Chromosomal-Level Genome Suggests Adaptive Constraints Leading to the Historical Population Decline in an Extremely Endangered Plant.

Increased human activity and climate change have significantly impacted wild habitats and increased the number of endangered species. Exploring evolutionary history and predicting adaptive potential using genomic data will facilitate species conservation and biodiversity recovery. Here, we examined the genome evolution of a critically endangered tree Pellacalyx yunnanensis, a plant species with extremely small populations (PSESP) that is narrowly distributed in Xishuangbanna, China. The species has neared extinction due to economic exploitation in recent decades. We assembled a chromosome-level genome of 334 Mb, with the N50 length of 20.5 Mb. Using the genome, we discovered that P. yunnanensis has undergone several population size reductions, leading to excess deleterious mutations. The species may possess low adaptive potential due to reduced genetic diversity and the loss of stress-responsive genes. We estimate that P. yunnanensis is the basal species of its genus and diverged from its relatives during global cooling, suggesting it was stranded in unsuitable environments during periods of dramatic climate change. In particular, the loss of seed dormancy leads to germination under unfavourable conditions and reproduction challenges. This dormancy loss may have occurred through genetic changes that suppress ABA signalling and the loss of genes involved in seed maturation. The high-quality genome has also enabled us to reveal phenotypic trait evolution in Rhizophoraceae and identify divergent adaptation to intertidal and inland habitats. In summary, our study elucidates mechanisms underlying the decline and evaluates the adaptive potential of P. yunnanensis to future climate change, informing future conservation efforts.