Research progress on diversity of androecium development of Orchidaceae

Orchidaceae is the most diverse family of angiosperms. The development of androecium varied greatly at the subfamily level, which has been a research focus of speciation and adatptive pollination biology. Progress on the development of androeceum in Orchidaceae was summarized according to a survey of literature. The results were as follows: (1) It was suggested that evolutionary trends on the decreasing number of fertile stamens was parallel with the increasing degree of coherence of pollens in the pollen dispersal units, which was present in the updated classification of Orchidaceae. (2) The reduced number of stamens and their functional differentiation would be related with the loss, delay, sub-fusion, sub-divsion of stamen primordia during the ontogeny of gynostemium in Orchidaceae. (3) Significant differences of the ultrastructural pollen existed in the four pollen dispersal units at the levels of taxonomic categories of Orchidaceae, including dry pollen, sticky pollen, sectile pollinium, and hard pollinium, which contains great implications of classifications. (4) Pollen may be dispersed in monads or grouped in pollen dispersal units by following three viscous substances (pollenkitt, elastoviscin, cohesion strands) derived from the tapetum. (5) Three main types of sectile pollinia in Orchidoideae were recognized based upon form and arrangement of massulae. Meanwhile, different numbers and varied morphology of pollinia in Epidendroideae were produced by the different numbers and orientations of sterile septum differentiated in the microsporangium during the early anther development based on anatomic evidence. (6) Some embryological features during anther development were diverse at the different categories, including the number of thecae, the type of anther wall development, the nuclei numbers of tapetum, the pattern of cytokinesis, the varied arrangements of microspore tetrads, and the nuclei number of pollens, which could provide a better understanding of taxonomy and evolution of Orchidaceae. However, considering the large number of orchid species, research data available seemed to be very limited. And then, it is necessary to clarify such fascinating questions as functional differentiation and developmental patterns of stamens and staminods, formation mechanisms for varied pollen dispersal units, and diverse embryological features of anther development. Therefore, much more efforts should be taken for the development of androecium in Orchidaceae, including expansive investigation of materials to be observed, using multidisciplinary technical methods, and rivising the common terminology of orchid floral morphology.