Ultrastructure of the fertilized embryo sac in the dwarf mistletoe Arceuthobium americanum (Viscaceae) and development of the caecum

Abstract

Post-fertilization changes in the seven-celled embryo sac of the parasitic angiosperm Arceuthobium americanum Nutt. ex Engelm. were investigated with light, fluorescence, and electron microscopy. Two embryo sacs, of which only one is fertilized, are found within the reduced ategmic ovule (the placental–nucellar complex). All cells of the fertilized embryo sac developed thickened cellulosic and (or) hemicellulosic bounding walls and, aside from the degenerative synergid, acquired ultrastructure reflective of metabolically active cells. The filiform apparatus became thickened in the persistent synergid, and a similar structure developed in the antipodals. Unlike the changes observed in the zygote and central cell, those in the persistent synergid and antipodals were unusual, as these cells regularly degenerate after fertilization in a typical angiosperm; therefore, in A. americanum, they likely play a role in providing nutrition to the zygote and (or) central cell. A pouch-like outgrowth (caecum) initiated from the central cell near the primary endosperm nucleus and, after vacuolar expansion and intercellular growth, reached the base of the placental–nucellar complex in three days. The observation of mitochondria clusters within the central cell, caecum, and adjacent placental–nucellar complex cells suggests the caecum functions in embryo sac expansion and haustorial nutrient absorption.Key words: Arceuthobium, caecum, cell walls, fertilized embryo sac, mistletoe, ultrastructure.