Palaeopolyploidy promoted ancient adaptation of Tibetan sclerophyllous oaks to aridity and cold triggered by mountain uplift

Hu, Jin-Jin

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

2026

691

Xing, YW; Zhou, ZK

Polyploidy is an important evolutionary force increasing plant diversity and environmental resilience. Reconstructing genome size (GS) and ploidy of plant fossils advances our understanding of plant evolution and stress adaptation. Palaeopolyploidy is usually inferred from molecular data, but precise timing of polyploidy events requires improvement. So far there has been no robust fossil evidence for palaeopolyploidy in angiosperms. Here, the GS of late Eocene (similar to 34.6 Ma, the Markam flora) and Middle Miocene (similar to 15 Ma, the Namling flora) sclerophyllous oak (Quercus sect. Heterobalanus) fossils from southern Tibet, and three Late Pliocene fossils from Yunnan, China, were derived using the strong positive correlation between guard cell length (GCL) and GS. The estimated GS of the Namling fossils was approximately double those of late Eocene and Late Pliocene fossils as well as extant samples, indicating that the Namling fossils probably experienced a polyploidisation event. These tetraploids seemingly conferred greater resilience to aridity and cold-induced stress after similar to 15 Ma, reflected by two distinct fossil assemblages preserved in this flora, coincident with uplift of the Gangdese. Our discovery of sclerophyllous oak tetraploids provides compelling fossil evidence for palaeopolyploids in angiosperms, and demonstrates plant adaptation at the genome level to regional climatic change triggered by mountain uplift. GCL analysis serves as a promising tool for investigating GS changes across land plant evolution. Future research of elucidating the drivers of GCL variation is essential for enhancing the precision and reliability of this proxy.