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Cytogenetics of forest tree species

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  1. Introduction.
    1. Plant breeding.
    2. Cytogenetic investigations of forest tree species.
    3. The main interest in forest tree cytogenetics in the early 1900's.
  2. Cell division and chromosomes.
    1. Cell division - nuclear (karyokinesis) and cytoplasmic (cytokinesis) division.
    2. Mitosis.
    3. Meiosis.
    4. The interkinesis stage.
  3. Variation in chromosome numbers.
    1. Polyploidy.
    2. Eneuploidy.
    3. Basic chromosome number.
  4. Slide preparation methodology.
    1. Uniform chromosome staining.
    2. Karyotype analysis.
    3. Banding methods.
  5. Applications of cytogenetics to basic genetic research in forest trees.
  6. Applications of cytogenetics to tree improvement.
  7. Conclusion.

The discipline of cytogenetics was ?rst de?ned by Sutton in 1903, as a ?eld of investigation which developed from the separate sciences of genetics and cytology. It is concerned with studies on the correlation of genetic and cytological (especially chromosomal) features characterizing a particular genetic system under investigation. With respect to forest trees, cytogenetic studies have generally been limited to chromosome studies, on the number, appearance, and behavior of chromosomes during mitosis and meiosis, chromosomal and karyotypic evolution, and the role of chromosomes in the transmission and recombination of genes. Plant breeding can be traced to the ancient Babylonians, but a clear understanding of genetics has its beginning in the nineteenth century with Mendel's hybridization experiments and their subsequent rediscovery by de Vries, Correns, and von Tschermack in 1900. Cytology required the invention of the microscope, and began when Robert Hook observed cork cells in 1665. Early scientists studied cell structure, organelles, and division. Nageli ?rst described chromosomes as visual bodies during cell division in 1844, and Fleming in 1882 described the complete process of mitotic nuclear division. However, it was not until the independent observations of Sutton and Boveri that chromosomes were ?rst linked with the emerging ?eld of genetics.

[...] Constitutive heterochromatin is chromosome-speci?c and species-speci?c and can be used for chromosome identi?cation; it is cold-sensitive, late-replicating, and genetically inert, and usually contains highly repetitive DNA sequences. After Pardue and Gall's paper in 1970 showed that Giemsa dye stained centromeres of mouse chromosomes more strongly than other chromatin, the Giemsa C-banding technique became the most widely used banding method for both animal and plant chromosomes. The ?rst successful Giemsa C-banding of a forest tree species was on Pinus nigra chromosomes by Borzan and Papes ? in 1978 on haploid chromosomes in the female gametophytic tissue. [...]

[...] Applications of Cytogenetics to Basic Genetic Research in Forest Trees Prior to the advent of molecular biology and in-situ hybridization of probes directly on chromosomes, physical gene mapping was essentially nonexistent in forest tree species. Agronomic and horticultural approaches that use chromosomal aberrations, e.g., translocations, or aneuploidy, such as monosomics or trisomics, in combination with breeding are generally not possible with coniferous species. Most conifers do not tolerate aberrations and aneuploid changes which usually affect growth and reproduction. With hardwood species, cytogenetic characterization of the different species was too limited to conduct mapping experiments. [...]

[...] The main interest in forest tree cytogenetics in the early 1900s was in discovering and interpreting the process of fertilization in pines. Ferguson conducted very detailed observations on the development of the egg cells, fertilization, and microsporogenesis in various pine species (Pinus strobus, P. nigra, P. rigida, P. resinosa, and P. uncinata). She determined the precise number of chromosomes in the haploid state for these pine species (n=12). Early embryological research, such as Ferguson's, also revealed that chromosomes of coniferous species are relatively large and easily investigated by techniques used at that time, including sectioning and chromosome smears. [...]

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