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-speciﬁc and species-speciﬁc and can be used for chromosome identiﬁcation; 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 ﬁrst successful Giemsa C-banding of a forest tree species was on Pinus nigra chromosomes by Borzan and Papes ˇ 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. [...]
[...] Variation in Chromosome Numbers Generally, each species has a characteristic number of chromosomes in each cell (except for gametes) referred to as the somatic number which is typically diploid. Higher organisms have one species-speciﬁc set of homologous chromosomes donated by the male (pollen) the gametic number which is typically haploid), and the other set by the mother (egg). Through evolutionary processes, the number of chromosomes can increase by whole sets (polyploidy) and/or increase or decrease by individual chromosomes (aneuploidy). Polyploidy can occur in different ways, spontaneously or induced. [...]
[...] Forest tree cytogenetic research over much of the twentieth century was dominated by somatic studies on coniferous species, particularly in Pinaceae and Taxodiaceae, using conventional staining methodology. Studies by Saylor, Khosho, Mergen and Burley, Mehra, Hizume, Muratova and Kruklis, Stebbins, Schlarbaum and Tsuchiya, Borzan, and Toda and other representative studies were quoted by Schlarbaum, in his review of cytogenetic studies of forest trees. Many studies were botanical in nature, investigating inter- and intraspeciﬁc variation, cyto-taxonomy, and phylogeny. In the latter part of the twentieth century, cytogenetic studies of trees exposed to air, heavy metal, and radioactive pollution were made under difﬁcult conditions, and demonstrated the effects of pollution on the meiotic and mitotic processes. [...]
[...] Cytogenetic improvement of hardwood species shows more promise than coniferous species. Some species have relatively short juvenile periods that would not greatly inhibit an integrated cytogenetic/ breeding approach to improvement. Ploidy changes, either natural or induced, are not a problem in many species, and euploid changes from the diploid state have been shown to increase yield in some species. Studies have shown that triploidy is the optimal level for growth in Populus and could be for some Quercus species. In general, cytogenetic manipulation of hardwood species is a vast reservoir of potential waiting to be explored. [...]
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