Tree Genetics and Molecular Breeding 2024, Vol.14, No.4, 194-205 http://genbreedpublisher.com/index.php/tgmb 198 M gene inhibits the development of stamens in flower buds. In contrast, the A gene suppresses female flower development and promotes the formation of male flowers. The interaction among the F, M, and A genes ultimately determines various sex phenotypes in cucumbers. However, gibberellins (GA) may promote masculinization in cucumbers via an ethylene-dependent pathway by altering the expression of the M (CsACS2) gene, ethylene receptor CsETR1, and ethylene response transcription factors. GA may also suppress female expression through an ethylene-independent pathway by regulating the expression of the class C floral homeotic gene CAG2. The fact that plant hormones like gibberellins (GAs) and ethylene promote the development of male and female flowers, respectively, highlights the significant role hormones play in regulating sex differentiation (Pelaz et al., 2000; Zhang et al., 2017). 4.2 Epigenetic modifications and their role in sex determination The essence of epigenetic regulation is to regulate the spatiotemporal activity of genes through structural changes in chromatin. Typical epigenetic events include DNA methylation, histone covalent modification, non-coding RNA (ncRNAs) silencing, and chromatin remodeling (mammalian epigenetics) (Tachibana, 2015). Recent studies have shown that epigenetic regulation plays an indispensable role in the sexual expression of many species. For example, in non-mammalian vertebrates, the gene cyp19a1, which encodes an important steroidogenic enzyme that converts androgens into estrogens, plays a vital role in the temperature-induced masculinization process. Exposure to high temperatures during the critical period of sexual determination in European sea bass leads to increased DNA methylation levels in the promoter region of the cyp19a1 gene, thereby inhibiting its expression and ultimately leading to masculinization. In the model plant melon of the Cucurbitaceae family, the gene CmACS-7, which encodes an ethylene biosynthetic enzyme, interacts with the transcription factor CmWIP1 to control the development of male and female flowers and hermaphrodites (Pawełkowicz et al., 2019). The transposon Gyno-hAT was inserted into the CmWIP1 promoter region, triggering the spread of DNA methylation to the CmWIP1 promoter, resulting in the transformation of male melon plants into female plants. In the gonadal somatic cells of wild-type XY mice, the enrichment of H3K9 demethylases induced a significant increase in the H3K9 dimethylation level and a decrease in the H3K4 trimethylation level at the Sry gene locus, resulting in the inhibition of Sry gene expression, thereby hindering the formation of testes and causing the mice to reverse from male to female sex (Liu et al., 2019). Epigenetic modifications, including DNA methylation and histone modifications, may play an important role in the sex determination of Eucommia ulmoides. Although the specific epigenetic mechanisms were not described in detail in the current study, the complex protein-motif composition and exon-intron structure of MADS-box genes suggest that epigenetic regulation may be a key factor. These modifications can affect gene expression patterns without changing the DNA sequence, leading to the sexual dimorphism observed in Eucommia ulmoides (Zhang et al., 2023). Future studies targeting the epigenetic landscape could shed more light on how these modifications regulate sex-specific gene expression. 4.3 Transcriptomic and proteomic analyses of male and female differentiation Transcriptomic analyses have revealed significant differences in gene expression between male and female E. ulmoides. Comparative transcriptome analyses identified 116 differentially expressed genes (DEGs) between males and females, with 73 male-biased and 43 female-biased genes. Notably, the male-biased DEGAPETALA3, a B class floral organ identity gene, has been implicated in sex determination. Proteomic analyses, although not explicitly detailed, would complement these findings by identifying sex-specific proteins and their post-translational modifications, providing a more comprehensive understanding of the molecular mechanisms underlying sex differentiation (Wang and Zhang, 2017). The integration of transcriptomic and proteomic data is essential for elucidating the complex regulatory networks involved in this process. 4.4 Role of microRNAs and other non-coding RNAs in sex differentiation MicroRNAs (miRNAs) and other non-coding RNAs (ncRNAs) are emerging as important regulators of gene expression in sex differentiation. These small RNA molecules can modulate the expression of target genes by binding to their mRNA transcripts, leading to degradation or inhibition of translation. Although specific miRNAs and ncRNAs involved in the sex differentiation of E. ulmoides have not been identified in the current studies, their
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