Comparative Transcriptome Analysis Reveals Sex-Biased Expression of Hormone-Related Genes at an Early Stage of Sex Differentiation in Red Bayberry (Morella rubra)

Abstract

The molecular mechanism of sex development and differentiation in the economically important dioecious fruit tree, red bayberry (Morella rubra), was revealed using next-generation transcriptome sequencing (NGS), and comparative analyses were used to identify differentially expressed genes (DEGs) in female and male flower buds. A total of 7029 of these DEGs were identified at two early development stages. KEGG pathway enrichment analysis revealed that plant hormone signal transduction was significantly overrepresented, and 91 genes related to hormones were identified. An analysis of 7029 DEGs revealed 161 hormone-related genes, with the 42 related to auxin and 26 related to ethylene being the most highly represented. A total of 62 genes were significantly up-regulated in females and 29 were in males, with 18 of them specifically expressed in females and 10 in males. A total of 415 transcription factors were identified, with 129 genes up-regulated in females and 53 in males. Moreover, 38 had female-specific expression and 18 had male-specific expression. Using weighted gene co-expression network analysis (WGCNA), two modules were found to be associated with sexual type. In the module coded light-green, there were five genes related to hormones, one to flower development and ten transcription factors with four genes specifically expressed in the males and four in females. The hub gene in the light-green module is MR0TCONS_00017483.1 (ACO), which is involved in ethylene biosynthesis and had male-specific expression. Among the transcription factors, three of the four male-specific expressed genes involved in flavonoid biosynthesis, including the MYB gene MR1TCONS_00020658.1 and two BHLH genes, MR6G001563.1 and MR8G020751.1, played important roles in male floral differentiation. In the dark-cyan module, six hormone-related genes, five transcription factors and three flower development genes were identified with the hub gene MR1G019545.1 (ETR1), which participates in the ethylene signaling pathway, and MR4G023618.1, which encodes the C3H zinc finger transcription factor. These results indicate that ethylene is the key hormone that interacts with other hormones and transcription factors to regulate sex differentiation in the red bayberry, which also provides new insights into the mechanism of sex determination and differentiation in the red bayberry.