Sinker Structure of Phoradendron californicum (Viscaceae) Confounds its Presumed Close Relationship to Other Acataphyllous Species

Abstract

Phoradendron is the largest genus of New World mistletoes, with about 250 species in two subgenera, Boreales and Aequatoriales, corresponding, respectively, to northern acataphyllous and southern cataphyllous groups. The typically acataphyllous P. californicum of western North America is controversial because recent phylogenetic work has nested it in the southern cataphyllous clade. Seedling establishment, stem anatomy, and endophytic system structure of this species were studied. Seedling haustorial holdfasts have gland cavities, structures considered absent in the Viscaceae clade of Santalales. The stem epidermis has a thick cuticle, deeply sunken stomata, and branched multicellular trichomes. The stem has an outer cortex of palisade chlorenchyma and an inner cortex of large isodiametric parenchyma cells. The boundary area between the outer and inner cortex contains druses and an unusual ring of small xylic bundles lacking protoxylem fibers and phloem. Sinkers are of two types: uniseriate, with only parenchyma that often has thick-walled transfer cells at its interface with vessels of the host; and multiseriate, with parenchyma and vessel elements that often are in direct contact and share simple perforation plates with vessels of the host. Sinker morphology is also dimorphic in the cataphyllous P. fragile but only unimorphic (multiseriate) in the acataphyllous P. juniperinum and P. serotinum. The dimorphic sinker system of P. californicum may be functionally partitioned, whereas these functions are combined in the unimorphic sinkers of other acataphyllous species. Differences in sinker morphology may reflect evolutionary trends in Phoradendron. This study also supports the hypothesis that P. californicum is more closely aligned with the mainly tropical cataphyllous species of the genus.