# Comparison of muscle structure and transcriptome analysis of eyed-side muscle and blind-side muscle in Cynoglossussemilaevis (Osteichthyes, Cynoglossidae)

## TL;DR

This study compares muscle structure and gene activity in the eyed and blind sides of a flatfish to understand why their muscles develop differently.

## Contribution

The first comparative analysis of muscle cell structure and transcriptome differences between eyed-side and blind-side muscles in Cynoglossus semilaevis.

## Key findings

- Eyed-side muscle has significantly more new muscle cells than blind-side muscle.
- 1177 differentially expressed genes were identified, including up-regulated myosin family genes in the eyed-side muscle.
- Glycolysis/gluconeogenesis pathway is suggested to be involved in muscle development differences.

## Abstract

Cynoglossussemilaevis (Osteichthyes, Cynoglossidae) is one of the most significant commercial marine fish species in China and has evolved a specialized asymmetrical body axis. In addition, C.semilaevis displays different muscle thickness between the eyed side and the blind side. However, the mechanisms underlying the muscle development difference between the two sides in C.semilaevis are unclear. In this study, we generated the first comparative investigation on the structure of muscle cells, and transcriptome analysis between the eyed-side muscle (ESM) and blind-side muscle (BSM) in C.semilaevis. Histological assays showed the obvious mosaic appearance of muscles on both the eyed side and blind side. However, the number of new muscle cells in ESM was significantly more than that in the BSM group. Comparative analyses of RNA-seq data showed that 1177 differentially expressed genes (DEGs) were identified between ESM and BSM groups, including 291 up-regulated and 886 down-regulated genes. The expression levels of myosin family genes (actin, myosin-binding protein C, titin, troponin, tnnil, and astrotactin-2) were significantly higher in ESM and might be a candidate regulator of muscle filament assembly in C.semilaevis. Murine double minute 2 (Mdm2) and cyclin A2 (ccna2) were also up-regulated in ESM, which indicates that the muscle development difference between ESM and BSM in C.semilaevis might be owing to the variation in myofibroblast proliferation. In addition, KEGG pathway enrichment analyses suggested that the glycolysis/gluconeogenesis pathway may be involved in the muscle development of C.semilaevis. Taken together, this study may provide useful information to understand the molecular mechanism of muscle development in flatfishes.