The evolutionary implications of interspecific hybridization among four species of zinnia sect. Mendezia (compositae)

Fertile interspecific hybrids were obtained among four species ofZinnia sect.Mendezia. The collection data and the segregation for ray color in F₁ hybrids suggest natural interspecific hybridization. Cytological data suggest that the descent of the four species from a common ancestor was accompanied by reciprocal translocations involving non-homologous chromosomes.     

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.     

Tracheary element differentiation

Differentiation of tracheary elements (TEs), characterized by formation of a visible secondary cell wall and autolysis, has been regarded as a model system for cytodifferentiation in plants. Fukuda and Komamine (Plant Physiol 65:57–60, 1980a) established an efficient experimental system for TE differentiation from isolated single mesophyll cells of Zinnia elegans. Physiological, cytological, and biochemical analyses of this system have revealed multiple aspects of TE differentiation regarding cell division, DNA synthesis, the cytoskeleton, autolysis, phytohormones, and cell wall components. In addition, molecular biological approaches have identified a number of Zinnia genes associated with TE-specific events. The knowledge obtained in the Zinnia system has been expanded to studies in other plants including Arabidopsis, leading to the identification of key regulators of TE differentiation.     

When interspecific prezygotic barriers break down: hybridization between two Potamogeton species (P. wrightii and P. perfoliatus) and comparison of the artificial and natural hybrids

Interspecific hybridization played an important role in speciation and evolution of angiosperms. Although the widespread occurrence of natural hybrids in the genus Potamogeton has been studied intensively, few successful experimental hybridization studies have been reported in this genus. In the present study, critical experimental hybridization was conducted using Potamogeton?×?intortusifolius, a natural hybrid widely distributed in China, and its parents (P. perfoliatus and P. wrightii). The absence of prezygotic barriers between P. wrightii and P. perfoliatus was observed, which contributes to the frequent hybridization of these species in nature. The pollen tube growth rates of P. perfoliatus were much faster than those of P. wrightii in the style of that species. However, the conspecific pollen tubes were competitively advantageous in P. perfoliatus styles. The interspecific hybridization could be applied bidirectionally, and 28 F1 hybrid individuals were successfully obtained from P. wrightii?×?P. perfoliatus, despite the low germination possibility of the hybrid seeds. Both the artificial and natural hybrids exhibited intermediate morphological characters but presented much lower fertility. The sterility of P.?×?intortusifolius is mainly due to its low female fitness. However, the offspring from P. wrightii?×?P.?×?intortusifolius indicated the potential for backcrossing in nature. This is the most successful attempt at artificial hybridization in this genus so far. The possible route for restoration of fertility and the fitness of the hybrids are also discussed in this paper.     

Chromosome evolution by robertsonian translocation in Gibasis (Commelinaceae)

The plant species Gibasis schiedeana (Kunth) D. R. Hunt sens. lat. contains two cytotypes viz. a self-sterile diploid with 2n=10 (x=5) and a selffertile cytological autotetraploid with 2n=16 (x=4). Single chromosome sets of these plants consist of 2 metacentrics +3 acrocentrics, and 3 metacentrics +1 acrocentric chromosomes respectively suggesting a Robertsonian relationship between them. Their artificial F₁ hybrids show the pairing of acrocentrics with metacentric arms confirming the supposed nature of the chromosome affinities. Both breeding systems and ploidy levels show that the direction of the change has been from x=5 to x=4 by a translocation of the Robertsonian type.     

The nature and status of Lysimachia X producta (Primulaceae)—II

Data from chromosome numbers and from morphological variation in wild plants and artificial hybrids further support the hypothesis thatLysimachia Xproducta is of hybrid origin and the taxonomic judgment that it is best treated at the status of interspecific hybrid rather than as a species of hybrid origin. Additional distribution data are presented.     

Interspecific hybridization and the analysis of meiotic chromosome pairing inDahlia (Asteraceae — Heliantheae) species with x = 16

The successful production of a large number of artificial hybrids betweenDahlia species based on x = 16 has allowed a detailed study of their genomic relationships. Chromosome behaviour in these artificial hybrids was extremely similar to that observed in parental species suggesting that there is a considerable degree of homology between the genomes of theseDahlia species. Using GISH it can be demonstrated that in these hybrids bivalent formation involved pairing only between parental genomes. The ability of GISH to differentiate between parental genomes in artificial hybrids was variable, indicating that molecular divergence of highly repeated sequences has accompanied the evolution of these species. However, the extent of chromosome pairing and chiasma formation in the hybrids does not reflect the differences that can be detected by GISH. Seyeral of the new hybrid combinations have resulted in horticulturally interesting plants.     

Intergeneric hybrids of the lower (42-chromosome) monkey species of the Sukhumi monkey colony

A cytogenetic study has been carried out of 2 intergeneric hybrids between Papio and Macaca genera and 17 hybrids between Papio and Theropithecus genera, obtained at the Sukhumi monkey colony for the period since 1949 up to 1964. Intermediate or mosaic-intermediate heredity of the phenotypic features of the initial parental species are noted in all the hybrids.Hybrids between Macaca and Papio genera resembled baboons according to the body build and colour of eyes, but acoording to the form of head, face, hair colour, sexual skin and ischial callosities they resembled macaques. More or less domination of the gelade phenotype (head form, location of nostrils, saddle-like nose, hair colour, body build) is seen in the Papio × Theropithecus hybrids. At the same time, the structure of sexual skin and ischial callosities had an intermediate type between baboons and gelades. Instead of the sexual skin, characteristic of gelades, the appearance of a new feature (hair-free nipple fields up to 6 cm in diameter) has been noted on the chest of the hybrids. The carotype of the hybrids between Macaca, Papio and Theropithecus genera consists of 42 chromosomes, as in the initial species, the homologues of most pairs being significantly similar.The results of crossings and chromosome study allow one to conclude that African species—hamadryas baboons and gelades—are genetically closer to each other than Asian rhesus macaques and African hamadryas baboons, having been isolated from one another geographically for a long period of time. In this connection, intergeneric hybrids between Papio and Theropithecus genera are obtained more easily and the hybrid retains fertility unlike the non-fertile hybrid between macaques and baboons, which is obtained with difficulty.     

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.     

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.     

Hybridization between Senecio smallii and S. tomentosus (Compositae) on the granitic flatrocks of the Southeastern United States

Senecio smallii andS. tomentosus occasionally hybridize on the granitic flatrocks of the Piedmont of the Southeastern United States. These hybrids are intersectional crosses between sectionsAurei andTomentosi. The parental species have different chromosome numbers withn = 22 inS. smallii andn = 23 inS. tomentosus. Meiosis in the hybrids is abnormal with 22ns and li or 2iis and 3is, and the percentage of stainable pollen is greatly reduced over that of the parents. The hybrid pollen grains are structurally abortive as demonstrated by the scanning electron microscope. Origin of the natural hybrids was documented by the production of artificial hybrids including F1’s, F2’s, and BC’s. Very few of the F2 seeds germinated, and all of the F2 seedlings later died, indicating hybrid breakdown. Seeds of the F1 and BC generations germinated well, and the seedlings were vigorous. Population samples indicate that there is some variation within the natural population systems; however, no evidence of introgression was detected. The isolating mechanisms and possible evolutionary effects of this natural hybridization are discussed.     

Enzymic and Photosynthetic Characteristics of Reciprocal F(1) Hybrids of Flaveria pringlei (C(3)) and Flaveria brownii (C(4)-Like Species)

The activities of key C₄ enzymes in gel-filtered, whole-leaf extracts and the photosynthetic characteristics for reciprocal F₁ hybrids of Flaveria pringlei (C₃) and F. brownii (C₄-like species) were measured to determine whether any inherited C₄-photosynthetic traits are responsible for their reduced CO₂ compensation concentration values (AS Holaday, S Talkmitt, ME Doohan Plant Sci 41: 31-39). The activities of phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, and NADP-malic enzyme (ME) for the reciprocal hybrids are only about 7 to 17% of those for F. brownii, but are three- to fivefold greater than the activities for F. pringlei. The low activities of these enzymes in the hybrids appear to be the result of a partial dominance of F. pringlei genes over certain F. brownii genes. However, no such dominance occurs with respect to the expression of genes for NADP-malate dehydrogenase, which is as active in the hybrids as in F. brownii. In contrast to the situation with the enzymes above, cytoplasmic factors appear to determine the inheritance of NAD-ME. The NAD-ME activity in each hybrid is comparable to that in the respective maternal parent. Pulse-chase ¹⁴CO₂ incorporation analyses at ambient CO₂ levels indicate that the hybrids initially assimilate 7 to 9% of the total assimilated CO₂ into C₄ acids as compared to 3.5% for F. pringlei. In the hybrids, the percentage of ¹⁴C in malate decreases from an average of 6.5 to 2.1% after a 60-second chase in ¹²CO₂/air. However, this apparent C₄-cycle activity is too limited or inefficient to substantially alter CO₂ exchange from that in F. pringlei, since the values of net photosynthesis and O₂ inhibition of photosynthesis are similar for the hybrids and F. pringlei. Also, the ratio of the internal to the external CO₂ concentration and the initial slopes of the plot of CO₂ concentration versus net photosynthesis are essentially the same for the hybrids and F. pringlei. At 45 micromoles CO₂ per mole and 0.21 mole O₂ per mole, the hybrids assimilate nearly fivefold more CO₂ into C₄ acids than does F. pringlei. Some turnover of the malate pool occurs in the hybrids, but the labelling of the photorespiratory metabolites, glycine and serine, is the same in these plants as it is in F. pringlei. Thus, although limited C₄-acid metabolism may operate in the hybrids, we conclude that it is not effective in altering O₂ inhibition of CO₂ assimilation. The ability of the hybrids to assimilate more CO₂ via phosphoenolpyruvate carboxylase at low levels of CO₂ than does F. pringlei may result in an increased rate of reassimilation of photorespiratory CO₂ and CO₂ compensation concentrations below that of their C₃ parent. If the hybrids do possess a limited C₄ cycle, it must operate intracellularly. They are not likely to have inherited an intercellular compartmentation of C₄ enzymes, since F. brownii has incomplete compartmentation of key C₃ and C₄ enzymes.     

Establishing in vitro Zinnia elegans cell suspension culture with high tracheary element differentiation

The Zinnia elegans mesophyll cell culture is a useful system for xylogenesis studies. The system is associated with highly synchronous tracheary element (TE) differentiation, making it more suitable for molecular studies requiring larger amounts of molecular isolates, such as mRNA and proteins and for studying cellulose synthesis. There is, however, the problem of non-uniformity and significant variations in the yields of TEs (%TE). One possible cause for this variability in the %TE could be the lack of a standardized experimental protocol in various research laboratories for establishing the Zinnia culture. Mesophyll cells isolated from the first true leaves of Z. elegans var Envy seedlings of approximately 14 days old were cultured in vitro and differentiated into TEs. The xylogenic culture medium was supplied with 1 mg/l each of benzylaminopurine (BA) and α-naphthalene acetic acid (NAA). Application of this improved culture method resulted in stable and reproducible amounts of TE as high as 76% in the Zinnia culture. The increase was mainly due to conditioning of the mesophyll cell culture and adjustments of the phytohormonal balance in the cultures. Also, certain biochemical and cytological methods have been shown to reliably monitor progress of TE differentiation. We conclude that, with the adoption of current improvement in the xylogenic Z. elegans culture, higher amounts of tracheary elements can be produced. This successful outcome raises the potential of the Zinnia system as a suitable model for cellulose and xylogenesis research.     

Chromosome studies in species and hybrids ofPetrorhagia sect.Kohlrauschia (Caryophyllaceae)

Cytogenetic investigations have been made in the fourPetrorhagia species and hybrids of the sectionKohlrauschia. The three diploid species show close similarities in chromosome number, size and morphology, with the exception ofP. velutina, where one pair of metacentric chromosomes is represented by a pair of telocentrics. Meiotic studies in hybrids indicate close genomic homology between the diploid species and also between the two floral forms ofP. prolifera. The tetraploidP. nanteuilii behaves as an allotetraploid forming only bivalents at meiosis and results suggest thatP. velutina andP. prolifera are the diploid progenitors of this species. Since meiosis in diploid and triploid hybrids results in extensive intergenomic pairing it is concluded that the natural tetraploid has a bivalent promoting mechanism that prevents pairing between the genomes of its diploid progenitors.     

The ploidy and genetic structure of hybrid populations of water frogs Pelophylax esculentus complex (Amphibia, Ranidae) of Ukraine fauna

The complex study, including allozyme variability and cytometry of hybrid populations of green frogs Pelophylax esculentus (L., 1758) complex has confirmed that the only region of Ukraine where allodiploid are encountered frequently is the Severski Donets basin (9% of all hybrids). In other areas, only two polyploidy hybrids (0.9%) and one probably autopolyploid individual of each parental species have been registered. According to allozyme specters, all three polyploidy hybrids from the Severski Donets basin were males and belonged to biotype P. esculentus (=lessonae) — 2 ridibundus, and their population in this region has halved during the past decade.     

Do they really hybridize? A field study in artificially established mixed populations of Euphrasia minima and E. salisburgensis (Orobanchaceae) in the Swiss Alps

Hybridization and introgression in the European species of Euphrasia depend on the relationships between the species, on flower size and habitat. Hybridization between Euphrasia minima and Euphrasia salisburgensis was investigated in their natural habitat using artificial sympatric populations of both species in the Swiss Alps. The insect behavior in the populations suggests, that cross-pollination is likely to occur. A number of putative hybrids were detected by morphological characteristics, and their hybrid origin was verified using RAPD analysis. The predominance of RAPD bands in one of the species and the occurrence of these bands in some plants of the second species point to earlier introgression events. The number of hybrids found in the artificial populations together with results of earlier studies indicate that insect visits and cross-pollination in small-flowered Euphrasia species in lower alpine regions may be more common than has been suggested in the past.     

Cytodifferentiation in isolated single cells

Isolated mesophyll cells ofZinnia elegans synchronously differentiate to tracheary elements at a high frequency when cultured in a liquid medium containing naphth-aleneacetic acid and benzyladenine. An experimental system with the isolatedZinnia mesophyll cells has been extensively used as an efficient model system for the study of cytodifferentiation in higher plants, providing many valuable results on cytodifferentiation. In this review, I introduce the results obtained with theZinnia system and discuss the induction and the sequence of differentiation. Recipient of the Botanical Society Award for Young Scientists, 1987.     

Introgressive hybridization in two southern California species of Rhus (Anacardiaceae)

Five populations ofRhus integrifolia andR. ovata were studied and, based upon morphological characteristics and pollen viability, a number of putative hybrids were found in those areas where the two species are sympatric. Diagnostic characteristics most useful for distinguishing the species and their hybrids are leaf morphology, sepal pubescence, petiole length, and petal size. Introgression was evident in at least three populations, which suggested that hybridization between the two species is an ancient phenomenon. Evolutionarily, hybridization between the two taxa has probably been of little consequence.     

Electrophoretic discrimination ofTribolodon species (Cyprinidae) and the occurrence of their hybrids

Electrophoretic discrimination was carried out on three species of the genusTribolodon (Cyprinidae),T. hakonensis, T. brandti andT. ezoe, by examining five loci controlling lactate dehydrogenase (LDH), phosphoglucomutase (PGM), phosphoglucose isomerase (PGI), and muscle protein (MP). Consequently, a clear-cut identification of the three species was possible even in 0⁺ young which were difficult to distinguish morphologically. Moreover, 1.0–19.9% of fish examined were detected electrophoreticaly as hybrids which had been previously undetected. The existence of such spontaneous hybrids has further complicated the classification of the genusTribolodon. In spite of imperfect reproductive isolation, the electrophoretic data supports the independent nature of the three species of the genusTribolodon.     

Reproductive isolation and hybridization in sympatric populations of three Dactylorhiza species (Orchidaceae) with different ploidy levels

Background and Aims

The potential for gene exchange between species with different ploidy levels has long been recognized, but only a few studies have tested this hypothesis in situ and most of them focused on not more than two co-occurring species. In this study, we examined hybridization patterns in two sites containing three species of the genus Dactylorhiza (diploid D. incarnata and D. fuchsii and their allotetraploid derivative D. praetermissa).

Methods

To compare the strength of reproductive barriers between diploid species, and between diploid and tetraploid species, crossing experiments were combined with morphometric and molecular analyses using amplified fragment length polymorphism markers, whereas flow cytometric analyses were used to verify the hybrid origin of putative hybrids.

Key Results

In both sites, extensive hybridization was observed, indicating that gene flow between species is possible within the investigated populations. Bayesian assignment analyses indicated that the majority of hybrids were F₁ hybrids, but in some cases triple hybrids (hybrids with three species as parents) were observed, suggesting secondary gene flow. Crossing experiments showed that only crosses between pure species yielded a high percentage of viable seeds. When hybrids were involved as either pollen-receptor or pollen-donor, almost no viable seeds were formed, indicating strong post-zygotic reproductive isolation and high sterility.

Conclusions

Strong post-mating reproductive barriers prevent local breakdown of species boundaries in Dactylorhiza despite frequent hybridization between parental species. However, the presence of triple hybrids indicates that in some cases hybridization may extend the F₁ generation.