Temporal expression profiles of tachykinin genes throughout the first reproductive cycle, male pregnancy, and brood pouch development in the big-belly seahorse (Hippocampus abdominalis).

The tachykinin family comprises highly conserved neuropeptides that regulate diverse physiological processes in vertebrates. However, the molecular characteristics and reproductive functions of this system remain poorly understood in seahorses, which exhibit the unique reproductive trait of male pregnancy. In the present study, we cloned the major tachykinin ligand genes of the big-belly seahorse (Hippocampus abdominalis) and investigated their temporal expression patterns across the first reproductive cycle, gestation, and brood pouch development via real-time quantitative PCR (RT-qPCR). Open reading frames of tac1 and tac3, as well as two transcript variants of tac4, namely tac4-S and tac4-L, were identified. Sequence alignment and phylogenetic analyses showed that these precursors were conserved among teleosts. Tissue distribution analysis showed that tac1 and tac3 were mainly enriched in the brain-pituitary complex, while tac4 transcripts exhibited marked male-biased expression and tac4-L had a wider peripheral distribution including the testis and brood pouch. Developmental profiling of these genes during the first reproductive cycle revealed distinct age-dependent and sex-dependent patterns, suggesting that tac1 may act mainly during early gonadal development, tac3 may function in the middle developmental stage of ovaries or testes, tac4 may primarily act in the late developmental stage of female ovaries or the middle and late developmental stage of male testes. During pregnancy, tac4-L expression was significantly decreased in the brain-pituitary complex but significantly increased in the testis and brood pouch of pregnant males, suggesting a close association between tac4-L and male pregnancy. Remarkable changes of tac3 and tac4 transcripts during the brood pouch growth revealed that tac3 and tac4-L may participate in regulating early and middle pouch development, while tac4-S may exert its effects during the late brood pouch growth. Taken together, these results reveal functional divergence among tachykinin ligands of the big-belly seahorse, which also provide a foundation for exploring roles of tachykinin system in regulating gonadal growth, male pregnancy, and brood pouch remodeling of seahorses.