**1. Introduction**

There is a hypothesis that the strengthening of hybridization processes in the secondary distribution range contributes to more successful existence of plants in their new homeland [1, 2]. Under unusual conditions, alien species can form hybrids with closely related native species, as well as with other alien plants inhabiting a given area. Often, hybrids are better adapted to secondary distribution range conditions than parent taxa [3–5], resulting in landscapes in a new home area. Successful recombination of genetic traits of parent species reduces the lag phase (a period of adaptation of an alien taxon to new conditions during which there is not yet active introduction into natural phytocoenosis and expansion of the secondary distribution range) and leads to the formation of new active "species transformers." 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 reaches 10% [7].

It is known that the highest invasive activity is exhibited by species of Asteraceae family [14], so we have focused our attention on representatives of this group of alien species. It is often impossible to say with certainty whether plants with intermediate morphological features are hybrids (between two species of the same genus). This may also be the case for new ecological forms, resulting from microevolution of species. To confirm or disprove the hypothesis about hybrid

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

origin of certain taxon, it is best to use molecular genetic methods.

them (**Table 1**). Phylogenetic trees were constructed using SplitsTree4.

*Bidens connata* Muhl. ex Willd. is a North American species whose natural area extends from Alaska in the north to Mexico in the south [17]. The species has high polymorphism within native area, and several varieties have been described: *B. connata* var. *ambiversa* Fassett, var. *anomala* Farwell, var. *fallax* (Warnstorf) Sherff, var. *gracilipes* Fernald, var. *inundata* Fernald, var. *petiolata* (Nuttall) Farwell, var. *pinnata S. Watson*, and var. *submutica* Fassett [18, 19]. In the second half of the twentieth century, American botanists made suggestions about the hybrid nature of *B. connata* based on morphological features [20]. This species was indicated as an alien for Europe [21]. However, European plants called "*B. connata*" are morphologically different from American samples. Their outer leaves are clearly leafshaped, well-developed, and 3–6 cm long, with no reedy flowers, and the first real leaves are less narrow and with more pronounced petioles than those of *B. connata* and fewer denticles on the leaves, and the denticles, in turn, are usually larger and less regularly located [22]. Plants from European populations have been described as *B. decipiens* Warnst. in 1895. The typical excrement material collected by Carl Warnstorf is stored in the herbaria of Edinburgh (E), Frankfurt (FR), and Charles University in Prague (PRC) [23]. In Europe, the locations of *B. decipiens* are few and far between. A map of the gradual eastward expansion of this species was previously compiled by the authors of this paper [24] and is shown in **Figure 1**. Previously, we studied morphological features of *B. decipiens* in Russia and found that features of this species are intermediate between the North American invasive

**2. Materials and methods**

*Hybridogenic Activity of Invasive Species of Asteraceae DOI: http://dx.doi.org/10.5772/intechopen.91370*

**3.** *Bidens decipiens*

**97**

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].

Hybridization increases the threat of extinction of many species due to introgression [12, 13]. High degree of introgression is often manifested by windpollinated 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 extinction increases significantly [11].

#### *Hybridogenic Activity of Invasive Species of Asteraceae DOI: http://dx.doi.org/10.5772/intechopen.91370*

It is known that the highest invasive activity is exhibited by species of Asteraceae family [14], so we have focused our attention on representatives of this group of alien species. It is often impossible to say with certainty whether plants with intermediate morphological features are hybrids (between two species of the same genus). This may also be the case for new ecological forms, resulting from microevolution of species. To confirm or disprove the hypothesis about hybrid origin of certain taxon, it is best to use molecular genetic methods.
