Genetic and metabolic insights into sexual dimorphism in the flexor carpi radialis of Asiatic toads (Bufo gargarizans) associated with amplexus behavior

BACKGROUND: Sexual dimorphism, a widespread phenomenon across the animal kingdom, encompasses differences between sexes in size, morphology, and physiological traits. In this study, we investigated sexual dimorphism in the flexor carpi radialis (FCR) muscle, which is critical for amplexus in Asiatic toads (Bufo gargarizans), using integrated transcriptomic and metabolomic approaches. RESULTS: Male toads exhibited significantly larger FCR muscles, reflecting enhanced muscle function required for sustained amplexus. Transcriptomic analysis identified 818 differentially expressed genes (DEGs) between sexes, with 389 upregulated and 429 downregulated in males, predominantly associated with muscle contraction, sarcomere organization, and energy metabolism. Metabolomic profiling revealed 69 differentially expressed metabolites (DEMs), with male-biased enrichment in pathways involved in protein synthesis and degradation, energy metabolism, and material transport. Integrated analysis pinpointed key metabolic pathways-such as glycine, serine, and threonine metabolism; alanine, aspartate, and glutamate metabolism; fatty acid degradation; and the tricarboxylic acid (TCA) cycle-as central to the observed sexual dimorphism. Among these, the genes AGXT, ACADL, ACAT1, MDH2, and SUCLG2 emerged as pivotal regulators. CONCLUSIONS: Collectively, these findings provide novel insights into the genetic and metabolic basis of sexual dimorphism in B. gargarizans, offering a deeper understanding of the evolutionary mechanisms driving sex-specific traits in vertebrates.