Developmental processes in flower differentiation of dioecious plants: insights from yerba mate (Ilex paraguariensis)

The dioecious species Ilex paraguariensis (yerba mate) holds significant economic and cultural value, yet key aspects of its reproductive development remain unresolved. This study combines histological, scanning electron microscopy (SEM), and cytogenetic analyses to elucidate floral ontogeny, microsporogenesis, and sexual differentiation in yerba mate. For both sexes, six arbitrary descriptive stages of floral development were established based on all available data. Our findings reveal that floral development follows a type I pattern, initiating as bisexual before diverging into unisexual flowers. In staminate flowers, functional androecium development coincides with early parenchymatization of the pistillode and suppression of megasporangial initiation-a potential adaptive strategy to minimise resource wastage. Cytogenetic analysis confirmed regular microsporogenesis, identified off-plate bivalents at higher frequencies than previously reported, and provided the first complete meiotic progression leading to gamete formation in this species. Besides, SEM revealed novel stephanocytic structures on pistillodes, interpreted as nectarostomata, suggesting a role in pollinator attraction. These findings challenge prior assumptions regarding nectary presence in yerba mate. In pistillate flowers, staminodes undergo abortion via tapetal degeneration and aberrant sporogenous tissue collapse before meiosis, aligning with previously defined categories of organ abortion. Sexual dimorphism emerges early and is governed by distinct mechanisms: constitutive pistillode sterility in males and selective microsporangial tissue abortion within the antherodes in females. This study advances the understanding of reproductive biology in a commercially important crop, providing key morphological and cytological insights that will guide future taxonomic, developmental, and evolutionary studies within Ilex.