SUMMARYThe ascus cytology of Podospora anserina (Ces.) Rehm, a secondarily homothallic Pyrenomycete fungus, has been followed under the light microscope from the crozier, through karyogamy and the three nuclear divisions in the ascus (the first two constituting a meiosis and the third a mitosis) to the first spore mitosis and spore maturation. The spindles remain widely separated at division 11, during which segregation for mating type is said to occur. After division 111, in which the spindles are transverse, the ascus contains four pairs of sister nuclei. The realignment of the nuclei so that two non-sister nuclei are included in each of the four spores is brought about by the movements of the centrosomes or, more likely, of their outer portions. Spore delimitation, which starts from these organelles, appears as a thin cleavage line separating the spore plasm from the epiplasm of the ascus. A mitosis in the spore initial followed by degeneration of one of the four daughter-nuclei in the primary appendage leads to an imbalance in the mating-type factors in the spore. The wall of the mature spore is separable into three layers, the middle one of which is pigmented. The mucilaginous secondary appendage formed at each end of the spore is already present before the primary appendage is cut off.
I N T R O D U C T I O NPodospora anserina (Ces.) Rehm is a Pyrenomycete fungus which has been the subject of many genetic studies. Its life-cycle was described by Ames (1932, 1934) and Dodge (1936) and some observations on its cytology were published by Moreau & Moreau (r951), Franke (1957Franke ( , 1962 and Heslot (1958). An attempt is made here to give a more complete account of the cytology.
METHODSCultures of Podospora anserina were grown in Petri dishes on a medium of sheep dung agar, at room temperature in continuous light. Small pieces of sterile filter paper were mixed with the medium to enhance perithecial production.Blocks of agar, approximately 3 mm3 and containing numerous young perithecia, were fixed for 15 min. under vacuum in either I : 3 (v/v) acetic acid : ethanol solution, or a I : 3 (v/v) propionic acid : ethanol solution, after which they were left in fixative at room temperature for periods of 24 hr to 6 weeks. The blocks were then washed for 2-24 hr under running water before treatment with a saturated solution of cytase (S/401 G.T. Gurr Ltd) for 10 hr. They were again washed in running water for I hr and then placed in a 400 ,ug./ml. solution of ribonuclease and incubated for I 2-24 hr at
SUMMARYThe ascospores of Podospora anserina (Ces.) Rehm are delimited by a double membrane system. The primary spore wall develops within this, the outer part of the double membrane being pushed out to form the spore membrane and the inner part forming the plasmalemma of the spore. Starting in the middle of the matrix of the expanding primary wall, a secondary wall is laid down and gradually extends to the outer periphery of the spore wall. Later, a thick tertiary wall is formed at the inner side of the secondary wall by blocks of electron-dense material between which channels of the primary wall matrix remain. This is the pigmented layer of the spore wall. On the innermost side of the spore wall, a part of the original primary wall remains.The primary appendage at the base of the spore arises as part of the spore initial, but, after it has been cut off by a septum, its contents degenerate and it is bounded only by the primary and secondary wall layers. The secondary appendages, formed at the apex of the spore and at the bottom of the primary appendage, are considered to be actively growing processes bounded by the spore membrane.
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