effects of the two parental species influenced community formation on hybrid classes. Line-cross analysis suggested that plant genetic factors responsible for structuring the herbivore community involved epistatic effects, as well as additive and dominance effects. We discuss the ramifications of these results in regard to the structure of insect herbivore communities on plants and the implications of our findings for the evolution of interspecific interactions.     

The effects of locus number,genetic divergence,and genotyping error on the utility of dominant markers for hybrid identification

In surveys of hybrid zones, dominant genetic markers are often used to identify individuals of hybrid origin and assign these individuals to one of several potential hybrid classes. Quantitative analyses that address the statistical power of dominant markers in such inference are scarce. In this study, dominant genotype data were simulated to evaluate the effects of, first, the number of loci analyzed, second, the magnitude of differentiation between the markers scored in the groups that are hybridizing, and third, the level of genotyping error associated with the data when assigning individuals to various parental and hybrid categories. The overall performance of the assignment methods was relatively modest at the lowest level of divergence examined (F_st ~ 0.4), but improved substantially at higher levels of differentiation (F_st ~ 0.67 or 0.8). The effect of genotyping error was dependent on the level of divergence between parental taxa, with larger divergences tempering the effects of genotyping error. These results highlight the importance of considering the effects of each of the variables when assigning individuals to various parental and hybrid categories, and can help guide decisions regarding the number of loci employed in future hybridization studies to achieve the power and level of resolution desired.     

PATTERNS OF HYBRIDIZATION IN THE PIRIQUETA CAROLINIANA COMPLEX IN CENTRAL FLORIDA: EVIDENCE FOR AN EXPANDING HYBRID ZONE

We have identified a broad zone of hybridization between two morphologically and ecologically distinct herbaceous perennial taxa (morphotypes) within the Piriqueta caroliniana complex, which extends more than 300 km across the central Florida peninsula. Phylogeographic analyses indicate that the caroliniana morphotype has been present in north and central Florida since the early Pleistocene and that the viridis morphotype has immigrated into southern Florida much more recently. We examine the distribution of diagnostic morphological characters and nuclear genetic markers to assess the extent and patterns of introgression in this system. The results are consistent with the hypothesis that this hybrid zone has expanded north in recent history as viridis alleles have introgressed into regions that were previously occupied by populations of caroliniana. Genetic markers diagnostic for caroliniana have consistently high frequencies across the hybrid zone, whereas markers for viridis are extremely variable among populations with frequency reversals in adjacent populations. The latter pattern is probably the result of the combined stochastic effects of dispersal and drift on viridis alleles as they introgressed northward. Additional evidence for the recent expansion of this hybrid zone comes from patterns of variation for morphological and genetic markers. As expected for an expanding hybrid zone, within-population morphological variation was greatest toward the advancing front of introgression and levels of genetic variation for neutral diagnostic markers were greatest in the region of initial contact and lower in areas of recent expansion. The observed patterns of variation suggest that at least some hybrid genotypes have high fitnesses, which has led to the expansion of the hybrid zone via the displacement of parental genotypes in central Florida.     

Genetic variation in monoploids of diploid potatoes and detection of clone-specific random amplified polymorphic DNA markers

Randomly amplified polymorphic DNA (RAPD) markers have been used to study the genetic variation among androgenetic monoploids of diploid Solanum species. Cluster analysis of pairwise genetic distances was used to construct a genetic relationship among anther donor and anther-derived potato plants. The clustering based on Rogers' distances resembled classifications based on parental origins and hybrid combinations. Six of the 32 RAPD primers used resulted in the selective amplification of DNA fragments which were polymorphic between the two S. phureja parental clones, 1.22 and A95. It should be possible to construct a genetic linkage map, without making crosses, using monoploids derived from a single heterozygous diploid clone and RAPD markers.     

Weak premating isolation between Clitarchus stick insect species despite divergent male and female genital morphology

Documenting natural hybrid systems builds our understanding of mate choice, reproductive isolation and speciation. The stick insect species Clitarchus hookeri and C. tepaki differ in their genital morphology and hybridize along a narrow peninsula in northern New Zealand. We utilize three lines of evidence to understand the role of premating isolation and species boundaries: (a) genetic differentiation using microsatellites and mitochondrial DNA; (b) variation in 3D surface topology of male claspers and 2D morphometrics of female opercular organs; and (c) behavioural reproductive isolation among parental and hybrid populations through mating crosses. The genetic data show introgression between the parental species and formation of a genetically variable hybrid swarm. Similarly, the male and female morphometric data show genital divergence between the parental species as well as increased variation within the hybrid populations. This genital divergence has not resulted in reproductive isolation between species, instead weak perimating isolation has enabled the formation of a hybrid swarm. Behavioural analysis demonstrates that the entire mating process influences the degree of reproductive isolation between species undergoing secondary contact. Mechanical isolation may appear strong, whereas perimating isolation is weak.     

Segregation of male‐sterility alleles across a species boundary

Hybrid zones may serve as bridges permitting gene flow between species, including alleles influencing the evolution of breeding systems. Using greenhouse crosses, we assessed the likelihood that a hybrid zone could serve as a conduit for transfer of nuclear male‐sterility alleles between a gynodioecious species and a hermaphroditic species with very rare females in some populations. Segregation patterns in progeny of crosses between rare females of hermaphroditic Schiedea menziesii and hermaphroditic plants of gynodioecious Schiedea salicaria heterozygous at the male‐sterility locus, and between female S. salicaria and hermaphroditic plants from the hybrid zone, were used to determine whether male‐sterility was controlled at the same locus in the parental species and the hybrid zone. Segregations of females and hermaphrodites in approximately equal ratios from many of the crosses indicate that the same nuclear male‐sterility allele occurs in the parent species and the hybrid zone. These rare male‐sterility alleles in S. menziesii may result from gene flow from S. salicaria through the hybrid zone, presumably facilitated by wind pollination in S. salicaria. Alternatively, rare male‐sterility alleles might result from a reversal from gynodioecy to hermaphroditism in S. menziesii, or possibly de novo evolution of male sterility. Phylogenetic analysis indicates that some species of Schiedea have probably evolved separate sexes independently, but not in the lineage containing S. salicaria and S. menziesii. High levels of selfing and expression of strong inbreeding depression in S. menziesii, which together should favour females in populations, argue against a reversal from gynodioecy to hermaphroditism in S. menziesii.     

The Rutilus alburnoides complex (Cyprinidae): evidence for a hybrid origin

A genetic survey of 18 presumptive enzyme loci was conducted on members of the diploid-triploid R. alburnoides complex in order to test the hypothesis of hybrid origin. Most specimens examined were heterozygous at a high proportion of loci, with seven loci showing virtually fixed heterozygosity in some or all populations. This observation strongly supports the hypothesized hybrid ancestry. Involvement in the origin of the R. alburnoides complex was examined by comparison of allels with those observed in the other diploid cyprinids that inhabit the same Portuguese drainages. Allelic composition at the homozygous loci IDDH* and LDH-B* seems to rule out the genus Chondrostoma and other members of the genus Rutilus as one of the ancestors, and supports Leuciscus sp. as one parental species involved in the putative hybrid origin of the R. alburnoides complex. Moreover, with few exceptions, all specimens exhibited at every locus one or two alleles also present in extant populations of Leuciscus. The indentity of the other parental taxon remains unclear. Five diploid males exhibited multilocus genotypes that fit to the hypothetical genotype(s) of the ‘missing’ ancestor. However, the possibility exists that a hybrid female could produce or re-create a genotype of a parental species.     

Epigenetic patterns newly established after interspecific hybridization in natural populations of Solanum

Interspecific hybridization is known for triggering genetic and epigenetic changes, such as modifications on DNA methylation patterns and impact on phenotypic plasticity and ecological adaptation. Wild potatoes (Solanum, section Petota) are adapted to multiple habitats along the Andes, and natural hybridizations have proven to be a common feature among species of this group. Solanum × rechei, a recently formed hybrid that grows sympatrically with the parental species S. kurtzianum and S. microdontum, represents an ideal model for studying the ecologically and evolutionary importance of hybridization in generating of epigenetic variability. Genetic and epigenetic variability and their correlation with morphological variation were investigated in wild and ex situ conserved populations of these three wild potato species using amplified fragment length polymorphism (AFLP) and methylation‐sensitive amplified polymorphism (MSAP) techniques. We observed that novel methylation patterns doubled the number of novel genetic patterns in the hybrid and that the morphological variability measured on 30 characters had a higher correlation with the epigenetic than with the genetic variability. Statistical comparison of methylation levels suggested that the interspecific hybridization induces genome demethylation in the hybrids. A Bayesian analysis of the genetic data reveled the hybrid nature of S. × rechei, with genotypes displaying high levels of admixture with the parental species, while the epigenetic information assigned S. × rechei to its own cluster with low admixture. These findings suggested that after the hybridization event, a novel epigenetic pattern was rapidly established, which might influence the phenotypic plasticity and adaptation of the hybrid to new environments.     

On Hybrid Formation in the Rock Fern Asplenium x alternifolium (Aspleniaceae,Pteridophyta)

Length polymorphism in a non-coding spacer (trnL_UAA-trnF_GAA) in the chloroplast DNA was used in the investigation of the origin of the most common and conspicuous European fern hybrid, Asplenium x alternifolium (Aspleniaceae, Pteridophyta). The origins of A. x alternifolium, the hybrid between A. trichomanes s.l. and A. septentrionale s.l. was studied at three ploidy levels, diploid, triploid and tetraploid. The cpDNA technique allowed us to investigate the mode of hybrid formation between sexual species for the first time over a wide geographic range and with a large sample size. Morphological variation in this hybrid has previously been attributed to different reciprocal parental combinations, and to the influence of chloroplast genes on morphogenesis. Our results demonstrate that one parent, A. septentrionale s.l., acts predominantly as the female parent in these hybrids, with only one population of A. x alternifolium showing reciprocal hybridity. The discovery of predominantly unidirectional hybrid formation in this hybrid may be explained by the different breeding systems of the parental taxa. The role of gametophyte ecology is also assessed.     

Hybrid speciation in sparrows I: phenotypic intermediacy,genetic admixture and barriers to gene flow

Homoploid hybrid speciation is thought to require unusual circumstances to yield reproductive isolation from the parental species, and few examples are known from nature. Here, we present genetic evidence for this mode of speciation in birds. Using Bayesian assignment analyses of 751 individuals genotyped for 14 unlinked, nuclear microsatellite loci, we show that the phenotypically intermediate Italian sparrow (Passer italiae) does not form a cluster of its own, but instead exhibits clear admixture (over its entire breeding range) between its putative parental species, the house sparrow (P. domesticus) and the Spanish sparrow (P. hispaniolensis). Further, the Italian sparrow possesses mitochondrial (mt) DNA haplotypes identical to both putative parental species (although mostly of house sparrow type), indicating a recent hybrid origin. Today, the Italian sparrow has a largely allopatric distribution on the Italian peninsula and some Mediterranean islands separated from its suggested parental species by the Alps and the Mediterranean Sea, but co‐occurs with the Spanish sparrow on the Gargano peninsula in southeast Italy. No evidence of interbreeding was found in this sympatric population. However, the Italian sparrow hybridizes with the house sparrow in a sparsely populated contact zone in the Alps. Yet, the contact zone is characterized by steep clines in species‐specific male plumage traits, suggesting that partial reproductive isolation may also have developed between these two taxa. Thus, geographic and reproductive barriers restrict gene flow into the nascent hybrid species. We propose that an origin of hybrid species where the hybrid lineage gets geographically isolated from its parental species, as seems to have happened in this system, might be more common in nature than previously assumed.