Hybridization and introgression between the exotic Siberian elm, Ulmus pumila, and the native Field elm, U. minor, in Italy

In response to the first Dutch elm disease (DED) pandemic, Siberian elm, Ulmus pumila, was planted to replace the native elm, U. minor, in Italy. The potential for hybridization between these two species is high and repeated hybridization could result in the genetic swamping of the native species and facilitate the evolution of invasiveness in the introduced species. We used genetic markers to examine the extent of hybridization between these two species and to determine the pattern of introgression. We quantified and compared the level of genetic diversity between the hybrids and the two parental species. Hybrids between U. pumila and U. minor were common. The pattern of introgression was not as strongly biased towards U. pumila as was previously observed for hybrids between U. rubra and U. pumila in the United States. The levels of heterozygosity were similar between U. minor and the hybrids and both groups had higher levels of heterozygosity relative to U. pumila. The programs Structure and NewHybrids indicated the presence of first- (F₁) and second- generation (F₂) hybrids and of backcrosses in the hybrid population. The presence of healthy DED resistant U. minor individuals combined with the self-compatibility of U. minor could help explain the presence of F₂ individuals in Italy. The presence of F₂ individuals, where most of the variability present in the hybrids will be released, could facilitate rapid evolution and the potential evolution of invasiveness of U. pumila in Italy.     

Detecting hybridization in mixed populations ofRhinanthus minor andRhinanthus angustifolius

Rhinanthus minor andRhinanthus angustifolius are known to hybridize in mixed populations in nature. These hybridization events can have important evolutionary consequences. The development and use of species-specific RAPD and ISSR markers allowed the detection of hybrid individuals not always distinguishable with morphological characters. Two mixed populations of different ages were studied. In a young mixed 2-year-old population, both individuals of the two parental species and F₁ hybrids were found using genetic analysis, showing that hybridization occurred rapidly. Flower morphology of F₁ hybrids was too variable to distinguish all these hybrids from the parental species. This morphological variability of F₁ hybrids was also confirmed in artificial crosses in the greenhouse. In an old and no longer mixed 30-year-old population, onlyR. angustifolius plants and a few genetically introgressed individuals close toR. angustifolius were present. Genetic markers showed traces of past hybridization and introgression. Unidirectional introgression ofR. minor intoR. angustifolius with the complete disappearance ofR. minor from this population was observed.     

Morphological Variation in Wild Marmosets (Callithrix penicillata and C. geoffroyi) and Their Hybrids

Evolutionary theory and observation predict wider phenotypic variation in hybrids than parental species. Emergent phenotypic novelty in hybrids may in turn drive new adaptations or speciation by breaking parental phenotypic constraints. Primate hybridization is often documented through genetic evidence, but knowledge about the primate hybrid phenotype remains limited due to a small number of available studies on hybrid primate morphology. Here, we examine pelage and morphometric variation in two Brazilian marmoset species (Callithrix penicillata and C. geoffroyi) and their hybrids. Hybrids were sampled in an anthropogenic hybrid zone in the municipality of Viçosa, Minas Gerais state, Brazil. We analyzed hybrid facial and body pelage color variation, and compared 13 morphometric measures between hybrids and parental species. Five different hybrid facial morphotypes were observed, varying from intermediate to parental-like. Hybrid facial morphotypes were biased towards C. penicillata, suggesting that the pelage of this species may be dominant to that of C. geoffroyi in this context, and indicating that mate preference, and therefore gene flow/introgression, may be biased towards C. penicillata within the hybrid zone. Hybrid morphometric features were on average intermediate to parental species traits, but transgressive hybrids were also observed, suggesting that morphometric variation for the studied traits is consistent with Rieseberg’s complementary allele model. Finally, we observed a decoupling of facial patterning and size/shape in hybrids, relative to parent phenotypes, suggesting that an important factor driving phenotypic novelty within the Viçosa marmoset hybrid zone might be the loosening of evolutionary constraints on phenotypic trait integration.     

Morphological and molecular evidence of natural hybridization in Shorea (Dipterocarpaceae)

Shorea (Dipterocarpaceae) is a large genus in which many closely related species often grow together in Southeast Asian lowland tropical rain forests. Many Shorea species share common pollinators, and earlier studies suggested occurrence of interspecific hybridization and introgression. Here, we show morphological and molecular evidence of hybridization between Shorea species. In the census of all the trees of Shorea curtisii, Shorea leprosula, and Shorea parvifolia (>30 cm dbh) within the 164-ha area of Bukit Timah Nature Reserve in Singapore, we found 21 morphologically recognizable hybrid individuals. All of the putative hybrids could be distinguished obviously from the parental species on the basis of vegetative characters. Population genetic analysis of DNA sequences of two nuclear (GapC and PgiC) and chloroplast (trnL-trnF) regions demonstrated that each of the three species had several species-specific mutations. The nuclear sequences of the putative hybrids were heterozygote at all the species-specific sites between two parental species. Hybrid between S. curtisii and S. leprosula was found most, while S. curtisii × S. parvifolia and S. leprosula × S. parvifolia hybrids were also found. Almost no shared polymorphism between populations of the parental species suggests rarity of introgression. The study indicated that natural hybridization between sympatric Shorea species should not be uncommon, but all of the hybrid individuals were F₁, and the post-F₁ hybrids were considerably rare.     

Bidirectional mitochondrial introgression between Korean cobitid fish mediated by hybridogenetic hybrids

Genomic introgression through interspecific hybridization has been observed in some species of the freshwater fish family Cobitidae. Within this family, a Cobitis hankugensis–Iksookimia longicorpa diploid–triploid hybrid species complex on the Korean peninsula is unique in displaying hybridogenesis, a unisexual reproduction mode that allows hybrids to mediate the transfer of mitochondrial DNA (but not nuclear DNA) between the two parent species. However, populations of the parental species in the wild have never been examined for the potential effect of introgression on their genomes. To address the genetic consequences of unisexual hybridization on the parental species, we examined genetic structure of the two parental species, C. hankugensis and I. longicorpa, in three independent natural habitats where they coexist with their hybrid complex using DNA sequence data of one mitochondrial gene and three nuclear genes. We found that mitochondrial introgression between the two species was extensive in all the examined localities, while there was no evidence of nuclear introgression across the species boundary. This result indicates that the hybridogenetic individuals mediate mitochondrial introgression from one species to the other, producing mito‐nuclear mosaic genomes such as C. hankugensis nuclear genomes associated with I. longicorpa mitochondrial DNA and the reverse. The direction and degree of introgression varied among the three localities, but the underlying mechanisms for this observation proved elusive. Introgression might depend on which species serves as the predominant sperm or ovum donor or the environmental conditions of the localities. The present study suggests that introgressive hybridization between pure C. hankugensis and I. longicorpa species is highly likely where the two species co‐occur with hybridogenetic individuals, but the consequence of introgression could be variable due to the history and environmental characteristics of particular populations across the parental species’ ranges.     

Genetic monitoring of two decades of hybridization between allis shad (Alosa alosa) and twaite shad (Alosa fallax)

Habitat alteration has been implicated in driving hybridization between the sympatric migratory shads Alosa alosa and Alosa fallax. Morphological and molecular evidence is consistent with hybridization across the overlapping range of these species, but the temporal extent of hybrid occurrence and genetic consequences for populations have not been explored. Using eight nuclear microsatellite loci and samples collected between 1989 and 2008 in the Solway Firth (UK), we genetically identified hybrids, studied temporal changes in their frequency, and explored changes in allele frequencies of parental populations. These molecular data confirmed the hybrid status of individuals identified using morphology (number of rakers on the outer gill arch), and enabled separation of hybrids from purebred individuals. Mitochondrial cytochrome-b sequencing revealed the presence of two haplogroups, each predominantly occurring in one species. Heterospecific haplotypes were found in 22.3 and 12.8% of A. alosa and A. fallax individuals, respectively, consistent with backcrossing and suggesting that hybrids are fertile. On average, microsatellite-identified hybrids comprised 12.7% of all samples, but when individuals with cytonuclear discordance were also considered introgressed on average 25.4% of individuals were of hybrid ancestry. Overall, allelic richness remained largely unchanged within species, but there were declines in the inbreeding coefficient (F _IS) of both species and episodes of significant temporal allelic frequency change. Hybrids sampled between 2004 and 2008 showed no evidence of lower fecundity relative to purebred individuals. Together, results suggest that hybridization between shad species in northern Europe is prevalent, and has been ongoing over at least two decades. The challenge is now to understand the extent to which observed patterns are linked to immigration from other populations, and the mechanisms that have prevented species collapse despite apparent hybrid fertility and longstanding introgression of neutral markers.     

Pollinator convergence and the nature of species' boundaries in sympatric Sardinian Ophrys (Orchidaceae)

Background and Aims

In the sexually deceptive Ophrys genus, species isolation is generally considered ethological and occurs via different, specific pollinators, but there are cases in which Ophrys species can share a common pollinator and differ in pollen placement on the body of the insect. In that condition, species are expected to be reproductively isolated through a pre-mating mechanical barrier. Here, the relative contribution of pre- vs. post-mating barriers to gene flow among two Ophrys species that share a common pollinator and can occur in sympatry is studied.

Methods

A natural hybrid zone on Sardinia between O. iricolor and O. incubacea, sharing Andrena morio as pollinator, was investigated by analysing floral traits involved in pollinator attraction as odour extracts both for non-active and active compounds and for labellum morphology. The genetic architecture of the hybrid zone was also estimated with amplified fragment length polymorphism (AFLP) markers, and pollination fitness and seed set of both parental species and their hybrids in the sympatric zone were estimated by controlled crosses.

Key Results

Although hybrids were intermediate between parental species in labellum morphology and non-active odour compounds, both parental species and hybrids produced a similar odour bouquet for active compounds. However, hybrids produced significantly lower fruit and seed set than parental species, and the genetic architecture of the hybrid zone suggests that they were mostly first-generation hybrids.

Conclusions

The two parental species hybridize in sympatry as a consequence of pollinator overlap and weak mechanical isolation, but post-zygotic barriers reduce hybrid frequency and fitness, and prevent extensive introgression. These results highlight a significant contribution of late post-mating barriers, such as chromosomal divergence, for maintaining reproductive isolation, in an orchid group for which pre-mating barriers are often considered predominant.Key words: AFLP markers, floral scent variation, hybrid zone, hybrid fitness, Ophrys iricolor, Ophrys incubacea, reproductive isolation, sexual deception     

Breakdown of species boundaries in Mandevilla: floral morphological intermediacy,novel fragrances and asymmetric pollen flow

• Phenotypic intermediacy is an indicator of putative hybrid origin and has provided the main clues to discovering hybrid plants in nature. Mandevilla pentlandiana and M. laxa (Apocynaceae) are sister species with clear differences in floral phenotype and associated pollinator guilds: diurnal Hymenoptera and nocturnal hawkmoths, respectively. The presence of individuals with intermediate phenotypes in a wild population raises questions about the roles of visual and olfactory signals (i.e. corolla morphology and floral fragrances) as barriers to interbreeding, and how the breakdown of floral isolation occurs.
• We examined phenotypic variation in a mixed Mandevilla population, analysing the chemical composition of floral fragrances, characterising floral shape through geometric morphometrics and assessing individual grouping through taxonomically relevant traits and an unsupervised learning algorithm. We quantified the visitation frequencies of floral visitors and tracked their foraging movements using pollen analogues.
• The presence of morphologically intermediate individuals and pollen analogue movement suggested extensive hybridisation between M. laxa and M. pentlandiana, along with asymmetrical rates of backcrossing between these putative hybrids and M. laxa. Floral volatiles from putative hybrid individuals showed a transgressive phenotype, with additional compounds not emitted by either parental species.
• Our results suggest the presence of a hybrid swarm between sympatric M. pentlandiana and M. laxa and indicate that initial hybridisation events between these parental species are rare, but once they occur, visits between putative hybrids and M. laxa are common and facilitate continued introgression.

     

One species or two? Multilocus analysis of nucleotide variation of Melastoma penicillatum and Melastoma sanguineum (Melastomataceae) in Hainan,China

Melastoma comprises more than 20 shrub species and is distributed in tropical Asia and Oceania. Melastoma penicillatum, the most narrowly distributed taxon of this genus in China, was recently incorporated into another morphologically similar species, M. sanguineum. Based on its distinct morphological traits, unique habitat and flowering time, we propose that M. penicillatum should be a distinct species. In this study, we sequenced three nuclear genes and four chloroplast intergenic spacers of M. sanguineum and M. penicillatum at two locations in Hainan to test this hypothesis. There was no sequence variation at all four chloroplast intergenic spacers within and between M. sanguineum and M. penicillatum, suggesting that they should have diverged recently. However, they showed strong divergence at all three nuclear genes and no shared haplotypes were observed between them. Bayesian clustering-based STRUCTURE analysis revealed that individuals of M. sanguineum and M. penicillatum were clustered into their own clades. Fst calculations showed that both taxa had marked population genetic differentiation, but population differentiation in M. penicillatum was stronger than that in M. sanguineum. Stronger population differentiation in M. penicillatum may be caused by its discontinuous high-elevation and understory habitats, which is disadvantageous for bird-mediated dispersal. Our analysis based on the isolation-with-migration (IM) model revealed bidirectional, asymmetrical gene flow in M. sanguineum and little gene flow in M. penicillatum. Taken together, our results indicate that M. penicillatum is a distinct species and should not be incorporated into M. sanguineum. The two species might have diverged recently, and lineage sorting has been incomplete. Differential adaptation to different elevations might have led to speciation in this case.     

Maintenance of strong morphological differentiation despite ongoing natural hybridization between sympatric species of Lomatia (Proteaceae)

Background and Aims

When species cohesion is maintained despite ongoing natural hybridization, many questions are raised about the evolutionary processes operating in the species complex. This study examined the extensive natural hybridization between the Australian native shrubs Lomatia myricoides and L. silaifolia (Proteaceae). These species exhibit striking differences in morphology and ecological preferences, exceeding those found in most studies of hybridization to date.

Methods

Nuclear microsatellite markers (nSSRs), genotyping methods and morphometric analyses were used to uncover patterns of hybridization and the role of gene flow in morphological differentiation between sympatric species.

Key Results

The complexity of hybridization patterns differed markedly between sites, however, signals of introgression were present at all sites. One site provided evidence of a large hybrid swarm and the likely presence of multiple hybrid generations and backcrosses, another site a handful of early generational hybrids and a third site only traces of admixture from a past hybridization event. The presence of cryptic hybrids and a pattern of morphological bimodality amongst hybrids often disguised the extent of underlying genetic admixture.

Conclusions

Distinct parental habitats and phenotypes are expected to form barriers that contribute to the rapid reversion of hybrid populations to their parental character state, due to limited opportunities for hybrid/intermediate advantage. Furthermore, strong genomic filters may facilitate continued gene flow between species without the danger of assimilation. Stochastic fire events facilitate temporal phenological isolation between species and may partly explain the bi-directional and site-specific patterns of hybridization observed. Furthermore, the findings suggest that F₁ hybrids are rare, and backcrosses may occur rapidly following these initial hybridization events.     

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

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

Introgressive hybridization as a promoter of genome reshuffling in natural homoploid fish hybrids (Cyprinidae,Leuciscinae)

Understanding the mechanisms underlying diversification and speciation by introgressive hybridization is currently one of the major challenges in evolutionary biology. Here, the analysis of hybridization between two pairs of Iberian Leuciscinae provided new data on independent hybrid zones involving Achondrostoma oligolepis (AOL) and Pseudochondrostoma duriense (PDU), and confirmed the occurrence of hybrids between AOL and Pseudochondrostoma polylepis (PPO). A multilevel survey combining morphological, genetic and cytogenomic markers on a vast population screening successfully sorted the selected fishes as admixed. Results were similar in both AOL × PDU and AOL × PPO systems. Overall, hybrid morphotypes, cytogenomic data and genetic profiling indicated preferential backcrossing and suggested AOL as a major genomic contributor. Moreover, results implied AOL as more permissive to introgression than PDU or PPO. Although PDU- and PPO-like individuals appeared more resilient to genome modifications, AOL appeared to be more involved and affected by the ongoing hybridization events, as chromosomal translocations were only found in AOL-like individuals. All hybrids analysed evidenced extensive ribosomal DNA (rDNA) polymorphism that was not found in parental species, but usually seen falling within the range of possible parental combinations. Yet, transgressive phenotypes that cannot be explained by normal recombination, including more rDNA clusters than expected or the occurrence of syntenic rDNAs, were also detected. Present results proved rapid genomic evolution providing the genetic novelty for species to persist. In addition, although the ultimate consequences of such apparently extensive and recurrent events remain unknown, modern genome-wide methodologies are of great promise towards answering questions concerning the causes, dynamics and impacts of hybridization.     

Genetic architecture and genomic patterns of gene flow between hybridizing species of Picea

Hybrid zones provide an opportunity to study the effects of selection and gene flow in natural settings. We employed nuclear microsatellites (single sequence repeat (SSR)) and candidate gene single-nucleotide polymorphism markers (SNPs) to characterize the genetic architecture and patterns of interspecific gene flow in the Picea glauca × P. engelmannii hybrid zone across a broad latitudinal (40–60 degrees) and elevational (350–3500 m) range in western North America. Our results revealed a wide and complex hybrid zone with broad ancestry levels and low interspecific heterozygosity, shaped by asymmetric advanced-generation introgression, and low reproductive barriers between parental species. The clinal variation based on geographic variables, lack of concordance in clines among loci and the width of the hybrid zone points towards the maintenance of species integrity through environmental selection. Congruency between geographic and genomic clines suggests that loci with narrow clines are under strong selection, favoring either one parental species (directional selection) or their hybrids (overdominance) as a result of strong associations with climatic variables such as precipitation as snow and mean annual temperature. Cline movement due to past demographic events (evidenced by allelic richness and heterozygosity shifts from the average cline center) may explain the asymmetry in introgression and predominance of P. engelmannii found in this study. These results provide insights into the genetic architecture and fine-scale patterns of admixture, and identify loci that may be involved in reproductive barriers between the species.     

Transgressive character expression in hybrid zones between the native invasives Tithonia tubaeformis and Tithonia rotundifolia (Asteraceae) in Mexico

Natural hybridization frequently promotes gene introgression among closely related species in sympatric populations, producing complex patterns of morphological variation. Therefore, a detailed understanding of the dynamics of interspecific gene flow and its morphological patterns is of widespread interest. We tested if introgressive hybridization promotes an increase in transgressive characters in comparison with the parental species. A sunflower species complex occurring in Mexico formed by two native invasive species, Tithonia tubaeformis and Tithonia rotundifolia, was analyzed using 46 morphological characters (leaf, flower and fruit) in five hybrid zones (N = 150 individuals) and two pure sites for each parental species (N = 80 individuals). In general, T. tubaeformis differed significantly from T. rotundifolia in all the examined characters, except six foliar and one inflorescence character. Morphological characters support the hypothesis of hybridization in this complex, even though both species remain morphological distinct in mixed stands. Individual hybrids appear to be a mosaic of parent-like (24.8 % of traits), intermediate (26.1 %) and transgressive (37.8 %) phenotypes (the remaining 11.3 % of the traits did not differ significantly from both parental species). Our results suggest that individuals from the same parental species were more similar among themselves than to putative hybrids, indicating occasional hybridization with segregation in hybrid types or backcrossing to parents. Evidence indicates a unidirectional pattern of gene flow toward T. rotundifolia.     

Evidence for strong assortative mating,limited gene flow,and strong differentiation across the blue‐footed/Peruvian booby hybrid zone in northern Peru

Hybrid zones represent natural laboratories in which the processes of divergence and genetic isolation can be examined. The generation and maintenance of a hybrid zone requires mispairing and successful reproduction between organisms that differ in one or more heritable traits. Understanding the dynamics of hybridization between two species requires an understanding of the extent to which they have diverged genetically, the frequency of misparing and hybrid production, and the extent of introgression. Three hundred and twenty one blue‐footed Sula nebouxii and Peruvian S. variegata boobies from the eastern tropical Pacific Ocean were analyzed using 19 putatively neutral genetic markers to evaluate interspecific differentiation, to classify morphological hybrids using Bayesian assignments, and to characterize hybridization using cline theory and Bayesian assignments. The species were well differentiated at mitochondrial and nuclear microsatellites, the hybrid zone was bimodal (contained a high frequency of each parental species but a low frequency of hybrids), and morphologically intermediate individuals were most likely F1 hybrids resulting from mating between female Peruvian boobies and male blue‐footed boobies. Clines in allele frequency could be constrained to share a common geographic centre but could not be constrained to share a common width. Peruvian and blue‐footed boobies hybridize infrequently, potentially due to strong premating reproductive isolation; however, backcrossing appears to facilitate introgression from blue‐footed to Peruvian boobies in this hybrid system.     

Genetic Evidence for Natural Hybridization between Species of Dioecious Ficus on Island Populations1

Natural hybrids between Ficus septica and two closely related dioecious species, F. fistulosa and F. hispida, were confirmed using amplified fragment length polymorphisms (AFLP) and chloroplast DNA markers. Ficus species have a highly species‐specific pollination mutualism with agaonid wasps. Therefore, the identification of cases in which breakdown in this sophisticated system occurs and the circumstances under which it happens is of interest. Various studies have confirmed that Ficus species are able to hybridize and that pollinator‐specificity breakdown can occur under certain conditions. This study is the first example in which hybrid identity and the presence of hybrids in the natural distribution of parental species for Ficus have been confirmed with molecular markers. Hybrid individuals were identified on three island locations in the Sunda Strait region of Indonesia. These findings support Janzen's (1979) hypothesis that breakdown in pollinator specificity is more likely to occur on islands. We hypothesized that hybrid events could occur when the population size of pollinator wasps was small or had been small in one of the parental species. Later generation hybrids were identified, indicating that backcrossing and introgression did occur to some extent and that therefore, hybrids could be fertile. The small number of hybrids found indicated that there was little effect of hybridization on parental species integrity over the study area. Although hybrid individuals were not common, their presence at multiple sites indicated that the hybridization events reported here were not isolated incidences. Chloroplast DNA haplotypes of hybrids were not derived solely from one species, suggesting that the seed donor was not of the same parental species in all hybridization events.     

Introgressive hybridization of human and rodent schistosome parasites in western Kenya

Hybridization and introgression can have important consequences for the evolution, ecology and epidemiology of pathogenic organisms. We examined the dynamics of hybridization between a trematode parasite of humans, Schistosoma mansoni, and its sister species, S. rodhaini, a rodent parasite, in a natural hybrid zone in western Kenya. Using microsatellite markers, rDNA and mtDNA, we showed that hybrids between the two species occur in nature, are fertile and produce viable offspring through backcrosses with S. mansoni. Averaged across collection sites, individuals of hybrid ancestry comprised 7.2% of all schistosomes collected, which is a large proportion given that one of the parental species, S. rodhaini, comprised only 9.1% of the specimens. No F1 individuals were collected and all hybrids represented backcrosses with S. mansoni that were of the first or successive generations. The direction of introgression appears highly asymmetric, causing unidirectional gene flow from the rodent parasite, S. rodhaini, to the human parasite, S. mansoni. Hybrid occurrence was seasonal and most hybrids were collected during the month of September over a 2-year period, a time when S. rodhaini was also abundant. We also examined the sex ratios and phenotypic differences between the hybrids and parental species, including the number of infective stages produced in the snail host and the time of day the infective stages emerge. No statistical differences were found in any of these characteristics, and most of the hybrids showed an emergence pattern similar to that of S. mansoni. One individual, however, showed a bimodal emergence pattern that was characteristic of both parental species. In conclusion, these species maintain their identity despite hybridization, although introgression may cause important alterations of the biology and epidemiology of schistosomiasis in this region.     

Asymmetric introgression between Magnolia stellata and M. salicifolia at a site where the two species grow sympatrically

In order to understand the ongoing evolutionary relationships between species, it is important to elucidate patterns of natural hybridization. In the zone where two species are sympatrically distributed, we examined 274 individuals of Magnolia stellata, Magnolia salicifolia, and their putative hybrids by means of 16 nuclear and three chloroplast microsatellite markers. Hybrid classes of individuals were estimated by admixture analyses. Morphological traits were also investigated for 64 of the 274 individuals. Admixture analyses revealed that 66 of the 274 individuals were classified as hybrids, comprising 17 F₁ and 19 F₂ individuals, 27 backcrosses to M. salicifolia, and 3 individuals of unknown origin. Morphological data from the 64 individuals agreed well with their genetic admixture rates. Spatial locations of F₁ and F₂ hybrids at the study site were intermediate between the two purebred species, indicating that the site preferences of hybrids are intermediate. The occurrences of F₂ and backcross hybrids indicate that F₁ hybrids are fertile. The chloroplast DNA haplotypes of all F₁ hybrids corresponded to those detected in M. salicifolia, so that maternal parents of the F₁ hybrids were all M. salicifolia. Furthermore, no hybrid individuals derived from a backcross to M. stellata were detected. These results suggest that the direction of hybridization and the subsequent introgression have been quite asymmetric and that the introgression occurred from M. stellata into M. salicifolia.     

Willow hybrids and herbivory: a test of hypotheses of phytophage response to hybrid plants using the generalist leaf-feeder Lochmaea caprea (Chrysomelidae)

To test hypotheses of herbivore response to hybrid plants, I determined the preference of the generalist beetle Lochmeae capreae for individuals of Salix caprea, Salix phylicifolia and F1 hybrids between these species. F1 hybrids and pure individuals were created by hand-pollination of willow individuals in the field. The preference tests were performed in 1994 and 1995 and in both years there was a tendency for higher leaf area consumption from hybrids than from the parental species. However, statistically significant differences in consumption were only found in 1995 between hybrids and S. phylicifolia. The result from this study therefore lends most support to the dominance hypothesis: hybrid susceptibility was similar to that of one of the parental species. This is one of the first strict preference tests showing that a generalist herbivore can detect differences in palatability between hybrids and parental species. Furthermore, the experimental design used in the study minimized environmental variation. The results should therefore primarily reflect genetically based quality differences between hybrids and parental species. Received: 4 March 1996 / Accepted: 12 August 1996     

Modeling of introgression between Elymus caninus and E. fibrosus (Poaceae) and their registration using one-dimensional SDS electrophoresis

Artificial sexual hybrids between Elymus caninus (L.) L. and E. fibrosus (Schrenk) Tzvel. were experimentally examined in generations F₁–F₅. The possibility of genetic introgression between these species was shown. Morphologically, the hybrid plants can be assigned to either of the parental species or to variety E. caninus var. muticus (Holmb.) Karlsson. Some traits (spike density, leaf blade width, leaf blade pubescence, awns of lemmas) exhibited considerable variation. Polypeptide spectra of endosperm proteins were characterized in the initial parental biotypes and the hybrid progeny, using a gel-buffer SDS electrophoresis system. It was suggested that successful interspecies hybridization requires backcrosses or normalizing crosses. The possibility of sexual genetic exchange enables to utilize the gene pools of the two species to transfer the required traits in selection forage forms.