A Natural Homoploid Hybrid between Centaurea horrida and Centaurea filiformis (Asteraceae) as Revealed by Morphological and Genetic Traits

Studies over the last two decades demonstrate that hybridization has played an integral role in the evolution of several sections of the genus Centaurea. Nevertheless, natural hybridization between narrow Mediterranean endemic Centaurea species has not been documented as yet. A population of fertile Centaurea individuals exhibiting intermediate morphological traits between two Sardinian narrow endemics, C. horrida and C. filiformis, was identified at the Tavolara Islet (Sardinia, Italy). Intermediate leaf length and head width characterized this population, suggesting its hybrid origin. The putative hybrid population was structured (i.e., composed of seedlings, saplings and adult individuals) and had a relatively high levels of seed production. The number of chromosomes was identical to that of the proposed progenitors (2n?=?18). Genotyping at five microsatellite loci showed that the putative hybrid possessed several alleles in common with the proposed parental species and intermediate values of genetic differentiation, as indicated by both F _ST and R _ST, between C. horrida and C. filiformis. We therefore conclude that the studied intermediate population is of hybrid origin, and discuss possible mechanisms of its reproductive isolation from the parental species, potential re-introgression, and evolutionary implications of this hybridization.     

Reevaluation of the biological status of Juniperus Deppeana var. sperryi Correll

Foliage and bark samples were collected from the tree that provided the type specimen forJuniperus deppeana var.sperryi Correll, as well as from trees from populations ofJ. pinchotii Sudw.,J. flacida Schl., andJ. deppeana Steud. var.deppeana. These four taxa were compared using terpenoid and morphological characters. The terpenoid data suggest thatJ. deppeana var.sperryi is most closely related toJ. deppeana var.deppeana; no evidence of relict or present hybridization withJ. flaccida was detected. The morphological data showedJ. deppeana var.sperryi to be intermediate in several characters betweenJ. deppeana var.deppeana andJ. flacdda. The probability of a hybrid origin for this taxon is discussed. Due to the scattered occurrence of trees referable toJ. deppeana var.sperryi, it is proposed that this taxon be reduced in rank toJ. deppeana formasperryi.     

Natural intersectional hybridization in perityle (Compositae)

A small number of supposed F₁ hybrids was discovered in an isolated sympatric population ofP. parryi (n = 17) andP. rupestris var.albiflora (n = 17) in the Chisos Mts. of Texas. Although the parental species are morphologically quite distinct, being assigned to separate taxonomic sections ofPerityle, their hybrids displayed relatively high fertility. Origin of the presumed natural hybrids was documented by the production of artificial hybrids betweenP. parryi andP. rupestris var.albiflora. Fertility data based on pollen stainability were accumulated for the available natural hybrids, and for artificial hybrids through F₃ and backcross generations. A brief discussion of biosystematic implications is included.     

Hybridization between Triodanis perfoliata and Triodanis biflora (Campanulaceae)

Experimental hybridizations betweenTriodanis biflora (Ruiz & Pavon) Greene andT. perfoliata (L.) Nieuwland result in vigorous offspring in the F₁ and F₂ generations as well as in backcrosses. Field studies of natural populations indicate a gradient between the two taxa with many individuals resembling the experimental hybrids. A partial isolating mechanism due to cleistogamy inT. biflora enables plants of that morph to be easily identified.Triodanis biflora is reduced toT. perfoliata var.biflora (Ruiz & Pavon) Bradley, comb. nov.     

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.     

Seed bank dynamics govern persistence of Brassica hybrids in crop and natural habitats

Background and Aims Gene flow from crops to their wild relatives has the potential to alter population growth rates and demography of hybrid populations, especially when a new crop has been genetically modified (GM). This study introduces a comprehensive approach to assess this potential for altered population fitness, and uses a combination of demographic data in two habitat types and mathematical (matrix) models that include crop rotations and outcrossing between parental species.Methods Full life-cycle demographic rates, including seed bank survival, of non-GM Brassica rapa × B. napus F₁ hybrids and their parent species were estimated from experiments in both agricultural and semi-natural habitats. Altered fitness potential was modelled using periodic matrices including crop rotations and outcrossing between parent species.Key Results The demographic vital rates (i.e. for major stage transitions) of the hybrid population were intermediate between or lower than both parental species. The population growth rate (λ) of hybrids indicated decreases in both habitat types, and in a semi-natural habitat hybrids became extinct at two sites. Elasticity analyses indicated that seed bank survival was the greatest contributor to λ. In agricultural habitats, hybrid populations were projected to decline, but with persistence times up to 20 years. The seed bank survival rate was the main driver determining persistence. It was found that λ of the hybrids was largely determined by parental seed bank survival and subsequent replenishment of the hybrid population through outcrossing of B. rapa with B. napus.Conclusions Hybrid persistence was found to be highly dependent on the seed bank, suggesting that targeting hybrid seed survival could be an important management option in controlling hybrid persistence. For local risk mitigation, an increased focus on the wild parent is suggested. Management actions, such as control of B. rapa, could indirectly reduce hybrid populations by blocking hybrid replenishment.     

A numerical study of the common narrow-leaved taxa of chenopodium occurring in the Western United States

A numerical taxonomic study was conducted on the common narrow-leaved members of the genusChenopodium that occur in the western United States. As many as six and as few as two species have been recognized. Thirty-five individual popu lations were used as OTU’s, and 35 characters were considered. The data were subjected to cluster (both the weighted-pair group method and the within-group dispersion method) and principal component analysis. The results of both analyses are in general agreement and suggest phenetic relationships that differ from current interpretations in the literature. It is concluded thatC. desiccatum var.leptophylloides is much more closely related phenetically toC. atrovirens (and may not be distinct from it) than it is toC. desiccatum var.desiccatum. Chenopodium hians andC. leptophyllum appear to be phenetically distinct despite the fact that they have sometimes been viewed as conspecific.Cheno podium subglabrum is not very similar toC. leptophyllum, and should probably be treated as a separate species rather than as a variety of the latter. Popu lations referable toC. incognitum form a rather loose cluster that appears some what intermediate between populations ofC. desiccatum var.leptophylloides,C. atrovirens, C. hians, andC. leptophyllum.     

Novelties in Hedeoma (Labiatae)

Hedeoma pilosum andH. microphyllum are described as new. The former, an endemic of western Texas, is closely related toH. pusillum andH. apiculatum; the latter is intermediate betweenH. costatum andH. nanum. H. oblongifolium var.mexicanum andH. drummondii var.crenulatum are presented as two new varieties, both of which may have originated as hybrids.Hedeoma crenulatum is proposed as a new name to replaceH. montanum (Brade) Epling & Játiva. Three new combinations are made in the genusHesperozygis, and new status is accorded to two species.     

An analysis of hybridization in Liatris

Liatris aspera andL. spicata are remotely related species, and rarely have an opportunity to hybridize because of different flowering periods and ecological tolerances. Seasonal and ecological barriers have been surmounted in a population in southern Cook County, Illinois, which has resulted in extensive hybridization. The population was characterized on the basis of morphological and chemical features, pollen fertility, and chromosome pairing relationships. Plants considered to be F₁ hybrids on the basis of morphology typically displayed the phenolic markers of both parental species, reduced fertility, and meiotic irregularities in the form of univalents and translocation figures. Plants judged to be backcrosses toL. aspera contained some of the phenolics ofL. spicata; however, backcrosses toL. spicata invariably lacked the phenolics ofL. aspera. The species-species phenolics ofL. aspera were inherited as a block, i.e., a hybrid had all or none of these compounds. The complex pattern of gene exchange within the population reflects the weakness of crossing and fertility barriers to hybridization.     

Hybridization between andropogon glomeratus var. pumilus and A. longiberbis (gramineae) in central Florida

In a mixed population ofAndropogon glomeratus var.pumilus andA. longiberbis in Lake County, Florida, there were six individuals morphologically intermediate between and less fertile than these two taxa. In a discriminant analysis using two characters, spikelet length and raceme sheath width, the putative hybrid plants are significantly different from both parents (P<0.01). Reproductive isolation of these taxa in other mixed populations comes, at least in part, from a combination of nonsynchronous flowering and cleistogamy.     

Proteomic analysis of F1 hybrids and intermediate variants in a Littorina saxatilis hybrid zone

Proteomic analysis was carried out on the Crab (upper-shore) and Wave (lower-shore) ecotypes of Littorina saxatilis from a hybrid zone at Silleiro Cape, Spain. Proteome profiles of individual snails were obtained. Protein expression in F₁ hybrid snails bred in the laboratory and snails with intermediate shell phenotypes collected from the mid-shore were compared with Crab and Wave ecotypes using analytical approaches used to study dominance. Multivariate analysis over many protein spots showed that the F₁ snails are distinct from both ecotypes but closer to the Wave ecotype. The intermediate snails are highly variable, some closer to the Crab and others to the Wave ecotype. Considered on a protein by protein basis, some proteins are significantly closer in expression to the Crab and others to the Wave ecotype for both F₁ and intermediate snails. Furthermore, a significant majority of proteins were closer in expression to the Wave ecotype for the F₁, consistent with the multivariate analysis. No such significant majority toward either the Crab or Wave ecotype was observed for the intermediate snails. The closer similarity of F₁ and Wave ecotype expression patterns could be the result of similar selective pressures in the similar mid-shore and low-shore environments. For a significantly larger number of proteins, intermediate snails were closer in expression to the ecotype having the lower expression, for both Crab and Wave ecotypes. This is somewhat unexpected as lower expression might be expected to be an indication of impairment of function and lower fitness. Proteomic analysis could be important for the identification of candidate proteins useful for gaining improved understanding of adaptation and barriers to gene flow in hybrid zones.     

A morphological and chemical study of Populus acuminata Rydberg

Evidence from morphology, flavonoid chemistry, and field observations suggests thatPopulus acuminata is of hybrid origin. The putative parents areP. angustifolia, the narrow leaf cottonwood, and deltoid leaved plants that are assigned toP. sargentii (P. deltoides var.occidentalis), P. fremontii, orP. wislizenii (P. fremontii var.wislizenii). Populus angustifolia exhibits a series of flavonol glycosides (kaempferol, quercetin, and myricetin) in its leaves. By contrast, the major leaf flavonoids of the broad leaved plants are flavone glycosides (apigenin and luteolin).Populus acuminata is intermediate between the suspected parents in morphological features. Additionally, the leaves of mostP. acuminata plants contain the exact summation of the flavonoid compounds characteristic of the putative parents. A diploid chromosome number of 2n = 38 was obtained for six plants, which confirms the one previous report for the species. Meiosis was regular in all cases. Correlated data indicate that the majority of plants ofP. acuminata represent F₁ hybrids and that complex hybridization is not common. Evidence from morphological and chemical studies is presented to show that in certain instances backcrossing to both parents has occurred. Results gathered in this study show thatP. ×andrewsii is undoubtedly “typical”P. acuminata, but the type specimen is from a sucker shoot, and thus has been interpreted as a backcross toP. sargentii. Populus acuminata var.rehderi is not considered worthy of taxonomic recognition.     

The role of compensatory neutral mutations in molecular evolution

A pair of mutations at different loci (or sites) which are singly deleterious but restore normal fitness in combination may be called compensatory neutral mutations. Population dynamics concerning evolutionary substitutions of such mutants was developed by making use of the diffusion equation method. Based on this theory and, also, by the help of Monte Carlo simulation experiments, a remarkable phenomenon was disclosed that the double mutants can easily become fixed in the population by random drift under continued mutation pressure if the loci arc tightly linked, even when the single mutants are definitely deleterious. More specifically, I consider two loci with allelesA andA’ in the first locus, and allelesB andB’in the second locus, and assign relative fitnesses 1, 1-s’, 1-s’ and 1 respectively to the four gene combinationsAB, A’B, AB’ andA’B’, wheres’ is the selection coefficient against the single mutants (s’ > 0). Letv be the mutation rate per locus per generation and assume that mutation occurs irreversibly fromA toA’ at the first locus, and fromB toB’ at the second locus, whereA andB are wild type genes, andA’ andB’ are their mutant alleles. In a diploid population of effective size N _e (or a haploid population of 2N _e breeding individuals), it was shown that the average time (T) until joint fixation of the double mutant (A’B’) starting from the state in which the population consists exclusively of the wild type genes (AB) is not excessively long even for large 4N _e s’ values. In fact, assuming2N _e v = 1 we have -T = 54Ne for 4Nes’ = 400, and -T = 128Ne for 4N _e s’ = 1000. These values are not unrealistically long as compared with -T~ 5N _e obtained for 4N _e s’ = 0. The approximate analytical treatment has also been extended to estimate the effect of low rate crossing over in retarding fixation. The bearing of these findings on molecular evolution is discussed with special reference to coupled substitutions at interacting amino acid (or nucleotide) sites within a folded protein (orrna) molecule. It is concluded that compensatory neutral mutants may play an important role in molecular evolution.     

Analysis of [Gossypium capitis-viridis × (G.hirsutum × G.australe)2] Trispecific Hybrid and Selected Characteristics

Speciation is always a contentious and challenging issue following with the presence of gene flow. In Gossypium, there are many valuable resources and wild diploid cotton especially C and B genome species possess some excellent traits which cultivated cotton always lacks. In order to explore character transferring rule from wild cotton to upland tetraploid cotton, the [G. capitis-viridis × (G. hirsutum × G. australe)²] triple hybrid was synthesized by interspecies hybridization and chromosome doubling. Morphology comparisons were measured among this hybrid and its parents. It showed that trispecific hybrid F₁ had some intermediate morphological characters like leaf style between its parents and some different characters from its parents, like crawl growth characteristics and two kind flower color. It is highly resistant to insects comparing with other cotton species by four year field investigation. By cytogenetic analysis, triple hybrid was further confirmed by meiosis behavior of pollen mother cells. Comparing with regular meiosis of its three parents, it was distinguished by the occurrence of polyads with various numbers of unbalanced microspores and finally generating various abnormal pollen grains. All this phenomenon results in the sterility of this hybrid. This hybrid was further identified by SSR marker from DNA molecular level. It showed that 98 selected polymorphism primers amplified effective bands in this hybrids and its parents. The genetic proportion of three parents in this hybrid is 47.8% from G. hirsutum, 14.3% from G. australe, 7.0% from G. capitis-viridis, and 30.9% recombination bands respectively. It was testified that wild genetic material has been transferred into cultivated cotton and this new germplasm can be incorporated into cotton breeding program.     

ECOLOGICAL SPECIATION IN STICKLEBACKS: ENVIRONMENT-DEPENDENT HYBRID FITNESS

“Ecological” speciation occurs when reproductive isolation evolves as a consequence of divergent selection between populations exploiting different resources or environments. We tested this hypothesis of speciation in a young stickleback species pair by measuring the direct contribution of ecological selection pressures to hybrid fitness. The two species (limnetic and benthic) are strongly differentiated morphologically and ecologically, whereas hybrids are intermediate. Fitness of hybrids is high in the laboratory, especially F₁ and F₂ hybrids (backcrosses may show some breakdown). We transplanted F₁ hybrids to enclosures in the two main habitats in the wild to test whether the distribution of resources available in the environment generates a hybrid disadvantage not detectable in the laboratory. Hybrids grew more slowly than limnetics in the open water habitat and more slowly than benthics in the littoral zone. Growth of F₁ hybrids was inferior to the average of the parent species across both habitats, albeit not significantly. The contrast between laboratory and field results supports the hypothesis that mechanisms of F₁ hybrid fitness in the wild are primarily ecological and do not result from intrinsic genetic incompatibilities. Direct selection on hybrids contributes to the maintenance of sympatric stickleback species and may have played an important role in their origin.     

Resistance toMelampsora larici-epitea leaf rust inSalix: analyses of quantitative trait loci

Quantitative resistance ofSalix toMelampsora larici-epitea leaf rust was studied in 2Salix mapping populations. One population was a backcross between aS. schwerinii ×S. viminalis hybrid andS. viminalis, and the other was an F₂ population betweenS. viminalis andS. dasyclados. A leaf disc bioassay was used to study the components of quantitative resistance (latent period, uredinia number, and uredinia size) to 3 isolates of the leaf rust. The analysis of quantitative trait loci (QTLs) revealed 9 genomic regions in the backcross population and 7 genomic regions in the F₂ population that were important for rust resistance, with QTLs explaining 8–26% of the phenotypic variation. An important genomic region was identified for the backcross population in linkage group 2, where QTLs were identified for all resistance components for 2 of the rust isolates. Four of the QTLs had overlapping mapping intervals, demonstrating a common genetic background for latent period, uredinia diameter, and uredinia number. QTLs specific to some rust isolates and to some resistance components were also found, indicating a combination of common and specific mechanisms involved in the various resistance components. Breeding implications in relation to these findings are discussed.     

CO(2) Exchange, Cytogenetics, and Leaf Anatomy of Hybrids between Photosynthetically Distinct Flaveria Species

Hybrids between the C₄-like species, Flaveria brownii, A. M. Powell and the C₃-C₄ intermediate species Flaveria linearis Lag., Flaveria floridana Johnston, and Flaveria oppositifolia (DC.) Rydb. exhibited bivalent chromosome pairing during meiosis and stainability of pollen was high, ranging from 51 to 95%. An F₂ population produced from an F. brownii × F. linearis F₁ hybrid, exhibited bivalent chromosome pairing and high pollen stainability indicating a high degree of fertility in the hybrid. Oxygen inhibition of apparent photosynthesis averaged 6.8% for F. brownii and 22.2% for the C₃-C₄ species (in two experiments), and F₁ hybrids exhibited inhibitions which were intermediate to their parents. Values of carbon dioxide compensation concentration determined at low irradiance were 4.0, 34.0, and 6.5 microliters per liter for F. brownii, F. linearis and their F₁ hybrid, respectively. The mean value at low irradiance for 33 F₁ plants was 6.8 microliters per liter, and individual values ranged only from 3.7 to 11.7 microliters per liter. Anatomical characteristics for the F₁ hybrid leaves were intermediate to those of the parents, and there was considerable variation among F₂ plants derived from F. brownii × F. linearis. In the F₂ population δ¹³C values ranged from −27‰ to −20‰. The expression of more C₄-like characteristics by the F₁ hybrids in this study and their apparent high fertility make them promising specimens for producing segregating populations for use in C₄ inheritance studies.     

Morphological,chemical, and cytogenetic characters of F1 hybrids betweenParthenium argentatum (Guayule) andP. fruticosum var.fruticosum (Asteraceae) and their potential in rubber improvement

Interspecific hybrids betweenParthenium argentatum, the guayule rubber plant, andP. fruticosum var. fruticosum were evaluated for their potential rubber content and quality. Fifteen-mo-old field-grown plants ofP. fruticosum var.fruticosum measured four times more in height and spread than those ofP. argentatum, but contained less than 0.05% rubber of low mol wt.Parthenium argentatum showed 2% rubber content, with a mol wt of about one million. Resin contents varied little among parents or hybrids. The same age F₁ hybrids were intermediate in height and spread and had low rubber content, but showed presence of high mol wt rubber like the guayule parent. This indicates that high mol wt rubber is expressed over the low mol wt rubber in F₁ hybrids. Despite low rubber content but favorable biomass production, F₁ hybrids revealed irregular meiotic chromosome behavior and low pollen and seed germination. These results suggest that interspecific F₁ hybrids may be used in backcross programs to increase biomass and rubber content in guayule.     

Stable Carbon Isotope Composition (deltaC), Water Use Efficiency, and Biomass Productivity of Lycopersicon esculentum, Lycopersicon pennellii, and the F(1) Hybrid

Three tomatoes, Lycopersicon esculentum Mill. cv UC82B, a droughttolerant wild related species, Lycopersicon pennellii (Cor.) D'Arcy, and their F₁ hybrid, were grown in containers maintained at three levels of soil moisture. Season-long water use was obtained by summing over the season daily weight losses of each container corrected for soil evaporation. Plant biomass was determined by harvesting and weighing entire dried plants. Season-long water use efficiency (gram dry weight/kilogram H₂O) was calculated by dividing the dry biomass by the season-long water use. The season-long water use efficiency was greatest in the wild parent, poorest in the domestic parent, and intermediate (but closer to the wild parent) in the F₁ hybrid. Instantaneous water-use efficiency (micromole CO₂/millimole H₂O) determined by gas exchange measurements on individual leaves was poorly correlated with season-long water use efficiency. However, the relative abundance of stable carbon isotopes of leaf tissue samples was strongly correlated with the season-long water use efficiency. Also, the isotopic composition and the season-long water use efficiency of each genotype alone were strongly negatively correlated with plant dry weight when the dry weight varied as a function of soil moisture.     

Genetic analysis of hybrid seed formation ability of Brassica rapa in intergeneric crossings with Raphanus sativus

A hybridization barrier leads to the inability of seed formation after intergeneric crossings between Brassica rapa and Raphanus sativus. Most B. rapa lines cannot set intergeneric hybrid seeds because of embryo breakdown, but a B. rapa line obtained from turnip cultivar ‘Shogoin-kabu’ is able to produce a large number of hybrid seeds as a maternal parent by crossings with R. sativus. In ‘Shogoin-kabu’ crossed with R. sativus, developments of embryos and endosperms were slower than those in intraspecific crossings, but some of them grew to mature seeds without embryo breakdown. Intergeneric hybrid seeds were obtained in a ‘Shogoin-kabu’ line at a rate of 0.13 per pollinated flower, while no hybrid seeds were obtained in a line developed from Chinese cabbage cultivar ‘Chiifu’. F₁ hybrid plants between the lines of ‘Shogoin-kabu’ and ‘Chiifu’ set a larger number of hybrid seeds per flower, 0.68, than both the parental lines. Quantitative trait loci (QTLs) for hybrid seed formation were analyzed after intergeneric crossings using two different F₂ populations derived from the F₁ hybrids, and three QTLs with significant logarithm of odds scores were detected. Among them, two QTLs, i.e., one in linkage group A10 and the other in linkage group A01, were detected in both the F₂ populations. These two QTLs had contrary effects on the number of hybrid seeds. Epistatic interaction between these two QTLs was revealed. Possible candidate genes controlling hybrid seed formation ability in QTL regions were inferred using the published B. rapa genome sequences.