The evolution of leaf traces in eupolypod ferns.

The expansion or reduction of structural parts is an aspect of phenotypic variation that leads to diverse form (e.g. digits in vertebrates, segments in arthropods, or petals in angiosperms). The number of petiole vascular bundles (leaf traces) in ferns is one such structure. Integrating traditional anatomy and developmental videography with phylogenetic comparative methods, we examined the evolution and development of fern leaf traces. Across the eupolypods - 80% of extant species richness - and their sister lineages, we observe four conserved vascular developmental units (VDUs) that consistently give rise to three leaf trace architectures. Transitions between leaf traces occur at low and equal rates, with the possibility of reversions, suggesting no directional trend toward increasing trace number. While vascular area is inexorably linked with leaf size, the actual number of leaf traces does not significantly predict leaf size across ferns, meaning the same vascular area can be packed in many ways, and leaf size evolves relatively independent of trace number. These findings suggest that leaf trace architecture in ferns is determined by consistent VDUs and underscores how impressive diversity in leaf size and morphology can evolve within a framework of conserved vascular trace architectures.