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Genetic systems of forest trees

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  1. Introduction.
  2. Cytological factors.
  3. Mating pattern and gene flow.
    1. The mating pattern.
    2. Outcrossing plant species.
    3. Metandry.
    4. Wind pollination - characteristic of gymnosperms.
    5. Self-incompatibility and embryonic lethals.
  4. Mode of reproduction.
  5. Isolation.
    1. Spatial isolation.
    2. Temporal isolation.
    3. Incompatibility.
    4. Hybrid sterility.
  6. Forest trees.
    1. Conifers.
    2. Angiosperms.
  7. Conclusion.

The term ?genetic system' was coined in 1932 by C.D. Darlington, one of the renowned pioneers of cytogenetics. His original de?nition was limited: Properties of heredity and variation, methods of reproduction and the control of breeding, we now realize, are in various ways bound up together in each group of organisms. They constitute a genetic system. The genetic systems of different groups of organisms differ widely. The concept and its de?nition have later been elaborated as follows. Genetic system refers to any of the species-speci?c ways of organization and transmission of the genetic material, which determines the balance between coherence and recombination of genes and control the amount and type of gene combinations. Evolution of the genetic systems means the evolution of those mechanisms effecting and affecting genetic variability. The genetic information in the nucleus is packed in structures called chromosomes. Each chromosome contains genes in a linear arrangement, with its genes linked together in a consistent sequence, such that the gene programming a given protein (and all its resulting functions) is at a particular position or locus within its chromosome. For higher plants the basic state is diploid, such that there are two homologous versions of each chromosome, one from the mother and another from the father.

[...] The chromosomes of broadleaved trees are very small, which does not mean that they contain less genetic information than the large chromosomes of conifers. The minute size causes problems in cytological studies. Even the counting of the exact number is tedious, and a detailed survey of meiosis is most dif?cult. The chromosome numbers vary widely. This is not surprising because the group consists of various taxonomic categories. Polyploidy has played an important role in species formation, and different levels of ploidy are found even within one genus (e.g., Betula). [...]

[...] The sustainability of forest ecosystems and the maintenance of genetic diversity may be threatened by exploitation and changes in land use. As the genetic system and its components determine the capability of a population to adapt and to undergo evolutionary changes, the components promoting genetic variability and regeneration are considered to be of utmost importance. From the biological point of view, however, isolation mechanisms must not be neglected. Introduced tree species may hybridize with autochthonous ones, which is usually undesirable (e.g., in black poplar, Populus nigra). [...]

[...] Conclusion The life-form and strategy of forest trees is typically coupled with high degrees of heterozygosity and potential to produce broad genetic variation in the offspring. The genetic system of successful species must have met this requirement. These features of the genetic system appear to have been crucial to the long-term success of forest tree species. The profuse production of pollen and seed becomes understand- able against this background. A sound knowledge of the structure and functioning of the genetic system of trees not only helps to understand trees' life but it is also essential when planning the management of genetic resources. [...]

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