Morphological basis for the development of pistillate and staminate florets in a dioecious species of Poa (Poeae, Pooideae, Poaceae)

The formation of unisexual florets in Poaceae occurs through diverse developmental pathways. However, the morphogenetic mechanisms controlling sex determination in dioecious grasses remain poorly understood. Here, we investigated the developmental basis of floral unisexuality in the dioecious forage species Poa lanigera (section Dioicopoa) to determine whether its developmental trajectory conforms to previously described patterns in grasses or represents a distinct pathway. We also compared stamen arrest in pistillate florets of P. lanigera with suppression of the third stamen in the model grass Brachypodium distachyon to assess potential similarities in organ arrest mechanisms. By using scanning electron microscopy and histological analyses, we reconstructed the sequence of floret organ initiation and differentiation in both species. Florets were initially established as perfect and followed a common sequence of organ initiation, with divergence arising shortly after ovary closure, stylar branch initiation, and the transition of anthers to the tetralobed stage. In pistillate florets of P. lanigera, androecium abortion involved early disorganization of hypodermal anther tissues, which prevented sporogenous differentiation and microsporogenesis, whereas, in staminate florets, gynoecium abortion occurred after sporogenous cell formation through localized tissue degeneration, which disrupted megasporogenesis. These developmental routes differ from previously described patterns in Poaceae, indicating a distinct variant of floral unisexuality. In contrast, suppression of the third stamen in B. distachyon occurred at the primordium stage without tissue degeneration, suggesting a mechanistically different process. Together, these findings provide a developmental framework to investigate the genetic regulation of sex determination in P. lanigera, with potential applications for pasture improvement.