Sex Determination in Sponges.

Sex determination in Porifera remains one of the least understood aspects of early metazoan biology despite the group's key phylogenetic position. Sponges display exceptional diversity in sexual systems-ranging from stable gonochorism to sequential hermaphroditism and sex reversal-yet lack morphological dimorphism and any discrete gonadal structures, blurring the boundary between sex determination and gametogenesis. Here we synthesize current knowledge on sexual systems, environmental and genetic influences on sexual fate, and the evolutionary origins of sexual systems in sponges. Evidence for environmental modulation is substantial, particularly temperature-driven shifts in sex ratios and reproductive timing, but environmental sex determination remains unsupported by causal evidence yet. Conversely, conserved molecular components of metazoan sex-determining pathways, such as fem-1 and Dmrt genes, are present in several sponge lineages, though their functional roles remain unresolved. We also evaluate the hypothesis that microbial symbionts contribute to sexual allocation by buffering the energetic costs of gametogenesis. Overall, sponges exhibit an evolutionarily labile system in which sexual fate may emerge from the interaction of conserved genetic modules, environmental cues, and physiological constraints. Clarifying these mechanisms will be essential for understanding the origins of sex determination in Metazoa and for predicting reproductive resilience under environmental change.