Tree Genetics and Molecular Breeding 2024, Vol.14, No.4, 194-205 http://genbreedpublisher.com/index.php/tgmb 195 showing male- or female-biased expression patterns. Furthermore, double-digest restriction site-associated DNA sequencing (ddRAD-seq) has been employed to identify sex-linked molecular markers, such as the male-specific locus MSL4, which can reliably distinguish male from female seedlings (Lan et al., 2019; Wang et al., 2023). These findings collectively enhance our understanding of the genetic and molecular basis of sex differentiation in E. ulmoides and provide valuable tools for its breeding and cultivation. This study aims to deeply analyze the mechanisms of sex differentiation in Eucommia ulmoides, identifying key sex-determining genes and molecular pathways. Through these investigations, the study seeks to provide new strategies for the breeding and commercial production of Eucommia, improving breeding efficiency and reducing production costs. Additionally, it aims to offer insights for the study of sex differentiation in other dioecious plants, further advancing the scientific understanding of plant sex determination mechanisms. 2 Botanical and Biological Characteristics of Eucommia ulmoides 2.1 General morphology and reproductive system Eucommia ulmoides, commonly known as the hardy rubber tree, is a dioecious plant species endemic to China, meaning it has distinct male and female individuals. This species is highly valued for its medicinal and economic benefits, particularly due to its production of α-linolenic acid and gutta-percha, a rubber-like substance (Wang et al., 2020; Du et al., 2023). Ecologically, E. ulmoides plays a crucial role in its native habitats, contributing to biodiversity and ecosystem stability. The species is well-adapted to various environmental conditions, which makes it a resilient component of the local flora (Xu et al., 2004). Morphologically, E. ulmoides is characterized by its unisexual flowers, which are produced on separate male and female trees from the earliest stages of stamen and pistil primordium formation. The male flowers typically contain stamens, while the female flowers contain pistils, and this differentiation occurs very early in the plant's development. This early differentiation is crucial for the plant's reproductive strategy and has significant implications for breeding and cultivation practices (He et al., 2014; Wang and Zhang, 2017). 2.2 The reproductive system of Eucommia ulmoides The reproductive system of E. ulmoides is complex and involves various genetic and molecular mechanisms. The flower, as the unique reproductive organ of seed plants, plays a critical role in seed plant breeding research. A complete flower consists of five whorls: sepals, petals, stamens, carpels, and ovules. However, in Eucommia ulmoides, the flower contains only stamens or carpels. Therefore, understanding the genes involved in the development of stamens and carpels in Eucommia may help uncover the genetic and molecular mechanisms behind sex differentiation in this species. Flower organ development is an extremely complex process. Currently, the most widely applied model for flower development is the ABCDE model or AE model, which is based on studies of model plants such as Arabidopsis, snapdragon (Antirrhinum majus), and petunia (Petunia hybrida (Hook.) E. Vilm.). In this model, A-class genes primarily regulate the development of the sepals and petals. In Arabidopsis, the representative A-class gene is APETALA1 (AP1). B-class genes regulate the development of petals and stamens and, in some species, also affect sepal development (e.g., in tulips). In Arabidopsis, the representative B-class genes are APETALA3 (AP3) and PISTILLATA (PI). C-class genes control the development of stamens, carpels, and ovules, although in some plants, they regulate only the stamens and carpels. In Arabidopsis, the representative C-class gene is AGAMOUS (AG). D-class genes control the development of ovules, and E-class genes inhibit the expression of A-class genes in petals and stamens. E-class genes are also involved in the formation of floral organs in each whorl and work together with A, B, and C genes to form the “quartet model” complex. In the AE model, these five classes of genes do not act independently to regulate flower organ development but instead function cooperatively across the five whorls of floral structures. Mutations in any of these gene classes can lead to abnormal floral morphology. Recent studies have identified several sex-biased genes and transcription factors that play critical roles in sex determination. For instance, MADS-box transcription factors have been shown to be involved in the regulation of floral organ identity and sex differentiation. Specific genes such as EuAP3 and EuAGhave been implicated in the
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