The mating system and the temporal dynamics of heterozygosity in five P. pumila natural populations from the Pacific region, optimal for the species, were studied using isozyme analysis. Seed samples were collected in northern Koryakia, southern Kamchatka (three samples), and the Kunashir Island (Kurils). The highest level of outcrossing was observed in population from Koryakia (tm=0.939). It may be due to the absence of dense stands which put obstacles in the way of pollen dispersal. In the case of old and continuous thickets (populations from Kamchatka and Kunashir) the outcrossing level is lower (tm=0.919 – 0.568). It may be caused by considerable pollen sedimentation at maternal trees. Wright’s fixation index was used to estimate the level of inbreeding in embryos from dormant seeds resulting from open pollination and in maternal plants. A substantial level of inbreeding was observed in the embryo samples but not in four out of the five adult samples. The inbreeding level at the reproductive age was higher only in the sample from Koryakia, which can be explained by a relatively young age of plants in the population frequently affected by fires. A general increase in heterozygosity in the course of ontogeny, characteristic of other conifers as well, was attributed to elimination of inbred progenies and by balancing selection in favor of heterozygotes, which is a key factor maintaining allozyme polymorphism in populations. The study is supported by the Programs of Fundamental Research (PFR) of the Presidium of Russian Academy of Sciences «Biological Diversity and the Gene Pools» and «The Origin of Biosphere and Evolution of Geo- and Biological Systems», by PFR of Division of Biological Sciences «Biological Resources of Russia».

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