Populus × moskoviensis in the south of Eastern Siberia

The results of a comparative morphological analysis of qualitative traits and phenotypic composition in populations confined to the zone of historical overlap between the ranges of P. laurifolia and P. suaveolens in the south of Eastern Siberia are presented. As preliminary studies showed, trees similar to P. suaveolens grow in the studied area, but their shoots were ribbed like those of P. laurifolia. Therefore, the authors of this work put forward the hypothesis that the studied plants currently represent a hybridogenic species (P. x moskoviensis), which was formed as a result of frequent crossings with each other and, possibly, P. suaveolens might be from both P. laurifolia and P. suaveolens. This work aims to study the features of the morphological characteristics of poplars and the phenotypic composition of the studied plantings, their comparative analysis with P. laurifolia and P. suaveolens to identify the nature, type of hybrid zone and its structure. To achieve this aim, the authors studied two populations of P. x moskoviensis located in the valley of the Irkut River (in the vicinity of Shelekhov) in the Irkutsk region. The comparative morphological analysis used materials from the studied populations of P. laurifolia from the territory of the Altai-Sayan mountainous country and P. suaveolens of the Chukotka Autonomous Okrug, remote from the hybridization zones (See Table 1). In each population, 30 reproductively mature trees were randomly examined. Herbarium material was collected from the southern side of the middle part of the crown. From each individual, 15 fully developed, undamaged leaves from shortened shoots were selected. At the same time, a complex of characteristics, which were considered qualitative by the authors, was studied, namely, the shape of the leaf blade, its apex and base, and the presence of basal glands (glands on top at the junction of the petiole and the leaf blade). F or each population, morph frequencies and indicators characterizing phenotypic variability were calculated. The belonging of individuals to a certain phenotype was determined by a combination of morphological qualitative characteristics. The investigation of the morphological characters of P. x moskoviensis individuals in the studied populations showed that they, like all representatives of the Tacamahaca section, including parental taxa, are characterized by differentiation of shortened crown shoots into discoblasts and leptoblasts (See Fig. 1). The elongated shoots of the crown are cylindrical, while the shoots of the coppice shoots are ribbed to the very base (See Fig. 2), ribs descend in threes from each leaf scar; in young individuals, ribbing is often visible on the shoots of the second year, which is generally characteristic of P. laurifolia, and according to this trait, P. x moskoviensis completely deviates from this parent species (See Fig. 3). The morphological characteristics of the leaf are intermediate in nature - the leaf blades on short shoots are ovoid or elliptical, with a rounded wedgeshaped base and a gradually pointed apex (See Table 2). Hybrids, like P. suaveolens, are characterized by the presence of a finely wrinkled, “shagreen” upper side of the leaf plastic, a feature not noted by P. laurifolia. The ovate-triangular plates typical ofP. laurifolia and the obovate plates typical of P. suaveolens were not observed, as is the short pointed apex characteristic of the latter. P. laurifolia is not characterized by the development of basal glands. On the contrary, in P. suaveolens stands, individuals with a pronounced trait predominated, although at the endogenous level it varies greatly from complete absence to 1-4 glands at the base of the plate. In P. x moskoviensis plantations, glands were found in only 10 individuals of one of the studied populations. Pairwise comparison of the phenotypic composition of all samples using the chisquare test and Zhivotovsky’s I-test of identity showed that the observed differences between the studied populations are reliable. At the same time, in terms of the phenotypic similarity of populations (r), the studied stands within the same species are 67.0-93.7% similar, and the populations of P. x moskoviensis are closer to P. laurifolia (See Table 3). Based on the combination of the studied qualitative morphological characters, 9 phenotypes were identified in the studied populations of P. laurifolia, 12 of P. suaveolens, and only 6 of P. x moskoviensis (See Table 4). The territory of the historical overlap of the ranges of P. laurifolia and P. suaveolens in the south of Eastern Siberia from the river. Kan in the northwest and eastern edge of the the Selenga River basin. The Selenga in the south is a zone of wide-type sympatric hybridization. The crossing zone between P. laurifolia and P. suaveolens identified in this study is an evolutionary new hybrid zone. This type of hybrid zone is not typical for Populus in Siberia. Throughout the entire area of overlapping habitats, the hybrid species P. x moskoviensis is widespread, resulting from numerous crosses between hybrids of different generations and their backcrossings with P. suaveolens, while P. laurifolia and, as a consequence, F1 hybrids are not recorded. In common hybrid individuals, while externally similar to P. suaveolens (primarily the “shagreen” upper side of the leaf blade), a set of qualitative morphological characteristics of the leaf, different from the parent species, is observed. A sign that indicates the parental participation of P. laurifolia is the ribbing of the surface of the shoots. It shows high heritability across different hybridization patterns in Populus species. The observed structure of the hybrid zone in the Baikal region might have been formed as a result of long-term asymmetric introgressive hybridization provoked by the expansion of P. suaveolens after glaciation into the territory previously inhabited by P. laurifolia. The article contains 3 Figures, 4 Tables and 38 References. The Authors declare no conflict of interest.

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