Ultrastructure of developing ascospores in Sordaria brevicollis

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Abstract
The ultrastructure of ascospore wall formation in the pyrenomycete S. brevicollis was studied in developing asci at progressive time intervals. From early spore delimitation through final stage of maturation, the wall of the ascospore differentiated into 4 composite layers, the periascosporium, the delineation ascosporium, the subascosporium and the endoascosporium. While ascospores were at the hyaline stage of development, they possessed only the periascosporium and delineation ascosporium as their wall components. At about 7 to 8 days from the initiation of the cross, the spores developed a yellow color, and this coloration was always associated with the elaboration of the subascosporium just internal to the ascosporium. As the spores continued to progressively darken in color, the subascosporium was seen to increase in complexity, electron density and thickness. Soon after the formation of the subascosporium, the endoascoporium began to develop de novo and was, therefore, the last wall layer formed as the spore approached maturity.