Comparative Transcriptome Analysis Revealed Genes Involved in Sexual and Polyploid Growth Dimorphisms in Loach (Misgurnus anguillicaudatus)

Sexual and polyploidy size dimorphisms are widespread phenomena in fish, but the molecular mechanisms remain unclear. Loach (Misgurnus anguillicaudatus) displays both sexual and polyploid growth dimorphism phenomena, and are therefore ideal models to study these two phenomena. In this study, RNA-seq was used for the first time to explore the differentially expressed genes (DEGs) between both sexes of diploid and tetraploid loaches in four tissues (brain, gonad, liver, and muscle). Results showed that 21,003, 17, and 1 DEGs were identified in gonad, liver, and muscle tissues, respectively, between females and males in both diploids and tetraploids. Regarding the ploidy levels, 4956, 1496, 2187, and 1726 DEGs were identified in the brain, gonad, liver, and muscle tissues, respectively, between tetraploids and diploids of the same sex. When both sexual and polyploid size dimorphisms were considered simultaneously in the four tissues, only 424 DEGs were found in the gonads, indicating that these gonadal DEGs may play an important regulatory role in regulating sexual and polyploid size dimorphisms. Regardless of the sex or ploidy comparison, the significant DEGs involved in glycolysis/gluconeogenesis and oxidative phosphorylation pathways were upregulated in faster-growing individuals, while steroid hormone biosynthesis-related genes and fatty acid degradation and elongation-related genes were downregulated. This suggests that fast-growing loaches (tetraploids, females) have higher energy metabolism levels and lower steroid hormone synthesis and fatty acid degradation abilities than slow-growing loaches (diploids, males). Our findings provide an archive for future systematic research on fish sexual and polyploid dimorphisms.