**2. Materials and methods**

Cross-pollinated plant species are the most predisposed to hybridization. Sympatric species are less likely to be cross-species hybridized than allopatric species or populations [6]. The share of hybrid taxa among invasive species of Middle Russia

*Invasive Species - Introduction Pathways, Economic Impact, and Possible Management Options*

For a long time, the most important factor in limiting hybridization was geographical isolation, but nowadays closely related taxa come into contact with each other through a multitude of anthropogenic "corridors" [8]. Thus, inter-regional immigration occurs by means of the introduction of plants, which can be frequent and repetitive, and therefore it significantly increases immigration flow [9]. If we consider the situation where conditions for the hybridization of closely related taxa already exist, there may be several possible developments that coexist: (a) New hybrid taxon may appear, and (b) native taxon disappears. During hybridization, genetic assimilation occurs, and new genes are injected into one or both parent species. Hybrids, even being fertile, can, however, be reproductively isolated from parent plants due to the effect of the selection on reproductive traits (allopolyploidy, heterozygous translocations, recombination, mitochondrial DNA-specific differences) and/or due to factors that predetermine crossing (flowering phenology, separation of ecological niches). Interspecific hybridization may also facilitate the naturalization of rare genotypes and cause an increase in their proportion by inverse crossbreeding with alien parent taxa or hybridization between the hybrids themselves. Greater selection advantages for alien alleles should lead to faster replacement of natural alleles through hybridization and slower replacement without hybridization [10]. The period of displacement (substitution) decreases significantly with increasing immigration flow and selective differentiation. Immigration and selection operate in a variety of ways: increasing immigration levels result in the substitution of native species by suppressing them, while increasing selective differentiation in favor of an alien species results in the substitution of an alien species by genetic assimilation without leaving "pure" native species. At moderate and high immigration rates, the loss of native species can be rapid with or without hybridization. Given the high number of species introduced by humans, the loss of native

species can increase only as a result of hybridization [11].

extinction increases significantly [11].

**96**

Hybridization increases the threat of extinction of many species due to intro-

pollinated species such as oaks. Hybridization and introgression can lead to a hybrid complex consisting of many hybrids due to a large number of loci. Thus, multiloci seem to increase the number of hybrid types and genetic complexes and accelerate the reduction of "pure" natural species. In addition, the large number of loci essentially reduces the probability of having a "pure" individual of any parent origin [11]. Without introgression, hybrids, being reproductively isolated, can quickly form a new species. With introgression, speciation slows down as inverse crosses with parent lines occur. The impact of hybridization and introgression on the rate of substitution of native species by closely related ones has been addressed by a mathematical model involving a one-loop bipartite inheritance scheme with different levels of cross-species hybridization [11]. Although the model did not take into account vegetatively propagating hybrids, the results showed that the substitution of natural taxa by alien ones could occur very quickly (in less than five generations). According to the results, hybridization and introgression can increase the degree of substitution of native species by non-native ones. Introgression increases species substitution with low immigration, but prevents substitution when an indigenous species has a significant advantage in selection as well as with higher immigration levels. However, as introductions are associated with increased frequency of hybrids, the impact on the indigenous taxon remains high, and the likelihood of

gression [12, 13]. High degree of introgression is often manifested by wind-

reaches 10% [7].

DNA was extracted from silica gel dried leaves of *Bidens*, *Solidago*, and *Erigeron* taxa according to the method by Rogers and Bendich [15]. The herbarium specimens are stored in the herbarium of the Tsitsin Main Botanical Garden (MHA). Polymerase chain reaction (PCR) was carried out in a DNA Engine Dyad Peltier Thermal Cycler amplifier (Bio-Rad, United States). For the nuclear ribosomal internal transcribed spacer (ITS) 1-2 (ITS1-ITS2), *nnc18s10* (forward) and *c26A* (reverse) primers with an annealing temperature of 50°С were used. For the chloroplast loci (*rpl32-trnL* and *trnL-trnF* intergenic spacers), primers were used at the annealing temperature from 0.3 to 65°С [16]. For the chloroplast locus *rpl32-trnL*, we used primers *rpl32F* (forward) and *trnL* UAG (reverse), and for the other chloroplast locus *trnL-trnF*, we used primers *c* (forward) and *f* (reverse). Purification of the PCR product for sequencing was carried out in a mixture of ammonium acetate with ethanol. The nucleotide DNA sequences were determined on an automatic sequencer (Syntol). Further processing of the nucleotide sequences was carried out in the BioEdit program. The data were sent to GenBank (2019), in which these nucleotide sequences can be found by the additional numbers assigned to them (**Table 1**). Phylogenetic trees were constructed using SplitsTree4.
