Systematic evaluation of scallop (Argopecten irradians) SoxH reveals its crucial role in spermiogenesis with limited evidence for sex determination.

The transcription factor Sox30 (SoxH), a conserved regulator of vertebrate spermatogenesis, remains poorly characterized in mollusks, despite emerging evidence of its potential role in sex regulation. In this study, we demonstrate that AiSoxH is a functional ortholog of vertebrate Sox30 in the hermaphroditic scallop Argopecten irradians. Developmental profiling reveals that AiSoxH expression is negligible during embryogenesis and larval stages. In adults, its expression is strictly testis-specific, confined to later developmental phases (growth and maturation stages) and localized to spermatids and spermatozoa under the control of a testis-enriched promoter. Knockdown of AiSoxH significantly disrupts spermiogenesis, leading to a reduction in mature germ cells and the downregulation of key spermiogenesis-related genes, including those encoding flagellar components, microtubule-associated proteins, and protein kinases. DNA affinity purification sequencing (DAP-seq) identifies 220 putative AiSoxH targets involved in microtubule binding, protein phosphorylation, and membrane trafficking. Dual-luciferase reporter assays confirm AiSoxH's direct activation of Limk1 and Eip74EF, two essential regulators of spermiogenesis. Collectively, our findings establish SoxH as a specific regulator of spermiogenesis, with limited evidence supporting a role in sex determination, positioning it as a potential target for fertility control in hermaphroditic scallops and possibly other mollusks.