4.3 | Formation process of natural hybrids
Under natural conditions, hybridization was one of the main evolutionary mechanisms of plants (Goulet et al., 2017; Soltis & Soltis 2009). Although some large families and genera were difficult to produce hybrids (Ellstrand et al., 1996), there were also frequent interspecific hybridizations of several genera, such as Asplenium (Lee et al., 2015), Senecio (Abbott & Lowe, 2004), Betula (Wang et al., 2014), Rhododendron (Yan et al., 2013), etc. Triticeae was a young group, there was a large possibility of random hybridization among the relative genera in the Triticeae (Barkwoth & Bothmer, 2009). The generation of natural hybrids was affected by the genomic constitution of species, pollination habits and environmental conditions. In addition to the overlapping or very close distribution, the parents of natural hybridization also needed to be close in flowering stage. For example, the flowering stage of Begonia crassirostrisand Begonia hemsleyana have lasted for several months, which increasing the possibility of flowering encounters with other species, so it was more likely to occur hybridization (Tian, 2017). The good open-air environment and close planting, favorable conditions were created for the occurrence of natural hybridization.
In this study, different genera species with different genome constitutions in Triticeae were planted in the experiment base of the SAGS, such as Roegneria (StY ), Elymus(StH ) etc. Due to these plants were perennials, they could be kept in the field for more than ten years. The two adjacent fields are planted with R . stricta and R. turczaninovii , respectively. After three years of planting, we collected their seeds respectively and individually planted them for expanding propagation. There were about 400 plants in the R. stricta field, and about 330 plants in the R. turczaninovii field. We found some putative natural hybrids randomly distributed in these fields. The natural hybrids morphologically combined some unique characteristics (Figure 1d-o) of the two species. R. stricta was similar to R. turczaninovii in flowering time and their distribution areas was also close, which provided conditions for the natural hybridization.
We selected 20 basic genomic species (representing the genomesSt , H , Ee ,Eb , W , P , I ,Ta , V , Ns , A , B , andD ) of Triticeae, hybrids and associated species growing around hybrids for phylogenetic analysis. The results indicated that 15 natural hybrids have close relationships with R . stricta andR. turczaninovii . We speculated that the hybrids were produced by the hybridization between R. stricta and R. turczaninoviiin this study. Their ploidy and the genomic constitution were the same, the distribution area was close, and the flowering time was synchronous, all of which form a favorable environment and factors for natural hybridization. In the process growing, their pollen pollinated each other and underwent natural hybridization to form natural hybrids.
From the perspective of hybridization rate, there were 23 hybrids out of the about 400 R. stricta plants, and the natural hybridization rate was about 5.75%, while among the about 330 R. turczaninoviiplants, there were 54 hybrids, natural hybridization rate was about 16.36%. It can be seen that natural hybridization rate of R. turczaninovii was about 3 times that of R. stricta . The reason may be that the source of the R. stricta parents was single and the genetic diversity was low, while the R. turczaninovii parent has higher genetic diversity. Large morphological differences were observed in the field of R. turczaninovii , which leaded to a higher natural hybridization rate. The genetic diversity of the R. stricta parents and R. turczaninovii parents needed to be further verified by molecular markers or other methods.