Evolutionary Characteristics of the Protamine-Like Protein (PLI) Gene Family in Syngnathidae Fishes and Its Potential Regulatory Role in Sperm Morphological Specialization.

Syngnathidae fishes possess an elongated sperm morphology that is adapted for fertilization within the brood pouch. Protamine-like proteins (PLs) present a class of DNA-binding proteins that replace histones and bind to DNA during the late stages of vertebrate spermatogenesis, promoting chromatin condensation into compact sperm nuclei. To verify the correlation between the specialized sperm morphology of Syngnathidae fishes and PLI (PL Type I) gene family evolution, this study analyzed sperm morphology in 61 teleost species and revealed that Syngnathidae fishes have relatively elongated sperm heads (head length/diameter, p < 0.01). Genomic analysis of 46 teleost species indicated that the PLI gene family underwent duplication events in the ancestor of Syngnathidae fishes, with some species retaining two copies and members of the genus Hippocampus carrying three copies (PLIa, PLIb, and PLIc). Further analysis of PLI sequence structures showed that Syngnathidae PLIs generally have a high arginine content (26%-58%, p < 0.001). Quantitative polymerase chain reaction (qPCR) results demonstrated that the three PLIs in seahorses were highly expressed during stage V of testis development, confirming the role of PLIs in Syngnathidae spermatid maturation. Our findings suggest that the PLI gene family in Syngnathidae undergoes specific gene duplication and sequence variation, which may enhance chromatin condensation by PLIs and play a crucial regulatory role in shaping the unique sperm head morphology of Syngnathidae.