Unisexual flowers as a robust synapomorphy in Cariceae (Cyperaceae)? Evidence for bisexual flowers in Schoenoxiphium

Cariceae, the largest tribe within Cyperaceae, comprises about 2000 species in five genera. Cariceae is usually considered to be distinct from other Cyperaceae by the presence of exclusively unisexual flowers and by the arrangement of the pistillate flowers in single-flowered spikelets that are enclosed by the flask-like spikelet prophyll (utricle or perigynium). The nature of several morphological features of the Cariceae inflorescence remains controversial. The staminate reproductive units, as well as earlier reported bisexual reproductive units in Schoenoxiphium have been considered to be reduced partial inflorescences, or flowers. Aims of this study are to test both interpretations, based on a floral ontogenetic investigation. Moreover, for the first time, detailed SEM micrographs are presented of the inflorescence and floral development and of bisexual flowers in Schoenoxiphium. We propose that ‘inhibition of bisexuality’ is a more robust synapomorphy in Cariceae than ‘presence of only unisexual flowers’.

► Bisexual flowers in Schoenoxiphium observed using SEM.
► Stamen position in a whorl, proximal to gynoecium, like bisexual cyperoid flower.
► Arrangement, position, absence of prophyll remnants and presence of glume-like structure indicate true bisexual flowers.
► Ontogenetic switch between axis, female spikelet and male flower hypothesised to cause expression of bisexual flowers.
► ‘Inhibition of bisexuality’ a more robust synapomorphy in Cariceae than ‘presence of only unisexual flowers’.