Plant Gene and Trait 2024, Vol.15, No.6, 285-294 http://genbreedpublisher.com/index.php/pgt 286 This study delves into the application potential of sex-specific molecular markers in E. ulmoides by analyzing existing genomic and transcriptomic data. The research summarizes several sex-related molecular markers and evaluates their reliability and stability in early sex identification. It focuses on reviewing current marker technologies and analyzing their application in the breeding of E. ulmoides, providing a theoretical foundation for future sex identification and efficient breeding strategies. By conducting an in-depth analysis of existing techniques, this study aims to offer valuable insights for optimizing breeding processes and resource allocation in E. ulmoides, while also exploring new directions for research and applications in other dioecious plants. 2 Biological Characteristics and Reproductive Biology of Eucommia ulmoides 2.1 Biological traits of Eucommia ulmoides Eucommia ulmoides, commonly known as the hardy rubber tree, is a dioecious perennial tree native to China. It is the sole species of the family Eucommiaceae and is highly valued for its medicinal and industrial applications, particularly for its rubber-producing capabilities (Wang et al., 2018; Wang et al., 2020). The species is characterized by its unisexual flowers, which are borne on separate male and female individuals from the earliest stages of stamen/pistil primordium formation (Zhang et al., 2023). The tree has a long juvenile phase during which it is difficult to distinguish between male and female plants based on morphological traits alone (Wang et al., 2011). 2.2 Reproductive biology of dioecious plants Dioecious plants, such as Eucommia ulmoides, have distinct male and female individuals, each producing only one type of reproductive organ. This separation of sexes can complicate breeding programs and commercial cultivation, as the sex of the plants cannot be determined until they reach reproductive maturity (Wang et al., 2011). The genetic mechanisms underlying sex determination in dioecious plants are complex and often involve sex-specific gene expression and chromosomal differences. In E. ulmoides, sex determination is likely genetically controlled, with specific genes and molecular markers associated with male and female plants (Wang and Zhang, 2017; Wang et al., 2020; Du et al., 2023). 2.3 Previous research on sex-specific markers in dioecious species Several studies have focused on identifying sex-specific markers in Eucommia ulmoides to facilitate early sex determination. For instance, a male-specific Amplified Fragment Length Polymorphism (AFLP) marker was developed and converted into a Sequence Characterized Amplified Region (SCAR) marker, which can be used for early sexual identification (Wang et al., 2011). Comparative transcriptome analyses have identified differentially expressed genes (DEGs) between male and female plants, including genes related to sex determination and sexually dimorphic traits (Wang and Zhang, 2017). Additionally, double-digest restriction site-associated DNA sequencing (ddRAD-seq) has been used to identify male-specific loci, with one locus, MSL4, being highly conserved and stable across male individuals (Wang et al., 2020). Random Amplified Polymorphic DNA (RAPD) markers have also been linked to sex determination, with a specific marker exclusive to pistillate plants (Xu et al., 2004; You et al., 2023). 2.4 Identified research gaps Despite the progress made in identifying sex-specific markers in Eucommia ulmoides, several research gaps remain. The genetic mechanisms of sex determination and differentiation in E. ulmoides are not fully understood, and further studies are needed to elucidate these processes. Additionally, while several sex-specific markers have been identified, their practical application in breeding programs and commercial cultivation requires further validation and optimization (Xu et al., 2004; Wang et al., 2011; Wang et al., 2020). There is also a need for more comprehensive genomic and transcriptomic analyses to identify additional sex-associated genes and markers, which could provide deeper insights into the molecular regulation of sex in E. ulmoides (Figure 1) (Wang and Zhang, 2017; Du et al., 2023; Zhang et al., 2023).
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