Development in the female flowers/fruit of the dwarf mistletoe Arceuthobium americanum (Viscaceae): Reproductive development, viscin tissue development, and cytogenetic analysis of tissue regions within the fruit

Arceuthobium americanum (dwarf mistletoe) is a dioecious flowering plant that is parasitic on Pinus banksiana (jack pine) in Manitoba. The purpose of this thesis was to elucidate reproductive developmental processes occurring in the female flowers and fruit of A. americanum in Manitoba using techniques of microscopy and image analysis. Reproductive development in the female flowers/fruit was documented from anthesis in early April to explosive discharge of the dispersal unit (ategmic seed or pseudoseed) at the end of August, seventeen months after anthesis. At anthesis, the ategmic reduced ovular structure, which was defined as a placental-nucellar complex (PNC), was fully developed, and two obliquely oriented hypodermal megasporocytes were evident in the tenuinucellate PNC. Multiple pollen tubes were already present in the style and around the base of each megasporocyte, and so synergids were not needed for pollen tube attraction in A. americanum. Both megasporocytes were capable of completing megasporogenesis. Megasporogenesis was bisporic, and the upper dyad from each megasporocyte (the dyad distal to the base of the PNC) became a functional megaspore. Only one functional megaspore was capable of completing megagametogenesis to form an unfertilized mature seven-celled embryo sac; the other functional megaspore degenerated.; The unfertilized mature seven-celled embryo sac possessed an egg apparatus (two synergids and an egg cell) at the lower pole of the embryo sac, three antipodals at the upper pole, and a central cell that occupied the remainder of the embryo sac. The two polar nuclei in the central cell fused shortly after their formation. One of the two synergids began to degenerate almost immediately after it was formed. Cell wall material was lacking at the interface between the unfertilized central cell and unfertilized egg apparatus. The unfertilized egg cell was attached to the upper side of the embryo sac, above and to the side of the two synergids. Only one pollen tube penetrated the mature embryo sac, entering via the degenerating synergid. Following double fertilization, cell wall material was deposited at the interfaces among all cells of the embryo sac. The egg cell and central cell become more metabolically active following fertilization, as evidenced by a loss of vacuolation and increase in organellar content. (Abstract shortened by UMI.)...