Preferential fertilization in Plumbago : Ultrastructural evidence for gamete-level recognition in an angiosperm

Abstract

Gametic fusion patterns in the angiosperm Plumbago zeylanica were determined by using cytoplasmically dimorphic sperm cells differing in mitochondrion and plastid content and then identifying paternal organelles through their ultrastructural characteristics within the maternal cytoplasm at the time of fertilization. The virtual absence of plastids within the sperm cell that is physically associated with the vegetative nucleus allows paternal plastids to be used to trace the fate of the two male gametes after fusion. Such paternal plastids were present in the egg in >94% of the observed cases, indicating the preferential fusion of the plastid-rich, mitochondrion-poor sperm cell with the egg. In only one instance did the opposite pattern occur. Since the possibility of this result occurring as the consequence of chance in random fusions is <1 in 7000, this represents strong evidence for the presence of a final putative recognition event occurring at the gametic level.Double fertilization is a phenomenon characteristic of flowering plants involving two separate fusion events that incorporate both sperm cells into developmentally different lineages, both of which are biologically significant. Although numerous pollen tubes compete for the opportunity to fertilize the female gametophyte, if successful, the pollen tube delivers only two male gametes and both fuse with separate female reproductive cells. In contrast with animals and non-seed plants, the angiosperms display a remarkable degree of conservation of gametic material at late stages of sexual reproduction. Of the two sperm cells, one fuses with the egg cell to produce the embryo. Nearly simultaneously, the second sperm cell fuses with the central cell in a second, separate fusion producing the nutritive endosperm. Whether the gametic fusion that initiates this process is a random event or one that results in a directed outcome has remained among the most refractory problems in angiosperm embryology (1, 2) and one that is addressed in the present study.Since sperm cells are outwardly similar cells derived from a single mitotic division of the precursor generative cell, the male gametes of angiosperms have traditionally been assumed to be essentially identical (3), with neither cell possessing a preferential tendency to fuse with the egg. Evidence contradicting this assumption has recently been reported in Plumbago zeylanica (4, 5), indicating cytoplasmic heteromorphism in size, shape, organelle content, and physical association with the vegetative nucleus. Although details of the fusion of gametes are concealed by nucellar cell layers surrounding the egg, ultrastructural studies have revealed that sperm cytoplasmic organelles in Plumbago are transmitted (6, 7) and can be identified by ultrastructural differences in the fertilized female gametophyte (7). Differences present in organelle content within the sperm cells can therefore be employed as an ultrastructurally observable probe, which can be followed with transmission electron microscopy. The prese...

Detection of changes in the nuclear phase and evaluation of male germ units by flow cytometry during in vitro pollen tube growth in Alstroemeria aurea

“…It is also conjectured that some physiological and/or morphological The MGU is thought to be related to double fertilization (Dumas et al 1984). Regarding the MGU of P. zeylanica, sperm cells that were not associated with vegetative nuclei preferentially fused with egg cells, and sperm cells associated with vegetative nuclei fused with central cells (Russell 1985). It was also reported that the expression of germ-line specific polyubiquitin gene differed between sperm cells in the MGUs of P. zeylanica (Singh et al 2002).…”

The Egg Cell: Development and Role in Fertilization and Early Embryogenesis.

“…The presente of unequal quantities of heritable organelles in the sperm is responsible for what has been termed "cytoplasmic heterospermy," whereas differences in nuclear content (such as the proportion of 6-chromosomes) cause "nuclear heterospermy" (Russell, 1985). Although cytoplasmic heterospermy is relatively common in flowering plants (see Russell, 1991), nuclear differences have been reported only in maize, in which 6-chromosomes in the generative cell frequently do not segregate during generative cell mitosis, resulting in aneuploid sperm cells (Roman, 1948).…”

“…In Plumbago, the plastid-rich Sua fuses with the egg 94% of the time (Russell, 1985) and, therefore, selectively transmits male plastids into the zygote. The ratio of male to female mitochondria, by contrast, remains approximately the same, 1:1000, in the egg and central cell; this occurs because differences in mitochondria in the two sperm cells nearly match differences in the mitochondrial content of the egg and central cell (Russell, 1987).…”

Ultrastructural Studies of Gametophytic Apomicts in Guinea Grass (Panicum maximum) I. Differentiation of Aposporous Initial Cell

“…The occurrence of vacuoles is nor- mal phenomenon in megosporogenesis process (Guan and Adachi 1994), and it is also an auspice of cell degeneration (Guan and Adachi 1997). The ultrastructure of dyad cells of P. maximum is comparable to that observed in other plants (Russell 1979(Russell , 1985. In case of P. maximum, the vacuole appearance may be one of auspices of sexual cell degeneration.…”