Identification of candidate genes and pathways involved in the establishment of sexual size dimorphism in the olive flounder (Paralichthys olivaceus) using RNA-seq

Abstract

Olive flounder (Paralichthys olivaceus) is a valuable mariculture fish that shows female-biased sexual size dimorphism (SSD), yet the molecular mechanisms responsible for this phenomenon remain poorly understood. In this study, the growth differences between females and males were compared, and comparative transcriptome analyses of brains/pituitaries, livers, muscles, and gonads from 13-month-old females and males were performed. The critical time window for SSD was from 13- to 14-month-old, with females reaching significantly larger sizes than males. Muscle cell size and plasma growth hormone (GH), insulin-like growth factor 1 and 2 (IGF1, IGF2), 11-ketotestosterone (11-KT), and 17β-estradiol (E2) levels had no significant differences between the sexes of 13- and 14-month-old flounders. Furthermore, the RNA-seq data revealed differential expression in the genes encoding the receptors for these hormones, suggesting that regulation may occur at the receptor level, potentially leading to SSD during the critical window. RNA-seq analysis identified 88, 645, 414, and 15,833 differentially expressed genes between females and males in brains/pituitaries, livers, muscles, and gonads, respectively. KEGG (Kyoto Encyclopedia of Genes and Genomes) and GSEA-based KEGG analyses revealed that some pathways associated with growth and development, including “prolactin signaling pathway” in brains/pituitaries, “cell cycle”, “oocyte meiosis”, and “DNA replication” in livers, “focal adhesion”, “ECM-receptor interaction”, “protein digestion and absorption”, “arginine and proline metabolism”, and “cysteine and methionine metabolism” in muscles, and “Hippo signaling pathway” and “JAK-STAT signaling pathway” in gonads may be involved in the establishment of sexual growth differences. Notably, females had lower steroid hormone synthesis abilities than males in brains/pituitaries, livers, and gonads, indicating the importance of sex steroid hormones in the establishment of SSD. Moreover, genes including frizzled3 (fzd3) and prolactin (prl) in brains/pituitaries, and growth hormone receptor (ghr), estrogen receptor (esr1), insulin-like growth factor binding protein-3 (igfbp3), bone morphogenetic protein 8A-like (bmp8a), leptin receptor (lepr), and lipoprotein lipase-like (lpl) in livers emerge as potential candidates for the establishment of SSD. In conclusion, this study may help clarify the molecular mechanisms involved in SSD-related growth traits in flounder and provide basic data for genetic breeding of flounder.