Tree Genetics and Molecular Breeding 2024, Vol.14, No.4, 194-205 http://genbreedpublisher.com/index.php/tgmb 199 potential role is supported by the differential expression of key regulatory genes such as MADS-box transcription factors. Future research focusing on the identification and functional characterization of miRNAs and ncRNAs will be crucial for understanding their contribution to the regulation of sex-specific gene expression in E. ulmoides (Devlin and Nagahama, 2002). 5 Environmental Influences on Sex Differentiation 5.1 Impact of environmental factors on sex expression Environmental factors such as temperature, light, and soil nutrients play an important role in the sex expression of Eucommia ulmoides. Studies on a variety of species have shown that temperature can affect the expression levels of sex determination-related genes, leading to changes in the sex ratio within a population (Schenkel et al., 2022). For example, in fish and scallops, temperature has been found to affect the fate of somatic and germ cells in the primitive gonads, indicating that environmental conditions can alter genetic sex determination pathways (Devlin and Nagahama, 2002; Edelaar et al., 2017). Papaya is a trisexual plant with three sex forms: male, female, and hermaphrodite. Although the sex of papaya is mainly determined by genes, the expression of its phenotypic sex can also be affected by environmental factors. Low temperature induces changes in the methylation level of histone H3-K9 in male flowers of papaya, prompting the reprogramming of plant hormone signal transduction pathways in hermaphrodite flowers and changing the expression levels of class I KNOX gene family, floral organ identity genes, and polarity-related genes, resulting in the conversion of male papaya plants to hermaphrodite plants (low temperature-induced sex reversal) (Lin et al., 2016). Similarly, in Eucommia ulmoides, phenotypic traits such as leaf, fruit, and seed size are associated with geographical and climatic factors, indicating that environmental conditions such as annual average temperature and rainfall significantly affect phenotypic changes and may affect sexual differentiation (Wang et al., 2023). 5.2 Interaction between genetic and environmental factors in sex differentiation The interaction between genetic and environmental factors is crucial in the sex differentiation process of Eucommia ulmoides. Mixed systems that incorporate both genetic and environmental components are more common than previously thought, and environmental effects on gene expression can lead to evolutionary divergence of sex determination mechanisms within species (Schenkel et al., 2022). In Eucommia ulmoides, genetic studies have identified key genes such as EuAP3 and EuAG that are involved in regulating sex differentiation (Du et al., 2023). These genetic components may interact with environmental factors, such as temperature and light, to influence the expression of sex-biased genes, thereby affecting the overall sex differentiation process. This interaction is also observed in other species, where environmental factors like temperature and social interactions can activate or suppress various male and female pathways depending on the genetic background. 5.3 Case studies and experimental findings Several case studies and experimental findings highlight the impact of environmental factors on sex differentiation in Eucommia ulmoides and other species. For example, a study on the housefly demonstrated that temperature-dependent expression levels of specific genes could predict the occurrence of different sex determination mechanisms along environmental gradients (Figure 2) (Schenkel et al., 2022). In Eucommia ulmoides, comparative transcriptome analyses have revealed differentially expressed genes between male and female individuals, indicating that both genetic and environmental factors contribute to sex differentiation. Additionally, genome-wide analyses of MADS-box transcription factors in Eucommia ulmoides have shown that certain genes exhibit sex-biased expression, which may be influenced by environmental conditions. These findings underscore the complex interplay between genetic and environmental factors in determining sex differentiation in Eucommia ulmoides and other species. 6 Biotechnological Approaches to Study and Manipulate Sex Differentiation 6.1 CRISPR/Cas9 and gene editing in sex determination studies The CRISPR/Cas9 system has revolutionized the field of genome editing, providing a powerful tool for precise manipulation of specific genomic elements. This technology employs a Cas9 nuclease guided by a customizable
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