The term ‘genetic system' was coined in 1932 by C.D. Darlington, one of the renowned pioneers of cytogenetics. His original deﬁnition 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 deﬁnition have later been elaborated as follows. Genetic system refers to any of the species-speciﬁc 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 difﬁcult. 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). [...]
[...] Neither of these systems is absolute; spontaneous selﬁng does take place, and controlled self-pollination results in some germinable seeds. The inbred seedlings usually display strong inbreeding depression, and most soon die under competition. Self-fertilization can occasionally produce offspring of full vigor, through fortuitous lack of genetic load in parents or fortuitous occurrences of balanced heterozygotes. In this respect the typical genetic system of a gymnosperm is highly ﬂexible in its stochastic (probabilitistic) discrimination against results of self- fertilization rather than self-incompatibility. [...]
[...] Mating Pattern and Gene Flow The mating pattern, or breeding system, is the second fundamental part of the genetic system. Mating pattern refers to the mode of combining haploid female and male gametes, which leads to the formation of a diploid zygote, embryogeny, and a new individual. The classical function of sexual reproduction is based on cross-fertilization, i.e., the female gamete and male gamete originate from different parents. This process requires cross- pollination, with pollen transported from one individual to the pollen- receptive site of the seed parent. [...]
[...] Because there are several archegonia and pollen grains in each ovule, a sound seed may still develop despite the abortion of one or more embryos, if there is at least one outcrossed zygote (or a ‘balanced heterozygote' from selﬁng). Self-incompatibility and embryonic lethals may be considered a part of the genetic load because they restrict seed production. In combination, however, they maintain high outcrossing rates despite partial self- pollination. Outcrossing and subsequent heterozygosity must be advantageous in long-lived plants, especially trees, as either self-incompatibility or embryonic lethals in combination with archegonial polyembryony are so predominant. [...]
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