Deciphering the complex organelle genomes of two Rhododendron species and insights into adaptive evolution patterns in high-altitude.

Background: The genomes within organelles are crucial for physiological functions such as respiration and photosynthesis and may also contribute to environmental adaptation. However, the limited availability of genetic resources, particularly mitochondrial genomes, poses significant challenges for in-depth investigations.

Results: Here, we explored various assembly methodologies and successfully reconstructed the complex organelle genomes of two Rhododendron species: Rhododendron nivale subsp. boreale and Rhododendron vialii. The mitogenomes of these species exhibit various conformations, as evidenced by long-reads mapping. Notably, only the mitogenome of R. vialii can be depicted as a singular circular molecule. The plastomes of both species conform to the typical quadripartite structure but exhibit elongated inverted repeat (IR) regions. Compared to the high similarity between plastomes, the mitogenomes display more obvious differences in structure, repeat sequences, and codon usage. Based on the analysis of 58 organelle genomes from angiosperms inhabiting various altitudes, we inferred the genetic adaptations associated with high-altitude environments. Phylogenetic analysis revealed partial inconsistencies between plastome- and mitogenome-derived phylogenies. Additionally, evolutionary lineage was determined to exert a greater influence on codon usage than altitude. Importantly, genes such as atp4, atp9, mttB, and clpP exhibited signs of positive selection in several high-altitude species, suggesting a potential link to alpine adaptation.

Conclusions: We tested the effectiveness of different organelle assembly methods for dealing with complex genomes, while also providing and validating high-quality organelle genomes of two Rhododendron species. Additionally, we hypothesized potential strategies for high-altitude adaptation of organelles. These findings offer a reference for the assembly of complex organelle genomes, while also providing new insights and valuable resources for understanding their adaptive evolution patterns.