Cambial Activity and Distribution of Rubber in Guayule (Parthenium argentatum)

Vascular cambium in Guayule, a rubber producing Mexican shrubof Asteraceae family is non-storied. Cambial activity variesperiodically, and the vascular cambium and its immediate derivativesdo not contain rubber. However, as the xylem and phloem parenchymacells derived from the vascular cambium age, rubber depositionstarts from the cell periphery along the walls and later towardstheir cell lumen. Though the sieve tubes and companion cellsof phloem contain no rubber, all parenchyma cells of xylem andphloem, show the presence of rubber, though its amount varies.However, certain lignified xylem ray cells and lignified pithcells are devoid of rubber accumulation. Microfluorescence studiesshow that the epithelial, phloem ray parenchyma, cortical andpith cells, in descending order, have the highest to lowestrubber content. The size and number of rubber particles observedin the parenchyma cells are greatest during the period of cambialdormancy than in an active cambial period Cambium, guayule, rubber     

Distribution, Development and Structure of Resin Ducts in Guayule (Parthenium argentatum Gray)

The distribution, development and structure of resin ducts inguayule (Parthenium argentatum Gray), the second best sourceof natural rubber, have been studied. Resin ducts are widelydistributed in stem, root, leaf, petiole and peduncle. The ductsin the primary tissues are initiated schizogenously and theirfurther development is schizolysigenous. The ducts in the cortexof the root do not have a well-defined epithelium. Ducts developedfrom the vascular cambium are initiated and develop schizogenously.Both resin and rubber are produced in the epithelial cells ofresin ducts. While resin is secreted into the duct lumen, rubberis stored within these cells. Epithelial cells store more thanneighbouring parenchyma cells. Guayule, rubber, resin, ducts, epithelial cells     

Cytogenetic study of F1 hybrids betweenCrepis dinarica andCrepis froelichiana (Asteraceae)

Crepis dinarica andC. froelichiana are two closely related species of theC. praemorsa complex. Even though they exhibit the same chromosome number (2n = 8) and similar idiogram shape, they differ widely in quantity and distribution of heterochromatin bands. The hybrids between these two species comprise three morphological types. Parental genomes were distinguished in hybrids by Giemsa differential staining (C-banding). Although meiosis presents only a few abnormalities (about 2.4%), the percentage of aborted pollen grains is very high (90%).     

Fecundity, phenology, and seed dormancy of F1 wild-crop hybrids in Sunflower (Helianthus annuus, Asteraceae)

Crop-to-wild hybridization has the potential to introduce beneficial traits into wild populations. Gene flow from genetically engineered crops, in particular, can transfer genes coding for traits such as resistance to herbicides, insect herbivores, disease, and environmental stress into wild plants. Cultivated sunflower (Helianthus annuus) hybridizes spontaneously with wild/weedy populations (also H. annuus), but little is known about the relative fitness of F1 hybrids. In order to assess the ease with which crop-to-wild introgression can proceed, we compared characteristics of F1 wild-crop progeny with those of purely wild genotypes. Two nontransgenic, cultivated varieties were crossed with wild plants from three different regions-Texas, Kansas, and North Dakota. Seed burial experiments in the region of origin showed that wild-crop seeds had somewhat higher germination rates (less dormancy) than wild seeds from Kansas and North Dakota, while no differences were seen in seeds from Texas. Progeny from each type of cross were grown in outdoor pots in Ohio and in a weedy field in Kansas to quantify lifetime fecundity and flowering phenology. Flowering periods of hybrid and wild progeny overlapped considerably, especially in plants from North Dakota and Texas, suggesting that these hybrids are very likely to backcross with wild plants. In general, hybrid plants had fewer branches, flower heads, and seeds than wild plants, but in two crosses the fecundity of hybrids was not significantly different from that of purely wild plants. In Ohio, wild-crop hybrids from North Dakota appeared to be resistant to a rust that infected 53% of the purely wild progeny, indicating a possible benefit of "traditional" crop genes. In summary, our results suggest that F1 wild-crop hybrids had lower fitness than wild genotypes, especially when grown under favorable conditions, but the F1 barrier to the introgression of crop genes is quite permeable.     

Morphological discriminants of Scottish wildcats (Felis silvestris), domestic cats (F. catus) and their hybrids

Skull characters of wildcats ( Felis silvestris ), classed as 'old', 'recent' or 'modern', depending on collection date, were compared with those of domestic cats ( F. catus ) and hybrids, using Fisher Linear Discriminant Functions, Principal Component Analyses and Canonical Variates. A key is derived for classification of individual (unknown) skulls. All analyses indicate that 'recent' and 'modern' wildcats were different from those collected earlier, and implied marked changes in the rate of hydridization during this century. We suggest that the older wildcat populations were a relatively 'pure' form but more recent populations contain a significant hybrid component. Although there may be a trend towards re-establishment of the earlier type, an alternative conclusion is that the 'pure' form of wildcat is effectively extinct in Scotland.     

Pollen fertility of synthetic intra- and interspecific F1 hybrids in Canary Island Tolpis (Asteraceae)

Pollen fertility was determined for synthetic F₁ hybrids between members of the genus Tolpis endemic to the Canary Islands. Mean fertility varies from 1 to nearly 100%. High hybrid sterility is unusual for island lineages. Surprisingly, the fertility of hybrids between morphologically distinct species was generally higher than some crosses within the morphologically variable group that includes the Tolpis laciniata and T. lagopoda complexes. Within this group, fertility was lower particularly in hybrids involving one recognized segregate species (T. webbii), but none of the four potentially new species had reduced hybrid fertility. Despite the overall fertility differences between groupings within the T. laciniata and T. lagopoda complexes there is variation in fertility within groups; sterility factors have a complex inter-populational distribution. The cause of hybrid sterility is unknown, but preliminary data suggest that both chromosomal and genetic factors are involved.     

Production,morphology, and cytogenetic analysis of Elymus caninus (Agropyron caninum) x Triticum aestivum F1 hybrids and backcross-1 derivatives

Summary Intergeneric hybrids were produced between common wheat, Triticum aestivum (2n=6x=42, AABBDD) and wheatgrass, Etymus caninus (Agropyron caninum) (2n=4x=28, SSHH) — the first successful report of this cross. Reciprocal crosses and genotypes differed for percent seed set, seed development and F₁ hybrid plant production. With E. caninus as the pollen parent, there was no hybrid seed set. In the reciprocal cross, seed set was 23.1–25.4% depending upon wheat genotype used. Hybrid plants were produced only by rescuing embryos 12–13 days post pollination with cv Chinese Spring as the wheat parent. Kinetin in the medium facilitated embryo germination but inhibited root development and seedling growth. The hybrids were vigorous, self sterile, and intermediate between parents. These had expected chromosome number (2n=5x=35, ABDSH), very little chromosome pairing (0.51 II, 0.04 III) and some secondary associations. The hybrids were successfully backcrossed with wheat. Chromosome number in the BC₁ derivatives varied 54–58 with 56 as the modal class. The BC₁ derivatives showed unusually high number of rod bivalents or reduced pairing of wheat homologues. These were sterile and BC₂ seed was produced using wheat pollen.     

Behavioral reproductive isolation inDrosophila silvestris,D. heteroneura,and their F1 hybrids (Diptera: Drosophilidae)

We investigated the role that courtship and aggressive interactions may have for the maintenance of reproductive isolation betweenDrosophila silvestris andD. heteroneura. We examined the behavioral bases of reproductive isolation between the parental species and we examined the courtship success of each sex of both reciprocal F₁ hybrids when paired with the parental species. We found reduced copulation success among heterotypic parental pairs compared to homotypic pairs, which was primarily due to the lack of courtship initiation betweenD. silvestris males andD. heteroneura females. When hybrid males from both reciprocal crosses were paired with parental females their copulation successes were not significantly different from that of parental males. In contrast, hybrid females from both crosses had reduced copulation success withD. silvestris males, which in turn was primarily due to a reduced success of reaching later stages of courtship. The time spent in copulation by hybrid males was intermediate between the two parental males. We studied aggression by observing the interactions of males of heterotypic pairs, both between the parental species and between the hybrids and parental males. A lack of aggressive interactions betweenD. silvestris males andD. heteroneura males in addition to the lack of courtship suggests thatD. silvestris males do not respond toD. heteroneura individuals of either sex. Hybrid males were equally successful in winning fights with bothD. silvestris andD. heteroneura males. These results indicate that the behavioral isolation betweenD. silvestris andD. heteroneura may be largely a consequence of the earliest stages of interactions. The two species may differ either in activity levels or in morphological or chemical traits that are important for species and mate recognition. The relatively high copulation and aggressive success of hybrids indicates that sexual selection against hybrids alone is unlikely to be a sufficient force to reduce gene flow and maintain species distinctions.     

Genetic,cytogenetic and morphological diversity in Helicrysum leucocephalum (Asteraceae) populations

Helichrysum (family Asteraceae) is a large, heterogeneous and polyphyletic genus. Most of the Helichrysum species have ornamental and medicinal values. Helicrysum leucocephalum Boiss., has wide geographical distribution in Iran and forms several local populations. The present investigation was performed to study inter populations, genetic, cytogenetic and morphological diversity of this medicinal species. The AMOVA test showed significant genetic difference among the studied populations but the Mantel test did not show correlation between the genetic distance and geographical distance of these populations. STRUCTURE and reticulation analyses showed some degree of genetic admixture and gene exchange among the studied populations. These populations had different chromosome numbers and also differed significantly in the size of their chromosomes.     

Morphological identification of Lucilia sericata,Lucilia cuprina and their hybrids (Diptera,Calliphoridae)

Hybrids of Lucilia sericata and Lucilia cuprina have been shown to exist in previous studies using molecular methods, but no study has shown explicitly that these hybrids can be identified morphologically. Published morphological characters used to identify L. sericata and L. cuprina were reviewed, and then scored and tested using specimens of both species and known hybrids. Ordination by multi-dimensional scaling indicated that the species were separable, and that hybrids resembled L. cuprina, whatever their origin. Discriminant function analysis of the characters successfully separated the specimens into three unambiguous groups – L. sericata, L. cuprina and hybrids. The hybrids were morphologically similar irrespective of whether they were from an ancient introgressed lineage or more modern. This is the first evidence that hybrids of these two species can be identified from their morphology. The usefulness of the morphological characters is also discussed and photographs of several characters are included to facilitate their assessment.     

Pollinator preferences for Nicotiana alata, N. forgetiana, and their F1 hybrids

The role of pollinators in plant speciation and maintenance of species boundaries is dubious because most plant species are visited by several types of pollinators, and most pollinator species visit several species of plants. We investigated pollinator preferences and their efficacy as ethological isolation mechanisms between two interfertile species, Nicotiana alata and N. forgetiana and their F1 hybrids. Hawkmoths pollinate N. alata, while primarily hummingbirds and occasionally small hawkmoths visit N. forgetiana. F1 hybrids are easily produced in the greenhouse and although the species grow in similar habitats, hybrids have not been found in nature. In Rio Grande do Sul, Brazil, near where both species are found, experimental plots were studied containing both species, and both species plus F1 hybrids. In the mixed-species plots, hawkmoths showed a strong preference for N. alata. Hummingbirds were less common and only visited N. forgetiana. Hybrid seed was produced but plants made significantly fewer hybrid offspring than predicted by the frequency of interspecific pollinator movements. Nicotiana forgetiana was the seed parent of 97% of the F1 offspring, suggesting an asymmetry in pollen delivery or postpollination processes. In plots containing F1 hybrids plus both parental species, hawkmoths preferred N. alata and undervisited the other two phenotypes, except that in the third plot they visited hybrids in proportion to the hybrid frequency. Hummingbirds strongly preferred N. forgetiana in all plots but also visited F1 hybrids in proportion to their frequency in the third plot. Overall, F1 hybrids were well pollinated and were frequently visited immediately before or after one of the parental species. Thus hybrids could facilitate gene flow between the parental species. We conclude that pollinator discrimination among species is strong but is an imperfect isolation mechanism, especially if hybrids are present.     

Vole preference for Salix caprea, S. repens, and their F1, F2, and backcross hybrids

In recent years, the influence of plant hybridization on plant–animal interactions has become an important issue; however, these studies have mainly focused on insects or slugs and to a large degree ignored mammalian herbivores despite their importance in many ecosystems. This study aims to determine the preference of voles for pure Salix caprea , S. repens , and their F1, F2, and backcross hybrids to evaluate whether voles select against hybrids in natural willow populations. To address this, we conducted two field studies and a cafeteria experiment in the laboratory with bank voles ( Clethrionomys glareolus ). We found no strong indications of reduced resistance in any of the hybrid taxa. Voles damaged more S. repens than S. caprea individuals. There was a general decreasing trend of utilization from pure S. repens to pure S. caprea , hybrid taxa being intermediate between the parents. Thus, voles seemingly do not have a more detrimental effect on hybrid fitness than on the fitness of pure individuals and probably will not select against hybrids in these willows populations.     

RAPD variations among pure and hybrid populations ofPicea mariana, P. rubens, andP. glauca (Pinaceae), and cytogenetic stability ofPicea hybrids: Identification of species-specific RAPD markers

Random amplified polymorphic DNA (RAPD) analysis was used to determine genetic relationships amongP. mariana (black spruce),P. rubens (red spruce), andP. glauca (white spruce) and to assess the degree of polymorphism within populations from different provenances and among spruce hybrids. Eleven arbitrary decamer primers were used to amplify genomic DNAs extracted from embryogenic cultures and seedlings. Species-specific RAPD markers were identified.Picea mariana andP. rubens showed similar RAPD profiles confirming their close genetic relationship. Species-specific RAPD markers were identified and were useful in distinguishing white spruce from black and red spruces. RAPD differentiation between populations within each species was small. The level of polymorphism was much higher in spruce hybrid populations than in the pure species. Cytological analysis ofP. mariana ×P. rubens hybrids showed normal mitotic behaviour at prophase, metaphase, anaphase, and telophase. All the hybrids analyzed from different cross combinations were euploids.     

Reproductive fitness of hybrids between Senecio jacobaea and S. aquaticus (Asteraceae)

Natural hybridization is increasingly recognized as an important process for the ecology and evolution of natural plant populations and species. There is a great need to initiate more studies based on natural populations in order to elucidate the possible role of hybrids in nature. The reproductive success of early generation hybrids can make or break hybrid lineages and may determine the genetic structure of hybrid swarms or the potential for gene flow through future generations, but studies of hybrid reproductive success are lacking. Here we measured components of male and female reproductive success in Senecio jacobaea and S. aquaticus (Asteraceae) species and F(1) hybrids between these species under laboratory conditions, and we measured reproductive output from crosses producing F(1), F(2), and backcross (BC) generation hybrids. F(1) hybrids were readily produced, and on average, the success of crosses producing subsequent generations (F(2), BC) decreased (though remained substantial), but the success of crosses was highly dependent on the genotypes involved. Also, F(1) hybrids were bigger, produced more flowers, and therefore produced more pollen than parental plants. Finally, crosses between parents were asymmetrical, such that S. aquaticus produced more and larger F(1) seeds than did S. jacobaea.     

Recent and recurrent polyploidy in Tragopogon (Asteraceae): cytogenetic, genomic and genetic comparisons

Tragopogon mirus Ownbey and T. miscellus Ownbey are allopolyploids that formed repeatedly during the past 80 years following the introduction of three diploids (T. dubius Scop., T. pratensis L. and T. porrifolius L.) from Europe to western North America. These polyploid species of known parentage are useful for studying the consequences of recent and recurrent polyploidization. We summarize recent analyses of the cytogenetic, genomic and genetic consequences of polyploidy in Tragopogon. Analyses of rDNA ITS (internal transcribed spacer) + ETS (external transcribed spacer) sequence data indicate that the parental diploids are phylogenetically well separated within Tragopogon (a genus of perhaps 150 species), in agreement with isozymic and cpDNA data. Using Southern blot and cloning experiments on tissue from early herbarium collections of T. mirus and T. miscellus (from 1949) to represent the rDNA repeat condition closer to the time of polyploidization than samples collected today, we have demonstrated concerted evolution of rDNA. Concerted evolution is ongoing, but has not proceeded to completion in any polyploid population examined; rDNA repeats of the diploid T. dubius are typically lost or converted in both allopolyploids, including populations of independent origin. Molecular cytogenetic studies employing rDNA probes, as well as centromeric and subtelomeric repeats isolated from Tragopogon, distinguished all chromosomes among the diploid progenitors (2n = 12). The diploid chromosome complements are additive in both allopolyploids (2n = 24); there is no evidence of major chromosomal rearrangements in populations of either T. mirus or T. miscellus. cDNA‐AFLP display revealed differences in gene expression between T. miscellus and its diploid parents, as well as between populations of T. miscellus of reciprocal origin. Approximately 5% of the genes examined in the allopolyploid populations have been silenced, and an additional 4% exhibit novel gene expression relative to their diploid parents. Some of the differences in gene expression represent maternal or paternal effects. Multiple origins of a polyploid species not only affect patterns of genetic variation in natural populations, but also contribute to differential patterns of gene expression and may therefore play a major role in the long‐term evolution of polyploids. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 485–501.     

Morphological variation among Betula nana (diploid), B. pubescens (tetraploid) and their triploid hybrids in Iceland

BACKGROUND AND AIMS: Introgressive hybridization between two co-existing Betula species in Iceland, diploid dwarf birch B. nana and tetraploid downy birch B. pubescens, has been well documented. The two species are highly variable morphologically, making taxonomic delineation difficult despite stable ploidy levels. Here an analysis is made of morphological variation within each ploidy group with an aim to establishing a reliable means to distinguish the species. METHODS: Plant materials were collected from 14 woodlands in Iceland. The plants were identified based on 2n chromosome numbers. Morphological variation in species-specific characters within each ploidy group was analysed qualitatively and quantitatively. The morphological index was based on eight discrete characters, whereas the multivariate analysis was based on nine leaf variables. KEY RESULTS: Of the 461 plants examined, 9.5 % were found to be triploid hybrids. The three ploidy groups were morphologically distinguishable but their variation overlapped. The diploid, triploid and tetraploid groups had average scores of 1.3, 4.1 and 8.3, respectively, in the morphology index scale from 0 (B. nana) to 13 (B. pubescens). A linear discriminant analysis also revealed significant separation among the three ploidy groups and the model assigned 96 % and 97 % of the B. nana and B. pubescens individuals correctly. The triploid hybrids were difficult to predict since only half of them could be assigned correctly. Leaf length was the most useful variable identifying triploid hybrids. Geographical patterns within the ploidy groups could partly be explained by differences in mean July temperature. CONCLUSIONS: Hybridization between B. nana and B. pubescens is widespread in Iceland. The species can be distinguished from each other morphologically, and from the triploid hybrids. The overlapping morphological variation indicates bidirectional introgression between the two species via triploid hybrids. Iceland could be considered a birch hybrid zone, harbouring genetic variation which may be advantageous in subarctic regions.     

Inheritance of RAPD markers in channel catfish (Ictalurus punctatus), blue catfish (I. furcatus), and their F1, F2 and backcross hybrids

The channel catfish ( Ictalurus punctatus ) has become the most important aquaculture species in the USA. A genetic linkage map in catfish is needed to improve efficiency of breeding by marker-assisted selection (MAS) and for identification of economically important genes such as disease resistance genes. To identify DNA-based genetic polymorphism, the present authors tested 42 randomly amplified polymorphic DNA (RAPD) primers for their utility in identifying genetic polymorphism in catfish. Out of these primers, 22 generated 171 highly reproducible RAPD markers, producing almost eight polymorphic bands per primer. The remaining 20 primers produced an additional 20 polymorphic bands. The RAPD markers were highly reproducible, transmitted to F1 hybrids, and segregated in F2 or backcross progeny in ratios that did not differ from Mendelian expectations. Because the interspecific hybrids of channel catfish and blue catfish are fertile, RAPD markers using the interspecific hybrid system will be useful for rapid construction of genetic linkage maps of catfish and for analysis of important quantitative trait loci.     

Crossability of Brassica tournefortii and B. rapa, and morphology and cytology of their F1 hybrids

The crossability between Brassica tournefortii (TT, 2n = 20) and Brassica rapa (AA, 2n = 20) and the cytomorphology of their F₁ hybrids were studied. Hybrids between these two species were obtained only when B. tournefortii was involved as a female parent. The hybrid plants were intermediate for most of the morphological attributes and were found to be free from white rust under field conditions. The F₁ plants showed poor pollen fertility, although occasional seed set was achieved from open pollination. Self-pollination or backcrosses did not yield any seeds in these plants. The occurrence of chromosome association ranging from bivalents (0–7), trivalents (0–2) to a rare quadrivalent (0–1) in the dihaploid hybrids indicates pairing between the T and A genomes. The homoeologous pairing coupled with seed set in the F₁ plants offer an opportunity for interspecific gene transfers from B. tournefortii to B. rapa and vice-versa through interspecific hybridization. Received: 3 July 2000 / Accepted: 22 September 2000