Altitudinal Patterns in Adaptive Evolution of Genome Size and Inter-Genome Hybridization Between Three Elymus Species From the Qinghai–Tibetan Plateau

Abstract

Genome size variation and hybridization occur frequently within or between plant species under diverse environmental conditions, which enrich species diversification and drive the evolutionary process. Elymus L. is the largest genus in Triticeae with five recognized basic genomes (St, H, P, W, and Y). However, the data on population cytogenetics of Elymus species are sparse, especially whether genome hybridization and chromosomal structure can be affected by altitude are still unknown. In order to explore the relationship between genome sizes, we studied interspecific hybridization and altitude of Elymus species at population genetic and cytological levels. Twenty-seven populations at nine different altitudes (2,800–4,300 m) of three Elymus species, namely, hexaploid E. nutans (StHY, 2n = 6x = 42), tetraploid E. burchan-buddae (StY, 2n = 4x = 28), and E. sibiricus (StH, 2n = 4x = 28), were sampled from the Qinghai–Tibetan Plateau (QTP) to estimate whether intraspecific variation could affect the genomic relationships by genomic in situ hybridization (GISH), and quantify the genome size of Elymus among different altitude ecological groups by flow cytometry. The genome size of E. nutans, E. burchan-buddae, and E. sibiricus varied from 12.38 to 22.33, 8.81 to 18.93, and 11.46 to 20.96 pg/2C with the averages of 19.59, 12.39, and 16.85 pg/2C, respectively. The curve regression analysis revealed a strong correlation between altitude and nuclear DNA content in three Elymus species. In addition, the chromosomes of the St and Y genomes demonstrated higher polymorphism than that of the H genome. Larger genome size variations occurred in the mid-altitude populations (3,900–4,300 m) compared with other-altitude populations, suggesting a notable altitudinal pattern in genome size variation, which shaped genome evolution by altitude. This result supports our former hypothesis that genetic richness center at medium altitude is useful and valuable for species adaptation to highland environmental conditions, germplasm utilization, and conservation.

Evaluation of chromosome diversity of Elymus nutans on the Qinghai‐Tibet plateau by FISH

“…In three subgenomes of four E. nutans populations, the number and location of S5 and AAG signals were observed to vary greatly in the Y and St subgenomes, but little in the H subgenomes. This is consistent with previous findings in E. nutans that the chromosomes of the Y and St subgenomes demonstrated higher polymorphism than that of the H genome (Chen et al, 2022; Liu et al, 2022b). It is generally believed that Triticeae plants with the StStYYHH genome have undergone two hybridization processes.…”

“…TE and tandem repeat sequences were used as FISH markers to distinguish E. nutans chromosomes. S5 is a DNA transposon probe developed from the St genome (Liu et al, 2019), and AAG is a microsatellite probe widely used in Triticeae (Cuadrado et al, 2008;Cuadrado & Jouve, 2011).…”