1 Introduction

The conservation of biodiversity is crucial for sustaining the stability and resilience of ecosystems. Genetic diversity constitutes a pivotal element in the long-term persistence of species, particularly for those facing extinction risks. Chieniodendron hainanense represents a rare and imperiled plant species characterized by extremely small populations (WPESP) and is endemic to the fragmented habitats of Hainan, China. Owing to the adverse consequences of habitat fragmentation, the genetic diversity of C. hainanense populations warrants significant attention in conservation initiatives. Comprehending the genetic architecture and diversity of these populations enables the development of suitable management approaches and facilitates species restoration efforts.
The genetic structure and diversity of a species play critical roles in their ability to withstand adverse environments and evolve over time. Studying the genetic makeup of wild plants with extremely small populations can provide insights into their evolutionary history, population dynamics, and response to environmental changes, enabling the development of effective conservation strategies (Yang et al., 2018; Zhang et al., 2021). Such research is also essential for advancing DNA sequencing technology and conservation biology (Windig et al., 2010).
Habitat fragmentation and overuse of resources have greatly impacted species’ genetic diversity, survival, adaptability, and biodiversity (Su et al., 2020). Habitat fragmentation reduces genetic diversity by isolating populations and restricting gene exchange, leading to an increase in genetic drift and inbreeding depression (Bijlsma et al., 2012; Wang et al., 2022). The reduced genetic diversity weakens the ability of species to adapt to environmental changes and increases the risk of species extinction (Luquet et al., 2012). Habitat fragmentation also reduces the number of plant populations, increases spatial isolation, and hinders the maintenance of population genetic diversity by disrupting dispersal, gene exchange, and inter-species interactions (Aguilar et al., 2008).
Wild plants with extremely small populations (WPESP) are endangered species under national key protection and require urgent rescue efforts. Their population sizes are smaller than the minimum viable and are usually distributed in a narrow area, making them highly vulnerable to extinction (Zang, 2020; Jain et al., 2013). The Chinese government has launched the National Project Plan for the Rescue and Protection of Wild Plants in Extremely Small Populations (2011-2015) to protect 120 WPESP species, most of which are endemic to China and have significant ecological, scientific, cultural, and economic value (Ma et al., 2013; Yang et al., 2020). The loss of biological and genetic values of WPESP due to their extinction can have significant adverse effects on human society and the ecosystem (Zhang et al., 2018). Therefore, it is crucial to prioritize research on WPESP conservation in current biodiversity conservation studies in China.
Chieniodendron hainanense is a second-class national key protected wild plant and a unique member of the Annonaceae family Chieniodendron genus in China. The species is an evergreen tree that grows up to 16 m tall with a DBH of about 50 cm, and its distribution is limited to the Guangxi Zhuang Autonomous Region and some areas of Hainan Province. Habitat fragmentation due to human activities such as logging has drastically reduced the distribution area of C. hainanense, and the wild resources of its population are scarce. Field surveys have shown that the existing populations of C. hainanense are mainly distributed in nine primary forest areas dominated by fragmented secondary rainforests in Hainan, with many original populations already disappeared (Jiang et al., 2021). The species has poor self-renewal ability and is highly sensitive to external disturbances, making its wild resources already endangered. Although some research reports have focused on the population structure, dynamic characteristics, leaf morphology, petal nodules development, and functional biochemical activities of C. hainanense, very few studies have investigated its endangerment mechanisms at the molecular level. To develop targeted and comprehensive conservation strategies, it is necessary to understand the causes of its endangerment, including its genetic diversity, and related components (Sork et al., 2006).
In this investigation, our objective is to evaluate the genetic diversity and population structure of C. hainanense across its fragmented habitats in Hainan, utilizing genotyping-by-sequencing (GBS) and single nucleotide polymorphism (SNP) techniques. Through the analysis of 35 specimens from six discrete cohort groups, we will scrutinize patterns of genetic variation, differentiation, and inbreeding both within and among the populations. The outcomes of this research will yield valuable insights into the conservation status and management of C. hainanense, promoting the formulation of efficacious strategies for the preservation and restoration of this critically endangered species. Additionally, our findings will augment the understanding of the wider implications of habitat fragmentation on genetic diversity in plant populations and inform conservation endeavors for other threatened plant species inhabiting fragmented ecosystems.