A comparative approach reveals diversity of floral developmental processes in Urticaceae

Most species of Urticaceae, the nettle family, have small and inconspicuous, diclinous flowers, in which the perianth, androecium and gynoecium tend to vary in number. Our objective was to study the morphology of the developing flowers of seven species of Urticaceae to understand the pathways that lead to the different patterns of floral reduction and the complex development of pseudomonomerous gynoecia. Buds and flowers were prepared for electron and light microscopy. Vascularization was studied via high resolution X-ray computed tomography micro-CT. Only one whorl of perianth organs is initiated, except for Phenax sonneratii, the flower of which is achlamydeous; variation in perianth merosity results from absence of organs from inception; dicliny results from the absence of stamens from inception (pistillate flowers) and from pistil abortion at intermediate developmental stages (staminate flowers). The gynoecium results from a primordium that divides partially forming two congenitally united primordia (most species) or from a single primordium that apparently does not divide. The gynoecium is served by a single (four species), or two vascular bundles. This second condition is expected for a pseudomonomerous gynoecium. Pistillode or rudimentary carpels occur in staminate flowers. The comparison among species shows that the developmental processes acting in the floral construction in Urticaceae is diverse.