Studies were conducted on the formation mechanisms of unreduced microspores in diploid, polyploid and mutant forms of birch (Betula), oak (Quercus), poplar (Populus), and elm (Ulmus). Unreduced gametes were observed to occur spontaneously, under the influence of high temperature during microsporogenesis or in meiotic mutant genotypes. Depending upon genotype and meteorological conditions, the frequency of unreduced microspores varies from 0.22 to 30.0 percent. Sharp temperature fluctuations during premeiotic periods and meiosis can cause production of abnormally high numbers of unreduced gametes. Formation in diploid species was shown to be controlled by genes that affect the meiotic process, e.g., synapsis. In autotriploids, 2x gametes were formed by the random distribution of all univalent chromosomes to one pole. Unreduced gametes are formed in allotriploids as a result of unequal segregation of the genomes. By subjecting branches undergoing meiosis to chemical mutagens or thermal treatment, 2n microspores develop as a result of abnormal chromosome synapsis and the failure of first meiotic division. Genotypes that produce high numbers of unreduced gametes have been isolated for use in the generation of triploids. Additionally, protocols have been developed to generate a high percentage (94%) of 2n pollen in a diploid Populus species.
Key words: hardwood, unreduced gamete, diploid, polyploid, meiosis.
Correspondence: Olga Mashkina, Research Institute of Forest Genetics and Breeding, Lomonosov str., 105, Voronezh, 394043, Russia