Researchers have conducted a conservation genomics study on two critically endangered island oaks—Quercus bawanglingensis and Quercus pseudosetulosa. They assembled high-quality chromosome-level genomes and revealed how natural selection and demographic history have shaped their markedly different genomic architectures.

The study, led by Prof. WANG Baosheng at the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences (CAS), was recently published in The Plant Journal.

Genetic diversity is key to evolutionary resilience. As populations decline, endangered species typically suffer from eroding genetic diversity, increased inbreeding, and accumulating genetic load. Yet, some endangered species with small populations maintain high genetic diversity and low inbreeding depression. This paradox remains poorly understood. Moreover, conservation genetics has largely focused on animals, leaving plants—especially endangered ones—severely understudied.

Oaks (Quercus) dominate many temperate and subtropical forests in the Northern Hemisphere. Although about 31% of oak species are threatened, research has been heavily biased toward North America and Europe. East and Southeast Asia, which harbor the most threatened oaks, remain poorly studied. Among these, some show severely reduced genetic diversity while others maintain high diversity under similar pressures. The mechanisms behind this contrast are unclear, hindering a deeper understanding of evolutionary processes shaping genetic resilience in threatened oaks.

The new study addresses this gap by focusing on two such species: Quercus bawanglingensis and Quercus pseudosetulosa.

Both species survive as single isolated populations with very few adult individuals. Q. bawanglingensis is restricted to the karst landscape of Exian Mountain on Hainan Island, while Q. pseudosetulosa persists only on weathered red soils of Dawanshan Island. Despite their similar conservation status, their genomic diversity differs dramatically. Q. pseudosetulosa shows reduced genetic diversity from a prolonged population decline, whereas Q. bawanglingensis retains high diversity—likely due to a larger historical effective population size (Ne) combined with its high outcrossing and longevity.

Despite its small N_e, Q. pseudosetulosa exhibits minimal genetic load, suggesting efficient purging of deleterious mutations. Population genomic analyses further reveal that the two species harbor different selected genes and show distinct sweep dynamics, reflecting their unique genetic backgrounds and habitat distributions.

The researchers say this work provides crucial scientific evidence for understanding evolutionary dynamics in small populations and for designing more effective conservation strategies.

SCBG's WANG Baosheng and LUO Wenjin are the corresponding author and first author, respectively. TU Tieyao from SCBG and Alison Kim Shan Wee from the University of Nottingham Malaysia also contributed to the study.

Geographic distribution, genomic characteristics, and genomic collinearity of Q. bawanglingensis and Q. pseudosetulosa. (Image by SCBG)