The meiospore ofPhysoderma maydis. The causal agent ofPhysoderma disease of maize

Summary The meiospore ofPhysoderma maydis (Phycomycetes, Chytridiales, Physodermataceae) has a nuclear cap enclosing the cellular ribosomes within a double membrane, and double membranes traversing the nuclear cap. Aggregates of ribosomes not incorporated into the nuclear cap are also enclosed by double membranes. A vesicular network is observed in the anterior portion of the spore in direct connection with the nuclear cap membrane and with a stacked parallel array of membranes, which itself is connected with the nuclear cap membrane.The meiospore ofP. maydis contains a side body complex of the type observed in spores of theBlastocladiales. Vesicles enclose the side body complex and these vesicles are connected to the nuclear cap membrane and the nuclear envelope, and form a network which partially encloses the kinetosomal apparatus.The nuclear cap membrane, stacked array of membranes, and the vesicles which surround the side body complex and the kinetosomal apparatus contain an electron-dense amorphous material. On the basis of their ultrastructural appearance, these membranes are interpreted as part of a highly divided microbody.The ultrastructural organization of the meiospore ofP. maydis is compared to the structural organization observed in spores of theChytridiales, Blastocladiales, Monoblepharidales, andHarpochytriales. It is concluded that the structural organization of the meiospores ofP. maydis is the same as observed for members of theBlastocladiales, and it is suggested that thePhysodermataceae should be transferred from theChytridiales to theBlastocladiales.