412 CLASS BASIDIOMYCETEAE 



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Fig. 138. Subclass Teliosporeae, Order Ustilaginales, Family Ustilaginaceae, 

 Ustilago zeae (Beckm.) Unger. (A) Germination of teliospore to form promycelium 

 and sporidia. (B) Two sporidia of opposite sexual phase, infecting epidermal cell of 

 host {Zea mays L.), the slender germ tubes uniting to form a stout multinucleate 

 hypha. (Courtesy, Hanna: Phytopathology, 19(5):415-442.) 



Family Ustilaginaceae. Promycelium transversely septate into 

 several, mostly four, cells. The teliospores arise from transformed di- 

 caryon cells of the mycelium. Sometimes the cell walls of the usually 

 tangled hyphae swell and undergo gelatinization, thus separating the 

 protoplasts to some distance. Around these then, within the gelatinized 

 walls, new walls are laid down and the cells enlarge while the gelatinized 

 walls disappear. At about the same time the two nuclei unite and the 

 teliospores come to maturity. The teliospores are produced singly or 

 united into spore balls. They are usually dark-colored, with the wall more 

 or less thickened and smooth or rough. From each cell of the promycelium 

 usually several sporidia are produced, for, unlike the majority of Uredi- 

 nales, the nucleus of the cell divides several times and one of the daughter 

 nuclei enters each sporidium. In some cases each promycelial cell grows 

 out into a slender germ tube instead of producing sporidia. This difference 

 in behavior is connected for some species with differences in temperature, 

 moisture, etc. In Ustilago nuda (Jens.) Kellerm. & Swingle and U. tritici 

 (Pers.) Rostr. sporidia are apparently never found. (Fig. 138A.) 



Sexual reproduction in this family is accomplished in various ways. In 

 several species it has been determined that two of the promycelial cells 

 represent one sexual phase and the remaining two the other phase. The 

 distribution of the two sexual phases in the four cells of the promycelium 

 may occur apparently in every possible order. In Ustilago zeae (Beckm.) 

 Ung., Hanna (1929b) showed that there occur four sexual phases. Prob- 

 ably there are two allelomorphic pairs of genes on separate chromosome 

 pairs that control this sexual behavior. If both chromosome pairs undergo 

 their reduction (disjunction) division in the first or in the second of the 

 two divisions occurring in the production of the promycelium two of the 

 promycelial cells will be of one sexual phase and two of the opposite 



