Dwarf males function as males but retain female-biased transcriptional profiles in an androdioecious barnacle

Kenji Toyota, Asami Kajimoto , Rie Kusunoki, Yoichi Yusa

Sexual systems exhibit remarkable diversity, yet how alternative reproductive strategies are implemented at the level of gene expression remains poorly understood. In androdioecious species, males coexist with hermaphrodites and function exclusively as sperm donors, raising the question of whether the males’ transcriptional profiles are fully masculinized or retain features of hermaphroditic ancestry. We investigated this question in the epizoic barnacle Octolasmis unguisiformis by comparing transcriptomes of male and female reproductive tissues within hermaphrodites and conspecific males that arrest growth at a young stage (dwarf males). Principal component analysis and correlation structure revealed that dwarf males cluster more closely with female tissue than with male tissue of hermaphrodites. Distributional analyses of log₂ fold changes showed minimal genome-wide divergence between dwarf males and female tissue. Multivariate distance comparisons and effect size analyses further supported that transcriptomic divergence was smallest in the dwarf male–female cpmparison. In contrast, male reproductive tissue of hermaphrodites exhibited strong enrichment of genes associated with flagellated sperm motility, ciliary structure and dynein complexes. These results indicate that specialization of hermaphroditic male tissue in this species is concentrated within a discrete sperm motility module, while dwarf males retain broadly female-biased transcriptional profiles at the genome-wide scale. Our findings demonstrate that functional male identity of dwarf males can arise without wholesale transcriptional masculinization and suggest that alternative male strategies in androdioecious systems may evolve through modular activation of male-specific gene sets within a shared transcriptional background.