SEXUAL REPRODUaiONS 12-8 



in an organism in which linkage is unknown and neither chromo- 

 some maps nor tetrad analysis is feasible. 



However, cytoplasmic exchange can be readily achieved in the 

 Hymenocycetes as Harder has so brilliantly demonstrated, and if 

 adequate chromosome maps were available in them, many of the 

 major questions which have been posed by Sonneborn's work might 

 find a less equivocal answer. 



SMUTS 



The smuts are parasites on grassy plants and are economically 

 important because they attack corn and small grains. The seeds 

 of an infected plant are replaced by a sooty dust called smut; the 

 soot is composed of chlamydospores, each of which contains a dip- 

 loid fusion nucleus. In the spring a reduction occurs and the chlamy- 

 dospore germinates to produce a promycelium containing four nu- 

 clei. The promycelium is divided into four cells and sporidia bud 

 from each of the cells. Sporidial cultures resemble yeasts and can 

 be grown in artificial media. There are two mating types in most 

 smuts and sporidial cultures copulate to produce the dicaryons 

 which infect the growing plant, which is almost always parasitized 

 by a dicaryon. Ustilago levis attacks only the oat plant while U. 

 hordei attacks only barley. The dicaryon produced by the copulation 

 of U. levis X U. hordei attacks both. Among the tetrapolar smuts, 

 Bauch discovered that an AB/ab dicaryon on an agar plate produces 

 Suchfaden which are recognized by their rapid filamentous exten- 

 sion across the plate; they correspond to the dicaryotic hjrphae 

 which infect the host. When two sporidial cultures with common 

 A factors but different B factors are mated, Wirrfaden grow out 

 of the mixed culture on the agar plate. Wirrfaden are twisted copu- 

 lation tubes resembling weak, distorted Suchfaden. Bauch dis- 

 covered that both the A and B mating type factors exist in a mul- 

 tiple allelic series producing a great variety of pathogens, and that 

 the same alleles were present in cultures isolated from England, 

 Germany, and Siberia. 



Bauch found that the chlamydospores from each naturally in- 

 fected plant were all of one genotype, but that neighboring plants 

 contained genetically different fungi. Christensen made an extended 

 study of the problem of solo -pathogens. In yeast they would be 

 called illegitimate diploids; solo -pathogens are monosporidial cul- 

 tures which are capable of infecting the plant by producing a dip- 

 loid or dicaryotic mycelium which grow throughout the plant and 

 finally produces chlamydospores. The chlamydospores do not, how- 

 ever always produce solo -pathogenic sporidia. 



Chlamydospores are rather easy to collect and can be germin- 

 ated on agar. Huttig has classified the types of promycelia pro- 

 duced by the different types of smuts. There are several varia- 



