Reversal in Ovary Position from Inferior to Superior in the Haemodoraceae: Evidence from Floral Ontogeny

Abstract

Floral anatomy and development were studied in Wachendorfia thyrsiflora and Lachnanthes caroliniana of the monocot family Haemodoraceae in order to evaluate the hypothesis that a superior-ovary position arose from an ancestral inferior-ovaried condition. In Wachendorfia three conduplicate carpel primordia develop at the floral apex, followed by postgenital fusion forming a superior ovary. The lower receptacular tissue at the central floral axis elongates at about the same extent as the peripheral receptacular tissue. In the extended receptacular region, two lateral commissures develop by differential tissue growth, followed by differentiation of inner epithelial cells to form septal nectaries. The receptacular commissures and nectaries are continuous with carpellary commissures of the superior ovary. Junction of the median outer tepal vein and dorsal carpellary vein occurs well below the septal nectaries at the base of the receptacle. In Lachnanthes three small, conduplicate carpel primordia develop at the floral apex. Growth of the lower receptacular tissue at the central floral axis is relatively retarded, such that the peripheral tissues elongate differentially, surrounding the now laterally positioned carpels. Subsequent postgenital fusion results in an inferiorovary position, relative to perianth insertion. Incomplete postgenital fusion of intercarpellary spaces results in the formation of septal commissures. These commissures later mature into septal nectaries, which extend to the apex of the inferior ovary. Junction of the median outer tepal vein and dorsal carpellary vein occurs just beneath the ovary locules. It is proposed that the floral receptacular region of Wachendorfia and of other superior-ovaried Haemodoraceae is homologous with an ancestral inferior ovary with regard to position, development, and vasculature. A superior ovary in the Haemodoraceae arose by the co-option of an ancestral inferior ovary into an elongated receptacle, along with extension of the intercarpellary commissures to the periphery of the receptacular tissue, and modification of those peripheral commissures to nectaries The adaptive significance of this evolutionary event is unclear but apparently related to a shift in pollination mechanism