Sexual Dimorphism in Vertebral Number: Developmental Implications Inferred From Vertebral and Body Proportions.

Segmentation is a fundamental feature of vertebrate body plans, and variation in vertebral number contributes substantially to morphological diversity. Although sexual dimorphism in vertebral number has been documented in several vertebrate lineages, its developmental basis remains poorly understood. Here, we report pronounced sexual dimorphism in vertebral number in the western Indian endemic ricefish Oryzias setnai, distributed across ~1800 km. Radiographic analyses of individuals collected throughout its range revealed that vertebral number increased with latitude in both sexes, while males consistently possessed more vertebrae than females irrespective of geographic origin. Size-adjusted analyses showed that this dimorphism is associated with sex differences in head size and position-dependent variation in vertebral length. However, positional allometric analyses demonstrated that both the magnitude and direction of these sex differences vary markedly during growth. In particular, among smaller individuals, posterior vertebrae were relatively shorter in males than in females, suggesting reduced somite length in the posterior body axis of male embryos and potentially reflecting sex-specific differences in the dynamics of somitogenesis. Given that this variation arises between males and females within a single species sharing an essentially identical genetic background, O. setnai provides a rare empirical system not only for linking early developmental processes to intraspecific sexual variation in vertebrate body plans but also as a powerful model to investigate the developmental basis of body plan variation across species.