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The fine-structure of asexual spore development in the Choanephoraceae and Cunninghamellaceae (Mucorales Higham, Michael Thomas


The development of sporangia and sporangioles is described in the mucoralean genera Choanephora, Blakeslea, Cunninghamella, and Mycotypha. The fine-structure of spore development is examined with transmission and scanning electron microscopy. Cytoplasmic cleavage in multisporous sporangia of Choanephora and Blakeslea involves fusion of cleavage vesicles to form a cleavage apparatus. Similar cleavage events occur in trisporous sporangioles of Blakeslea but the cleavage apparatus is oriented with the three longtitudinal "suture" lines of the sporangiole wall. The outer layer of spore walls is derived from a fibrous coating on the membranes of the cleavage apparatus. The inner wall layer is formed after deposition of the outer layer and is associated with the presence of granular vesicles in the spore cytoplasm. In monosporous sporangioles of Choanephora, Cunninghamella, and Mycotypha no cleavage apparatus is produced. Spores of all genera studied possess a bilayered wall with the outer layer demonstrating much greater electron-density than the inner layer. The relative thicknesses of the two wall layers varies greatly among genera. The outer layer is deposited in ridges and furrows in spores of Choanephora and Blakeslea. The structure of bipolar spore appendages is identical in these two genera. The ultrastructure of spines on sporangial surfaces is described. Spines on sporangioles of Cunninghamella exhibit a fine-structure different from that of Choanephora and Blakeslea. No spines are produced on spore walls of any of the genera studied. The identification of cellular components is examined using plastic-section histochemistry and light-microscopy. The cleavage apparatus initially contains carbohydrate which disappears as the spores mature. Bipolar spore appendages are shown to be composed of carbohydrate, with their bases staining for protein. In all spores, the cytoplasm shows intense staining for protein and weak staining for carbohydrates. Spore walls are difficult to stain with any of the procedures used. Based on the observed comparative ultrastructural development of asexual spores, recommendations and comments are made concerning the taxonomy of the Mucorales.

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