COMPARATIVE GYNOECIUM STRUCTURE AND MULTIPLE ORIGINS OF APOCARPY IN CORYPHOID PALMS (ARECACEAE)

Within palms, recent phylogenetic evidence suggests that the free-carpellate (apocarpous) condition that occurs in mature gynoecia of some species represents a derived condition, in contrast to traditional views. This article examines the comparative morphology and ontogeny of several palm genera representing a broad phylogenetic range within the diverse subfamily Coryphoideae (Arecaceae/Palmae). The resulting data are optimized onto a recent supertree phylogeny to clarify the evolutionary history of gynoecial characters within Coryphoideae. The investigation shows that coryphoid palms possess unusually diverse gynoecium morphology, especially with respect to carpel fusion, nectary presence, and nectary location. Our observations indicate three evolutionary transitions within Coryphoideae to a free-carpellate condition (apocarpy): in Cryosophileae, Phoeniceae, and subtribe Rhapidinae of Trachycarpeae. All three groups of apocarpous coryphoids appear to lack gynoecial nectaries, even at the ovary base. The monocarpellate condition that is present in one group of genera within the tribe Cryosophileae has apparently evolved from the free-carpellate condition. Some coryphoids possess a gynoecial condition that is apparently unique to Trachycarpeae among monocots, in which three carpels become postgenitally fused only at their styles. Based on our optimization, the relatively unusual conditions of apocarpy, a monocarpellate gynoecium, and partial congenital syncarpy are all derived from the plesiomorphic condition of postgenital syncarpy, which occurs in the syncarpous clade of Coryphoideae. Taking into account the genus Nypa (subfamily Nypoideae), apocarpy appears to have been independently derived four times within palms.