Natural Interspecific Hybrids Research Articles

Occurrence of unreduced pollen in diploid blueberry species, Vaccinium sect. Cyanococcus

A total of 1475 individuals belonging to 43 natural populations of seven diploid (2x) blueberry species (Vaccinium section Cyanococcus) and two natural interspecific 2x hybrid populations were evaluated for unreduced pollen production. Significant differences were found in the frequency of unreduced pollen producers between species and within and between populations of the same species. Individuals with 1% or more unreduced pollen were considered unreduced pollen producers. The average frequency of unreduced pollen producers in these diploid species was 13.5%, ranging from 7.4% (V. corymbosum) to 18.4% (V. darrowii). The frequency of unreduced pollen grains in individual clones varied from <-1% to 28.6%. The production of unreduced pollen was not associated with male fertility. The widespread occurrence of unreduced pollen in the diploid species should allow the introgression of this germ plasm to the tetraploid level via unilateral sexual polyploidization.

Utilization of wild sunflower species for the improvement of cultivated sunflower

Abstract The genus Helianthus is composed of 49 species and 19 subspecies with 12 annual and 37 perennial species. These diverse species represent considerable genetic variability which can be utilized for the improvement of cultivated sunflower. The taxonomy of Helianthus is somewhat confusing due to the complicated natural interspecific hybridization and different ploidy levels of several species. A germplasm collection of 2000 accessions, mostly annuals, has been assembled. Interspecific hybridization has become important as a means of introducing genetic variability into the cultivated sunflower. This has been facilitated by the use of embryo culture and chromosome doubling with colchicine to increase fertility. The wild species continue to serve as a source of cytoplasmic male sterility for cultivated sunflower. Molecular techniques of restriction endonucleases of mitochondrial DNA can be used to differentiate CMS sources. The greatest impact the wild species have made on cultivated sunflower has been in the area of genes for disease resistance and, to a lesser extent, resistance to insects. The recent discovery of genes for high tolerance to the disease Phomopsis/Diaporthe helianthi Munt. Cvet. is an excellent example. Considerable variability has been reported in oil content and fatty acid concentrations in the wild species as well as agronomic and morphologic traits of plant height, days to flowering and tolerances to stress, especially salt.

Genetic diversity and alkaloid production in Catharanthus roseus, C. trichophyllus and their hybrids

Inter and intraspecific variation was analyzed in two Catharanthus species with regard to isozyme polymorphism and indole alkaloid content in roots and leaves. No significant differences in alkaloid production were observed in three groups of C. roseus plants individualized for their flower color. Conversely, comparisons between C. trichophyllus and C. roseus, showed large differences of alkaloid profiles in both roots and leaves. Specific isozyme markers on four presumed loci were found allowing us to establish that natural hybridizations could occur between the two species when grown together. Experimental hybridizations confirmed that introgressions were feasiblebut suggested that a reproductive barrier was acting and involved interspecific incompatibility. The identification and assay of the main alkaloid compounds in natural interspecific hybrids displayed such a high hybrid vigor that interspecific hybridization may present a new and successful way of improving alkaloid production in Catharanthus species.

Cytology of Parthenogenesis inBacillus whiteiandBacillus lynceorum(Insecta Phasmatodea)

Summary We investigated the maturation divisions of two obligate parthenogenetic natural interspecific hybrids: the diploid B. whitei and the triploid B. lynceorum. B. whitei shows two maturation divisions which produce 4 viable haploid nuclei. At the telophase II two non-sister nuclei come into contact and fuse to restore the diploid chromosome number and the maternal genetic structure. On the other hand, in the triploid B. lynceorum, pachytenic oocytes undergo an extra DNA doubling which allows the formation of a somatic number of autobivalents. Two divisions in rapid succession result in a pronucleus with the somatic number of unistranded chromosomes and two quickly degenerating polar bodies: this mechanism maintains the maternal cytological and genetic structure in the offspring. These cytological findings fully support the genetic features ascertained for the two hybrids and provide evidence of their different and very likely independent meiotic adaptations to parthenogenetic reproduction.

Experimental documentation of natural hybridization inCactaceae: Origin of Lloyd's Hedgehog Cactus,Echinocereus �lloydii

The origin ofEchinocereus ×lloydiiBritt. & Rose, pro sp. (Lloyd's Hedgehog Cactus) was investigated using comparative morphology, cytology, biochemistry, and particularly, artificial hybridization. Numerous artificial crosses between the putative parentsE. coccineusEngelm. (a species of claret-up cactus) andE. dasyacanthusEngelm. (Texas Rainbow Cactus) were successful, resulting in the production of hundreds of seeds with hybrid embryos. The F1 hybrid progeny (i.e., syntheticE. ×lloydii) grew to sexual maturity in about four and one-half years, whereupon successful backcrosses and F2 generation hybrids were also obtained. The known F1 hybrids closely approximated naturalE. ×lloydii. The fertility of these syntheticE. ×lloydii was high, like their natural counterparts. The populations ofE. ×lloydii in Pecos County, Texas are inferred to have originated as the result of natural interspecific hybridization. It is assumed thatE. ×lloydii or similar plants may arise wherever the parental taxa grow sympatrically.

Possibility of Natural Hybridization between Cyperus iria L. and Cyperus microiria Steud.

No experimental evidence of hybridization between Cyperus iria L. and C. microiria Steud. has heretofore been documented, though occurrence of natural interspecific hybrids in the genus Cyperus has been reported by Koyama (1953, 1961). The authors have obtained new types of plants from an experimental field after a mixed planting of these two species. The new plants showed an intermediate form between the two in shape of floral scale and floret density. They were longer in spikelet length and had a greater number of florets, though seed fertilities were evidently lower than those of the parent species. The seeds of new plants were dormant in each of the reciprocal crossings between C. iria and C. microiria, and the dormancy could be broken by a stratification at 15°C for one month. This character was considered to have been inherited from the dormant parent C. microiria. Progenies of the new type plants also showed a clear segregation in the characters of spikelet and floret morphology. It can thus be confirmed that these plants obtained from mixed planting in the experimental field are to hybrids between C. iria and C. microiria.

Assessment of natural interspecific hybridization of blue and Engelmann spruce in southwestern Colorado

In a partial diallel mating design among 20 blue and 20 Engelmann spruce parents, the interspecific crosses were successful only with Engelmann spruce as the female parent. No viable seed were obtained from the reciprocal cross among the 60 full-sib families attempted. Under the conditions of artificial pollination and a controlled germination environment, an average of 0.3% of the seed germinated on a total seed basis across all 20 Engelmann spruce females. Many abnormalities were observed among the germinating hybrid seed, suggesting hybrid inviability also contributes to the low crossability between these two species. Isozyme analysis was used to confirm the interspecific hybrids between blue and Engelmann spruce based on the unique genotypic compositions of the hybrids relative to the two species. No natural F1 hybrids between blue and Engelmann spruce were observed in this study based on isozyme analysis of mature individuals or their seedling progeny. Analyses included samples of open-pollinated seed from blue and Engelmann spruce females located in an area where both species are present in close proximity, often side-by-side, and where pollen shed and female strobilus receptivity in the two species are coincident. In addition, there was evidence of possible gametic selection or hybrid inviability among the full-sib progeny based on deviations of observed from expected segregation ratios for progeny isozyme genotypes. Deviations tended to favor the allele more common to both species rather than the allele unique or more common to only one species. Also, the elevationally allopatric blue and Engelmann spruce subpopulations were less divergent genetically than the sympatric subpopulations.

Natural Interspecific Hybridization in Eastern North American Claytonia

Natural Interspecific Hybridization in Eastern North American Claytonia

Nonreciprocal Hybridization Failure in Crosses Between Annual Wild-Rice Species (Zizania palustris x Z. aquatica: Poaceae)

Natural interspecific hybridization has been proposed to account for morphological variation in the annual members of the genus Zizania, commonly known as wild-rice, although isozyme data, as reported in a single study, have disclosed no evidence of introgression. Experimental hybridizations were conducted in all possible combinations among the five varieties of the two annual species of wild-rice. Meaningful intraspecific variation in crossability was not detected, which is in agreement with morphological and isozyme data. A unilateral interspecific crossability barrier was found between the two annual species. Pistillate florets of Z. palustris typically failed to produce viable fruits when pollinated by Z. aquatica. The reciprocal interspecific cross was successful at a low level of crossability. This evidence further strengthens the taxonomic separation of the two annual species previously defined by spikelet morphology, ultrastructure, and isozyme varia- tion. Examination of pollinated stigmas of the infertile types of crosses disclosed that pollen tubes had penetrated the entire length of the stigma. The crossing barrier is therefore not due to a pollen/ stigma interaction that severely interferes with pollen tube growth. Interspecific F, hybrids ex- pressed the spikelet morphology of the Z. aquatica parent exclusively, although other morphological characters were typically expressed as intermediate to those of the parent species. The potential for natural hybridization between Z. aquatica and Z. palustris is discussed.

MICROSPOROGENESIS IN MALE‐STERILE AND HERMAPHRODITIC PLANTS OF NINE GYNODIOECIOUS TAXA OF HAWAIIAN BIDENS (ASTERACEAE)

Microsporogenesis was investigated in hermaphroditic and male‐sterile plants in nine gynodioecious taxa of Hawaiian Bidens. Normal microsporogenesis in hermaphrodites and the onset of abortion in male steriles were similar in all taxa and in a hybrid between two gynodioecious species. The early abnormal vacuolation of tapetal cells is the first visible evidence leading to premeiotic abortion of microsporogenesis in male steriles. The sporogenous cells disintegrate rapidly after the vacuolation of the tapetum, resulting in a shrunken, indehiscent anther which is composed of only the epidermal layer with some remnant cells of the endothecium and the connective at anthesis. In hermaphrodites, the tapetal cells remain dense and undergo karyokinesis to become binucleate during meiosis I. The tapetum becomes plasmodial after microspores are released from tetrads and gradually disappears during pollen formation. The genetic factor(s) which cause the abortion act with remarkable precision and consistency in all taxa investigated. This suggests that gynodioecy in all Hawaiian Bidens is homologous and the establishment of male sterility in Hawaiian Bidens occurred only once. The spread of the genetic male‐sterile factor(s) may be the result of adaptive radiation of the original gynodioecious species or natural interspecific hybridization.

Taxonomy of Xanthocephalum (Compositae: Astereae)

The eight taxa of Xanthocephalum, including X. durangense, X. gymnospermoides var. intermedium, and X. gymnospermoides var. eradiatum, are characterized by glabrous recep- tactes. goblet-shaped, pubescent disc corollas, deltate style branch appendages with short, rounded collecting hairs, and essentially glabrous achenes with very little or no pappus. The taxa are dis- tributed in damp, generally montane valleys from central Mexico northwest through the Sierra Madre Occidental to southern Arizona and western Texas. The chromosome number of all species is n = 6. Two natural interspecific hybrids are known. Xanthocephalum has affinities with other entities of Astereae that have x = 6, particularly Grindelia and Olivaea. Xanthocephalum has recently been reduced in number of species via the transfer of five taxa

Isoenzyme Verification of American-Chinese Hybrids of Liquidambar and Liriodendron

Control-pollinated interspecific hybrids between Liquidambar styraciflua and L. formosana and between Liriodendron tulipifera and L. Chinese were verified by comparison of isoperoxidase banding patterns developed by starch gel electrophoresis of cambial extracts. The relationships between some of the trees of eastern Asia and eastern United States (Li, 1952) are especially evident in Liriodendron, in which the only two species, the American Liriodendron tulipifera L. and Liriodendron Chinese Helmsley, occur in these regions. There are only four species in Liquidambar: Liquidambar styraciflua L. occurs in eastern United States, with disjunct populations in Mexico and Central America, and Liquidambar acalycina and Liquidambarformosana Hance are native to China. A fourth species is found in Turkey. These two genera were among the first to be investigated after the initiation of a project on cytogenetics, breeding, and improvement of landscape trees at the U.S. National Arboretum in 1967. Santamour (1972a) reported successful crossing among three Liquidambar species, using leaf morphology to verify interspecific hybridity. Because verification of young hybrids would be more difficult in Liriodendron, in which leaf shape of the two species is very similar, Santamour used the biochemical evaluation of leaf flavonoids (Santamour, 1972b) to establish the true hybridity of seedlings from crosses of L. Chinese made in 1970. This was thought to be the first controlled hybridization between these species. However, similar research in China was unknown in the West. According to the Nanjing Technological College of Forest Products (1980), they and the Jiangsu Institute of Botany had successfully crossed the two tuliptrees first in 1963, and then several times between 1963 and 1980. They also reported that the hybrids were superior in growth to L. Chinese at 12 years of age. In addition, Huang and Chen (1979) verified both natural and artificial interspecific hybrids by analyses of isoenzyme banding patterns from dormant bud extracts. They found that by acrylamide gel electrophoresis of peroxidase isoenzymes they could distinguish the hybrids that contained the isoenzyme bands of both parents as well as two new bands. Inasmuch as our joint research efforts at the U.S. National Arboretum involved considerable work in isoenzyme electrophoresis, we decided to investigate the potential of using this technique to verify the Liquidambar and Liriodendron hybrids produced at the U.S. National Arboretum and the Liriodendron hybrids produced in China. MATERIALS AND METHODS All of the parent trees and the progenies derived from controlled crosses at the National Arboretum were available for study. The first material received from China included dormant twigs from one tree each of the parent species and one putative hybrid. A second shipment contained dormant twigs from three putative hybrids. Although most isoenzyme studies in plants have utilized leaf tissue, Santamour and Demuth (1980) found that cambial tissue was equally as effective for isoperoxidases. Furthermore, studies of cambial isoenzymes can be made at any time of the year and dormant twigs can be shipped halfway around the world with no adverse effects. The methods used for starch gel electrophoresis and staining of cambial peroxidase isoenzymes were the same as those reported by Santamour (1982) and have been successful in a wide range of woody genera (Santamour & Demuth,

POLLINATION BIOLOGY AND CHEMOTAXONOMY OF THE ECHINOCEREUS VIRIDIFLORUS COMPLEX (CACTACEAE)

The Echinocereus viridiflorus complex is morphologically distinct from other members of the genus. Temporal differences in flowering preclude natural interspecific hybridization between taxa of the E. viridiflorus complex and sympatric congeners. All taxa within the complex are xenogamous and self‐incompatible. Considerable taxonomic disagreement exists among herbarium and field botanists with regard to species boundaries and infraspecific classification of the taxa constituting the E. viridiflorus complex. One author views the complex as consisting of a single species, E. viridiflorus while other authors additionally recognize E. chloranthus, E. russanthus, and E. davisii. Pollination in E. viridiflorus is by solitary halictid bees. Photographs taken with a filter transmitting long‐wave ultraviolet (UV) light revealed two floral patterns among entities of the complex: entirely UV‐absorbing tepals and tepals with UV‐reflective outer segments and UV‐absorbing centers (target‐type blossoms). We suggest that UV‐floral patterning increases pollinator visitation and efficiency. Six flavonol‐O‐glycosides and two dihydroflavonol‐7‐O‐glycosides were localized in tepal tissue. No qualitative differences were seen either between differentially UV‐patterned flowers or among E. viridiflorus, E. chloranthus, E. davisii, and E. russanthus. Flavonoid evidence indicates that these taxa are conspecific.

Natural and experimental interspecific hybridization between populations ofDolichopoda cave crickets

Evidence of hybridization between 2 pairs of allopatricDolichopoda species is provided by electrophoretic analysis. Occurrence of hybrids was revealed by laboratory crosses and in nature both by occasional co-existence due to passive dispersal and by transplantation experiments.

CYTOGENETICS OF SOME NATURAL INTERGENERIC HYBRIDS BETWEEN ELYMUS AND AGROPYRON SPECIES

The occurrence of natural interspecific hybrids between Elymus and Agropyron species was detected and verified by cytomorphological studies. The hybrids were highly vigorous and resembled Elymus species in vegetative growth habit, although their spikes were sterile, and intermediate between the putative parents in morphology. Chromosome pairing in the hybrids was highly irregular with 14 to 28 univalents per cell. The formation of up to seven bivalents, with mean frequencies of 2.86 to 3.14 per cell in different hybrids, was attributed to homoeologous relationships existing between the genomes of E. innovatus (JJXX) and those of Agropyron species (SSHH).

Natural hybridization inDrosophila

Although at least 159 cases of interspecific hybridization between closely related species ofDrosophila have been obtained under laboratory conditions, only 7 cases of natural interspecific hybridization have been recorded. We report yet another case, concerningD. malerkotliana andD. bipectinata.

Cytogenetics of semi-arid plants. III. A natural interspecific hybrid of Cucurbitaceae (Citrullus colocynthis Schrad*C. vulgaris Schrad).

Appearance of a natural interspecific hybrid between C. colocynthis and C. vulgaris have further reinforced the close phylogenetic relationship among the two species. On the basis of available evidence it was concluded that C. vulgaris is a cultivated form of C. colocynthis evolved through mutation and selection particularly for the fruit character. The hybrid's behaviour in nature has been discussed from evolutionary point of view and also its possible exploitation for the improve-ment of C. colocynthis whose emmense economic value have been realised in recent past.

Natural Hybridization between a Sympatric Pair of Hawaiian Drosophila

Natural interspecific hybrids between two closely related species of Hawaiian Drosophila, heteroneura and silvestris, have been collected from the Kahuku Ranch area on the Island of Hawaii. Despite full fertility of both sexes of these hybrid specimens, hybridization is apparently not extensive. Investigations of the degree of introgression between these two species and of environmental factors which might contribute to the breakdown of isolating mechanisms are continuing.

Biochemical Genetics of Sunfish. II. Genic Similarity between Hybridizing Species

Recent improvements electrophoretic and biochemical staining techniques have allowed a quantification of levels of genic similarity between various taxa and have stimulated renewed interest the role of speciation converting intraspecific to interspecific variation (Lewontin 1967). Mutationists the early 1900s claimed that certain types of single mutations could give rise to new species. However, an examination of the tremendous amount of variability interspecific F2 hybrids soon showed that even closely related species could differ at a large number of genetic loci (Harland 1936; Sumner 1932; Darlington 1940; Timofeeff-Ressovsky 1940). Electrophoretic studies have generally supported the latter observation. Populations belonging to closely related species usually show differences allelic composition at a far higher percentage of genetic loci than conspecific populations. Studies on sibling species of Drosophila (Hubby and Throckmorton 1965; Nair et al. 1971; Ayala et al. 1970), Peromyscus (Smith, Selander, and Johnson, preparation), kangaroo rats (Dipodomys) (Johnson and Selander 1971), and cotton rats (Sigmodon) (Johnson et al. 1972) indicate that even sibling species show major differences at between 20%o and 50% or more of their genetic loci and that in spite of their morphological and ecological similarity and their evolutionary affinity these species have very different gene pools (Ayala et al. 1970). These and similar studies provide a base with which genic similarities between other species may be compared. In this study, we examine protein differences a group of fishes whose propensity to hybridize is well known. Natural interspecific hybridization is more common fishes than any other group of vertebrates (Lagler et al. 1962), and no group of fishes is more renowned for its ability to hybridize (with the possible exception of certain cyprinids) than are the centrarchids and particularly species of Lepomis (Birdsong and Yerger 1967). Theoretically the 11 species of Lepomis can produce 55 F1 interspecific hybrid types, and of these, at least 21 have been found nature (Childers 1967). Artificially raised hybrids are frequently fertile (West 1970), and Hubbs (1955) has noted that introgressive hybridization may occasionally occur nature.

Natural interspecific hybridization between cultivated safflower (Carthamus tinctorius) and the wild C. Tenuis

A description is given of the natural hybridization between cultivated safflower and the unrelated wild C. tenuis. Although the observed hybrids were completely sterile, it is supposed that continuous pollination may have produced viable seeds and introgression between both species.