Flower development and morphoanatomy of Dendrophthora costaricensis Urb. (Viscaceae) and its implications in the taxonomy of Phoradendreae

Dendrophthora is a genus of stem-parasitic plants that are commonly known as mistletoes. It exhibits a notable morphological similarity to Phoradendron, which makes differentiation difficult. Traditionally, the two genera are grouped within the tribe Phoradendreae. Molecular analyses have called into question the monophyly of each genus but confirm the monophyly of Phoradendreae. The unique distinction between genera is the number of locules in the anther. However, this feature introduces uncertainties when defining the two genera. Therefore, this study examines the development and floral morphoanatomy of Dendrophthora costaricensis to identify characters that distinguish it from Phoradendron. Standard histological and scanning electron microscopy techniques were employed. Staminate flowers of D. costaricensis have monosporangiate anthers throughout development, while the pistillode develops structures resembling a style and stigmodium. In pistillate flowers, the carpels are arranged in a ring with an internal placenta before ovule formation; there is no vestigial androecium. As the carpels close, a sessile stigma and two ategmic ovules develop, aligned in the dorsal-ventral plane of the flower. For the first time, a nectary has been observed in the gynoecium and pistillode of Dendrophthora. Anther wall development is monocotyledonous and basic, with dehiscence through an apical pore. We describe the development of pollen and unusual embryo sacs. While similarities exist in the development and floral morphoanatomy of the two genera, the presence of monosporangiate anthers and an Allium type of embryo sac development are distinctive features of Dendrophthora. Nonetheless, more research on floral development in Phoradendreae is needed for accurate comparisons.