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.