The flower has always aroused great interest of researchers because it is considered the key innovation in the evolution of plants, featuring a very special group, the Angiosperms. It is conceptualized as a specialized stem apex or as a lateral branch with shortened internodes and appendices hypothetically homologous to leaves, modified for reproductive functions in sepals, petals, stamens and carpels. The predominance of angiosperm species on Earth is due probably to the success of the appearance of the flower, which enabled more effective ways in interbreeding. This fact is closely related to pollinators, which, along with the flowers, have diversified, becoming sometimes specialized in the pollination of a single plant species. A group of well-known plants, the urticalean rosids, composed of Cannabaceae, Moraceae, Ulmaceae and Urticaceae, surprises by displaying a flower of unusual morphology: small, inconspicuous, diclinous (= single-sex), perianth single or none, androecium with few stamens (1-5) arranged in one whorl, pseudomonomerous gynoecium, a single functional ovule inserted in different ways, in addition to a stigma with various forms. Thus, this flower, if compared to that of other rosids (Rosaceae, for example), can be considered reduced. Although morphological features of the flower of urticalean rosids are recorded since a long time (about 150 years), comprehensive studies on floral development, which could elucidate how this floral reduction takes place in this group are missing. This study aims to show how the ontogeny can explain the variations in flower structure of urticalean rosids. Flower morphology and pollination syndrome are also addressed. Our interpretations are based on data from at least 20 species obtained by analyses of surface in scanning electron microscopy and of histology in conventional light microscopy. The main points we raised are summarized in: (A) The condition perianth single (Fig. 1) or none (Fig. 2) results from the absence of the corolla organs or of both calyx and corolla organs from the inception. The interspecific variation in the calyx number of organs is also due to the absence of organs from the inception and not by organ abortion throughout the development. (B) The synorganization can be congenital (rare) (Fig. 3) or postgenital in the perianth and congenital in the gynoecium and androecium (rare). (C) Male and female flowers result of stamen and carpel abortion (Fig. 4) in most cases. Thus female flowers bear a pistillode and the male flowers a staminode. (D) The ontogeny of pseudomonomerous gynoecium deserves to be highlighted: one primordium arises in the center of the floral meristem, divides into two, but only one forms an ovule, although the other also participates in the style and stigma structure. (E) The arrangement of floral organs and pollination syndrome is strongly related in some species of Urticaceae (anemophily) and Moraceae (anemophily and entomophily). The inflate pistillode together with the anthers arrested by sepals in the staminate flowers of Urticaceae, for example, compose an explosive mechanism of pollen release which is then transported by the wind to a pistillate flower. In the few genera of Moraceae with entomophilous species, the union of several stigmas of different flowers within the inflorescence (fig), forming platforms (Ficus), or the offer of pollen and various exudates to pollinator insects (Artocarpus, Castilla, Dorstenia) ensures the formation of seeds. We conclude that the floral reduction in urticalean rosids takes place by different ontogenetic pathways, even among members of the same family. This finding contradicts the current paradigm which predicts that the floral ontogeny is conserved in related groups. Floral specializations, observed late in development, appear as responses to selection pressures exerted by different pollinators observed in the urticalean rosids.

We thank to FAPESP (process number 2014/07453-3, 2013/19459-3, 2012/15644-8), CNPq (process number 303493/2015-1) and CAPES for financial support.

Figures:

Fig 1. Staminate developing flower of Trema micrantha (L.) Blume (Cannabaceae) (SEM). Note the perianth single formed by five sepals (s), the initiation of stamen (st) and carpel (c) primordia. The carpel primordium will develop in a pistillode. Scale bar = 50 µm.

Fig. 2. Pistillate developing flower of Phenax sonneratii (Poir.) Wedd. (Urticaceae) (SEM). Note that no perianth or stamen primordia are initiated. Scale bar = 50 µm.

Fig. 3. Staminate developing flower of Cecropia pachystachya Trécul (Urticaceae) (transverse section - LM). Note that the perianth formed a congenitally united calyx. No carpel primordium initiated. Symbols: s = sepal, st = stamen. Scale bar = 50 µm.

Fig. 4. Staminate flower of Brosimum gaudichaudii Trécul (Moraceae) (SEM). Note that no perianth is formed but a rudimentar carpel (*) remained at the flower basis. Symbols: a = anther, f = filament. Scale bar = 100 µm.

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EMC Abstracts - https://emc-proceedings.com/abstract/ontogeny-resolves-the-unusual-flower-morphology-of-urticalean-rosids/