Delimitation of the Gametangia of Rhizopus sexualis (Smith) Callen: an Electron Microscope Study of Septum Formation

Abstract

The development of the septa which cut off the gametangia of Rhizopus sexualis from the suspensor cells was studied by electron microscopy. The edge of the ingrowing septum extends by the incorporation of coalesced vesicles which were previously aligned in the plane of the developing septum. The membranes surrounding these vesicles fuse to form the plasmalemmas of the gametangium and suspensor cell and are eventually continuous with the plasmalemma of the original progametangium. New wall material is laid down between these plasmalemmas. Associated with this process is a considerable increase in complexity of the endoplasmic reticulum which forms broad, irregular bands on either side of the septum. Fine tubules, resembling plasmodesmata, pass through the wall at intervals. Thickening of the septum proceeds more rapidly on the gametangial side. I N T R O D U C T I O NSoon after a pair of progametangia of Rhizopus sexualis make contact at their apices, the terminal parts (gametangia) become cut off from the suspensor cells by the formation of transverse septa. This paper describes the process of formation of these septa as revealed by the electron microscope. METHODSThe fungus was grown on cellophane laid on the surface of malt-agar plates. Young colonies with all stages of zygospore development present were fixed in 2 yo (w/v unbuffered KMn04 for 30 min. at room temperature, and washed several times in distilled water. The hyphae became brittle during fixation and the zygospores were readily detached from the mycelium during washing. Zygospores were then collected in centrifuge tubes, dehydrated, and embedded in Epon or Vestopal. Sections were cut at approximately 450-550A thickness. RESULTSThe first sign of septum development detectable by light microscopy is a slight decrease in density of the cell contents on the inner side of the enveloping walls of the progametangium at the site from which the septum would be expected to develop. The electron microscope revealed, however, that in such a specimen a thin wall (about 135oA thick at the junction with the progametangial wall and tapering to about 385A thickness at the advancing inner edge) was already parti ally developed, extending inwards from the peripheral wall and leaving a central gap of about one third of the

Plasmodesmata and the supracellular nature of plants

“…Among the many thousands of fungal species, plasmodesmata occur in only a few species of Geotricum, Endomyces (the perfect stage of the imperfect Geotrichum), Endomycopsis, Gilbertella, Rhizopus and the chytrids Entophlyctis and Rhizophydium (see Hashimoto, Kishi & Yoshida, 1964;Hawker & Gooday, 1967;Powell, 1974;Marchant, 1976). The plasmodesmata in Entophylyctis and Endomyces contain an appressed ER that is identical in appearance to that observed in higher plants (Fig.…”

“…Cytokinesis in the fungi occurs either by annular furrowing of the cell wall (Bracker & Butler, 1963), or by centripetal development of a cell plate through vesicle fusion (in contrast to the centrifugal mode in the advanced algae and higher plants) (Hawker & Gooday, 1967). Again, there seems to be little correlation between the existence of plasmodesmata and the mode by which cytokinesis is achieved.…”

A New Basidiomycetous Septal Type: The Multiperforate Septum in Kriegeria Eriophori

“…Septal development in species of the filamentous chytridiomycete Allomyces may leave a ring of pores in the septum periphery (Meyer and Fuller, 1985). Gametangial septa in the Zygomycota are commonly multiperforate (Hawker andGooday, 1966, 1967;O'Donnell, Hooper, and Fields, 1976;O'Donnell et aI., 1977b;Gibson, Kimbrough, and Benny, 1986), and vegetative septa may also be multiperforate (O'Donnell et aI., I 977a;Jeffries and Young, 1983). Multipored septa occur in the medullary hyphae ofa number oflichenized ascomycete genera (Wetmore, 1973).…”

Multicellularity and the Need for Communication—A Systematic Overview on (Algal) Plasmodesmata and Other Types of Symplasmic Cell Connections

“…The diameter of these membrane-lined cytosolic channels (20–70 nm) resembles that of canonical PD and in some species a solid ‘core’ ( Figure 1 R) or even an ER-associated ‘desmotubule’ traverses the PD/PDLCs ( Figure 1 A). For the cytokinesis of Rhizopus sexualis , it was shown that the centripetally growing septum is elongated by fusion of linearly aligned vesicles just like in a centrifugally growing plant cell plate [ 116 ]. Thus, the formation of true PD among these vesicles appears feasible.…”