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PubMed1975

The mating system and microevolution.

Studies of natural and experimental plant populations have revealed that genotypic frequency distributions are highly structured in predominantly selfing species. This high degree of genetic organization is manifested in intense correlations in allelic state over loci and also in striking micro-geographical heterogeneity. Both aspects of this structure are facilitated by self-fertilization. Development of non-random associations of alleles within populations is facilitated because inbreeding reduces heterozygosity and thus also the randomizing effect of recombination. Spatial differentiation is facilitated because self-fertilization retards gene flow from population to population. The effect of organizing the entire populational genotype into a sort of giant supergene is to increase the frequency in the population of genotypes which confers high fitness and hence to increase adaptation to the local environment. However the recombinational potential remains substantial. As a result considerable free genetic variability remains in the population and it is available for long-term response to natural selection. Thus the organization of genetic variability within populations provides for high immediate fitness and also for flexibility to meet longer term evolutionary needs. At the same time selfing is a barrier to migration and it promotes the development and maintenance of different multilocus organizations in adjacent populations occupying unlike habitats. In total, therfore, a pattern of genetic differentiation develops in space which is an almost exact overlay of the environmental heterogeneity. The plant genetic and plant breeding literature contains extensive evidence that the mating system in plants can be modified simply and drastically by selection and that different populations within the same species often practice very different amounts of inbreeding. Considering the ease with which the mating system can be altered, and the benefits of adjusting genetic variability through regulation of the mating system, it is not surprising that a high proportion of flowering plants self-fertilize to some extent and that at least one-third of species have adopted predominant selfing as a strategy in ecogenetic adaptation.

Adaptation, Biological↗
PubMed2006

A meta-analysis of genetic correlations between plant resistances to multiple enemies.

Genetic correlations between plant resistances to multiple natural enemies are important because they have the potential to determine the mode of selection that natural enemies impose on a host plant, the structure of herbivore and pathogen communities, and the success of plant breeding for resistance to multiple diseases and pests. We conducted a meta-analysis of 29 published studies of 16 different plant species reporting a total of 467 genetic correlations between resistances to multiple herbivores or pathogens. In general, genetic associations between resistances to multiple natural enemies tended to be positive regardless of the breeding design, type of attacker, and type of host plant. Positive genetic correlations between resistances were stronger when both attackers were pathogens or generalist herbivores and when resistance to different enemies was tested independently, suggesting that generalists may be affected by the same plant resistance traits and that interactions among natural enemies are common. Although the mean associations between resistances were positive, indicating the prevalence of diffuse selection and generalized defenses against multiple enemies, the large variation in both the strength and the direction of the associations suggests a continuum between pairwise and diffuse selection.

Animals↗
PubMed2000

An efficient screen for reproductive pathways using mature seeds of monocots and dicots.

Seed samples of 32 species (obligate and facultative sexuals and apomicts of monocots and dicots) were investigated by flow cytometry to reveal the pathway of reproduction. Ten different pathways of seed formation could be reconstructed considering whether the female and/or male gametes were reduced or unreduced, the embryos arose via the zygotic or parthenogenetic route and the endosperm via the pseudogamous or autonomous route. The screen is suited to select sporophytic or gametophytic mutants in sexual species, to identify pure sexual or obligate apomictic genotypes from facultative apomictic species, and to analyze the inheritance of the individual reproductive processes. Corresponding unique results are presented for Arabidopsis, Arabis, Hypericum and Poa. The screen of mature seeds by flow cytometry yielded more information about the reproductive behavior of individual plants than any other available test, and is very useful both in basic research and plant breeding.

Plant Physiological Phenomena↗
PubMed1995

Origin and evolution of viroids and viroid-like satellite RNAs.

Viroids, the smallest and simplest agents of infectious disease, cause a number of economically important diseases of crop plants. Present evidence indicates that most of these diseases originated recently (in the 20th century) by chance transfer of viroids from endemically infected wild plants or by use of viroid-infected germplasm during plant breeding. Modern agricultural practices, such as widespread monoculture of genetically identical plants, and worldwide distribution of planting material, has made it possible for the pathogens to maintain themselves in the crop plants and to conquer new territories. Phylogenetic analysis of their nucleotide sequences indicates that viroids and satellite RNAs represent a monophyletic group, with all but the two self-cleaving viroids forming one cluster and the satellite RNAs another. The two self-cleaving viroids are phylogenetically distant from either cluster; they may represent ancestral forms. Results from site-directed mutagenesis experiments indicate that, upon exposure to selective pressures, viroids can evolve extremely rapidly, with another, fitter, component of the quasi-species often becoming dominant within days or weeks. This extreme plasticity of their nucleotide sequences establishes viroids as the most rapidly evolving biological system known.

Base Sequence↗
PubMed2006

Heterosis associated gene expression in maize embryos 6 days after fertilization exhibits additive, dominant and overdominant pattern.

Heterosis is important for conventional plant breeding and is intensively used to increase the productivity of crop plants. Genetic processes shortly after fertilization might be of particular importance with respect to heterosis, because coordination of the diverse genomes establishes a basis for future performance of the sporophyte. Here we demonstrate a strong crossbreeding advantage of hybrid maize embryos as early as 6 days after fertilization in a modern maize hybrid and provide the first embryo specific analysis of associated gene expression pattern at this early stage of development. We identified differentially expressed genes between hybrid embryos and the parental genotypes by a combined approach of suppression subtractive hybridization and differential screening by microarray hybridizations. Association of heterosis in embryos with genes related to signal transduction and other regulatory processes was implied by the enrichment of these functional classes among the identified gene set. Quantitative RT-PCR analysis validated the expression pattern of 7 of 12 genes analysed and revealed predominantly additive, but also dominant and overdominant expression patterns in hybrid embryos. These patterns indicate that gene regulatory interactions among parental alleles act at this early developmental stage and the genes identified provide entry points for the exploration of gene regulatory networks associated with the specification of the phenomenon heterosis in the plant life cycle.

Base Sequence↗
PubMed2006

Ferredoxin limits cyclic electron flow around PSI (CEF-PSI) in higher plants--stimulation of CEF-PSI enhances non-photochemical quenching of Chl fluorescence in transplastomic tobacco.

We tested the hypothesis that ferredoxin (Fd) limits the activity of cyclic electron flow around PSI (CEF-PSI) in vivo and that the relief of this limitation promotes the non-photochemical quenching (NPQ) of Chl fluorescence. In transplastomic tobacco (Nicotiana tabacum cv Xanthi) expressing Fd from Arabidopsis (Arabidopsis thaliana) in its chloroplasts, the minimum yield (F(o)) of Chl fluorescence was higher than in the wild type. F(o) was suppressed to the wild-type level upon illumination with far-red light, implying that the transfer of electrons by Fd-quinone oxidoreductase (FQR) from the chloroplast stroma to plastoquinone was enhanced in transplastomic plants. The activity of CEF-PSI became higher in transplastomic than in wild-type plants under conditions limiting photosynthetic linear electron flow. Similarly, the NPQ of Chl fluorescence was enhanced in transplastomic plants. On the other hand, pool sizes of the pigments of the xanthophyll cycle and the amounts of PsbS protein were the same in all plants. All these results supported the hypothesis strongly. We conclude that breeding plants with an NPQ of Chl fluorescence increased by an enhancement of CEF-PSI activity might lead to improved tolerance for abiotic stresses, particularly under conditions of low light use efficiency.

Amino Acid Sequence↗
PubMed2002

Three criteria for establishing the usefulness of biotechnology for reducing micronutrient malnutrition.

The fundamental reason that plant breeding using either conventional breeding or biotechnology is so cost-effective is that the benefits of a one-time investment at a central research location can be multiplied over time across nations all over the world. Supplementation and fortification incur the same recurrent costs year after year in country after country. However, each intervention has its own comparative advantages, such that a combination of several interventions is required to substantially reduce micronutrient malnutrition. Improving the density of trace minerals in plants also reduces input requirements and raises crop yields. A simulation model for India and Bangladesh demonstrated that $42 million invested in conventional breeding in developing and planting iron- and zinc-dense varieties of rice and wheat on only 10% of the acreage used for these crops would return $4.9 billion in improved nutrition (including a total of 44 million prevented cases of anemia over 10 years) and higher agricultural productivity.

Biotechnology↗
PubMed2005

Early Mendelism and the subversion of taxonomy: epistemological obstacles as institutions.

This paper presents and discusses a series of hybridization experiments carried out by Nils Herman Nilsson-Ehle between 1900 and 1907 at a plant breeding station in Svalöf, Sweden. Since the late 1880s, the Svalöf station had been renowned for its 'scientific' breeding methods, which basically consisted of an elaborate system of record-keeping through which the offspring of individual plants were traced over generations while being meticulously described. This record system corresponded to a certain breeding technique (the so-called 'pedigree method') and certain theoretical convictions (mutational rise of new varieties, and non-productivity of selection). Inspired by Tschermack's translation of Mendel's Pisum-paper, Nilsson-Ehle began his experiments in 1900 and published a first, major synthesis of his findings in 1908. If one compares these experiments as documented in the breeding records, with their representations in print, one encounters discrepancies in terms of procedure and presentation of data. This can be explained by the fact that Nilsson-Ehle was obliged to follow the recording and breeding procedures institutionalised at Svalöf, and these procedures, grounded in a taxonomic discourse, left little room for Mendelian hybridisation experiments. The twists and turns that this story takes are analysed in terms of Bachelardian philosophy of science, where the 'epistemological obstacle' functions as a central, analytic category. In contrast to Bachelard, however, I will characterise these obstacles as being of an institutional, rather than mental, nature. Thus characterized, moreover, they turn out to have been prerequisites as much as barriers to scientific progress.

Academies and Institutes↗
PubMed1992

A viewpoint on the taxonomy of potyviruses infecting sugarcane, maize, and sorghum.

The value of taxonomy lies in its ability to communicate concepts and relationships. Our present concepts of the poaceous potyviruses, based on their biology, serology, and biochemistry, identify four viruses that can be distinguished by each characteristic. Identifying these as four distinct viruses has important implications for disciplines such as epidemiology, plant breeding, and diagnostics.

Edible Grain↗
PubMed1983

Prospects in plant genetic engineering.

The functional expression of a novel gene in a genetically engineered plant has not yet been reported. One major barrier in movement toward this goal is our limited understanding of the molecular bases of gene expression. Attempts to establish genetic engineering as a practical facet of plant breeding are also complicated by the fact that genes for most important plant characteristics have not yet been identified. However, the benefits to be gained from all aspects of plant improvement are stimulating research into both the development of plant transformation technology and the isolation and characterization of genes responsible for valuable traits. As scientists develop greater knowledge of plant molecular genetics, we can expect to see practical applications in such diverse areas as improvement of plant nutritional quality, decreases in fertilization requirements, and increases in resistance to environmental stresses and pathogens.

Agriculture↗
PubMed2000

[RAPD-markers linked to the locus for resistance to the race 4 pathogen for black rot, Xanthomonas campestris pv. campestris (Pamm.) Dow., in Brassica rapa L].

Association between the RAPD markers and the resistance to race 4 of the black rot causative agent was studied in Brassica rapa L. Experiments were carried out using doubled haploid lines, obtained via crosses between the race 4-susceptible fodder turnip and resistant pak-choi, and the F2 progeny of the crosses between the doubled haploid lines with contrasting resistance. The WE(22)980 RAPD marker inherited from the pak-choi and associated with the clubroot susceptibility was also linked to the locus responsible for the resistance to race 4 of Xanthomonas campestris pv. campestris. The two other RAPD markers were linked to susceptibility to black rot. Simultaneous association of the same DNA markers with the resistance/susceptibility to two different obligate pathogens favored the hypothesis on cluster organization of the resistance genes in plants. The markers described can be used in plant breeding and in further investigation of the genetic bases of resistance in plants.

Base Sequence↗
PubMed2005

Microspore-derived embryogenesis in pepper (Capsicum annuum L.): subcellular rearrangements through development.

Background information. In vitro-cultured microspores, after an appropriate stress treatment, can switch towards an embryogenic pathway. This process, known as microspore embryogenesis, is an important tool in plant breeding. Basic studies on this process in economically interesting crops, especially in recalcitrant plants, are very limited and the sequence of events is poorly understood. In situ studies are very convenient for an appropriate dissection of microspore embryogenesis, a process in which a mixture of different cell populations (induced and non-induced) develop asynchronically.Results. In the present study, the occurrence of defined subcellular rearrangements has been investigated during early microspore embryogenesis in pepper, an horticultural crop of agronomic interest, in relation to proliferation and differentiation events. Haploid plants of Capsicum annuum L. (var. Yolo Wonder B) have been regenerated from in vitro anther cultures by a heat treatment at 35 degrees C for 8 days. Morphogenesis of microspore-derived embryos has been analysed, at both light and electron microscopy levels, using low-temperature-processed, well-preserved specimens. The comparison with the normal gametophytic development revealed changes in cell organization after embryogenesis induction, and permitted the characterization of the time sequence of a set of structural events, not previously defined in pepper, related to the activation of proliferative activity and differentiation. These changes mainly affected the plastids, the vacuolar compartment, the cell wall and the nucleus. Further differentiation processes mimicked that of the zygotic development.Conclusions. The reported changes can be considered as markers of the microspore embryogenesis. They have increased the understanding of the mechanisms controlling the switch and progression of the microspore embryogenesis, which could help to improve its efficiency and to direct strategies, especially in agronomically interesting crops.

Capsicum↗
PubMed2002

Rate and pattern of mutation at microsatellite loci in maize.

Microsatellites are important tools for plant breeding, genetics, and evolution, but few studies have analyzed their mutation pattern in plants. In this study, we estimated the mutation rate for 142 microsatellite loci in maize (Zea mays subsp. mays) in two different experiments of mutation accumulation. The mutation rate per generation was estimated to be 7.7 x 10(-4) for microsatellites with dinucleotide repeat motifs, with a 95% confidence interval from 5.2 x 10(-4) to 1.1 x 10(-3). For microsatellites with repeat motifs of more than 2 bp in length, no mutations were detected; so we could only estimate the upper 95% confidence limit of 5.1 x 10(-5) for the mutation rate. For dinucleotide repeat microsatellites, we also determined that the variance of change in the number of repeats (sigma(m)2) is 3.2. We sequenced 55 of the 73 observed mutations, and all mutations proved to be changes in the number of repeats in the microsatellite or in mononucleotide tracts flanking the microsatellite. There is a higher probability to mutate to an allele of larger size. There is heterogeneity in the mutation rate among dinucleotide microsatellites and a positive correlation between the number of repeats in the progenitor allele and the mutation rate. The microsatellite-based estimate of the effective population size of maize is more than an order of magnitude less than previously reported values based on nucleotide sequence variation.

Alleles↗
PubMed2004

Overexpression of the acidic dehydrin WCOR410 improves freezing tolerance in transgenic strawberry leaves.

Progress in freezing tolerance (FT) improvement through plant breeding approaches has met with little success in the last 50 years. Engineering plants for greater FT through plant transformation is one possible way to reduce the damage caused by freezing. Here, we report an improvement of the selection procedure and the transfer of the wheat Wcor410a acidic dehydrin gene in strawberry. The encoded protein has previously been shown to be associated with the plasma membrane, and its level of accumulation has been correlated with the degree of FT in different wheat genotypes. The WCOR410 protein was expressed in transgenic strawberry at a level comparable with that in cold-acclimated wheat. Freezing tests showed that cold-acclimated transgenic strawberry leaves had a 5 degrees C improvement of FT over wild-type or transformed leaves not expressing the WCOR410 protein. However, no difference in FT was found between the different plants under non-acclimated conditions, suggesting that the WCOR410 protein needs to be activated by another factor induced during cold acclimation. These data demonstrate that the WCOR410 protein prevents membrane injury and greatly improves FT in leaves of transgenic strawberry. A better understanding of the limiting factors allowing its activation may open up the way for engineering FT in different plant organs, and may find applications for the cryopreservation of human tissues and organs.

Journal Article↗
PubMed2001

[Studies of the meiosis of 2n gamete apomictic wheat grass (Elymus rectisetus)].

The formation of polyploid was mainly through the combination of 2n gamete, and the apomixis played an important role in plant evolution and plant breeding. In general, the formation of male gamete and female gamete in apomictic plants are obviously different, i.e., the formation of embryo sac was through apomixis (form into 2n female gamete); however, the formation of male gamete was through normal meiosis (form into 1n male gamete). The 2n male gamete apomictic wheat grass was more important than 1n male gamete apomictic wheat grass, because the 2n male gamete can bring all genes including apomictic gene into other plant. The meiosis of 2n male gamete apomictic wheat grass (Elymus. rectisetus) was observed through Olympus AH3 microscope; the results showed that the meiosis of 2n gamete apomictic wheat grass was very abnomal. In the interphase, lots of mini-nucleus were found. Inverted cycle, multibivalent and lagging chromosome were also found in the prophase. In metaphase, chromosomes frequently show unequal division tendency; In anaphase I and telophase I, completely unequal division, modified meiosis, the tendency of nucleus fusion and other abnormal phenomena were found. These phenomena directly lead to the production of 2n gamete. In meiosis II, the abnormalities mainly lie in the proceeding of microspore, dyad (55.7%), triad (23.7%) and tetrad (21.6%) were found. The abnormal of those proceeding also lead to the production of 2n gamete.

Meiosis↗
PubMed2003

DNA, chromosomes, and in situ hybridization.

In situ hybridization is a powerful and unique technique that correlates molecular information of a DNA sequence with its physical location along chromosomes and genomes. It thus provides valuable information about physical map position of sequences and often is the only means to determine abundance and distribution of repetitive sequences making up the majority of most genomes. Repeated DNA sequences, composed of units of a few to a thousand base pairs in size, occur in blocks (tandem or satellite repeats) or are dispersed (including transposable elements) throughout the genome. They are often the most variable components of a genome, often being species and, occasionally, chromosome specific. Their variability arises through amplification, diversification and dispersion, as well as homogenization and loss; there is a remarkable correlation of molecular sequence features with chromosomal organization including the length of repeat units, their higher order structures, chromosomal locations, and dispersion mechanisms. Our understanding of the structure, function, organization, and evolution of genomes and their evolving repetitive components enabled many new cytogenetic applications to both medicine and agriculture, particularly in diagnosis and plant breeding.

Animals↗
PubMed1996

Comparative analysis of plant genome architecture.

Many genes are similar in most plants and it is clear that the ordering of genes is highly conserved across wide taxonomic groupings. Repetitive DNA, consisting of sequence motifs between 2 and 10,000 base pairs long, repeated many hundreds or thousands of times in the genome, represents the majority of most plant genomes and defines some of the differences between species. Some sequences are highly conserved in many species, while other sequences show species or even chromosome specificity. Different types of sequences have markedly contrasting genomic distributions; even among tandem repeats, some are sub-terminal, some paracentromeric and others intercalary. The reasons for these different distributions are largely unknown, and mechanisms of homogenization, dispersion and amplifications are the subject of much speculation. Aspects of plant genome architecture-the organization of repetitive and single-copy DNA sequences along the chromosomes, and the positioning of those sequences within the nucleus at interphase-have important consequences for plant genetics. Models of large scale genome organization may be useful in learning the function of different components of the genome, in evolutionary studies and in plant breeding.

Biological Evolution↗
PubMed2005

Allelic reduction and genetic shift in the Canadian hard red spring wheat germplasm released from 1845 to 2004.

Analysis of genetic diversity changes in existing gene pools of cultivated crops is important for understanding the impact of plant breeding on crop genetic diversity and developing effective indicators for genetic diversity of cultivated plants. The objective of this study was to assess genetic diversity changes in 75 Canadian hard red wheat (Triticum aestivum L.) cultivars released from 1845 to 2004 using 31 simple sequence repeats (SSRs) markers. A total of 267 SSR alleles were detected, and their allelic frequencies ranged from 0.01 to 0.97, with an average of 0.14. Significant allelic reduction was observed at only four SSR loci for the cultivars released from 1970 onwards. However, 51 alleles (about 19%) present in pre-1910 cultivars were undetected in cultivars released after 1990 and were spread over 27 SSR loci. The proportion of SSR variation accounted for by six breeding periods was 12.5%, by four ancestral families, 16.5%, and by eight breeding programs, 8.4%. The average genetic diversity measured by three different band-sharing methods did not change significantly among cultivars released from different breeding periods, breeding programs, and ancestral families. However, genetic shift was obvious in the cultivars released over the six breeding periods, reflecting well the various breeding efforts over years. These results clearly show the allelic reduction and genetic shift in the Canadian hard red spring wheat germplasm released over time. Consequently, more effort needs to be made to broaden the wheat breeding base and conserve wheat germplasm.

Analysis of Variance↗