Comparative anther and pollen tetrad development in functionally monoecious Pseuduvaria trimera (Annonaceae), and evolutionary implications for anther indehiscence

Abstract

The multiple evolutionary origins and diverse morphologies of unisexual flowers in angiosperms indicate that many different developmental mechanisms [sporophytic and (or) gametophytic tissues] underlie patterns of sex differentiation, yet, these mechanisms leading to unisexuality remain largely unresolved. In Pseuduvaria trimera (W.G. Craib) Y.C.F. Su & R.M.K. Saunders, morphologically hermaphroditic flowers are functionally female due to indehiscent anthers, but the developmental and anatomical mechanisms preventing their dehiscence are still unknown. Anther and pollen development were compared in both male and functionally female flowers using histological observations to test whether anther indehiscence results from a sporophytic and (or) gametophytic default. The epidermis, endothecium, middle layers, and pollen development were identical in the two floral morphs, but variations occurred in the tapetum and stomium regions. In male flowers, concurrently with the binucleate tapetal cell degeneration, the appearance of intercellular spaces and lysis of the stomium region cells lead to anther dehiscence. Conversely, in the functionally female flowers, trinucleate tapetum appears with delayed degradation, and the persistent cells with a highly vacuolated cytoplasm and stomium region remain intact at maturity. Sporophytic tissues with tapetum abnormalities and stomium integrity are, thus, responsible for anther indehiscence. Lack of microspore rotation in P. trimera might indicate a different evolutionary origin of pollen tetrad formation in this family.