SEXUAL REPRODUCTIONS 12-10 



they have not been exploited to this end, presumably because the 

 workers in this field are more directly concerned with problems 

 of economic importance. It may be, however, that the problems 

 could be more quickly solved if a genetical analysis of the organ- 

 ism were available. The photographs presented by Stakman, Kern- 

 kamp, King, and Martin show that this analysis would not be dif- 

 ficult. 



An idea which has persisted in smut literature is the view that 

 segregation may occur in the third division. This view is based 

 primarily on the extreme variability of many of the sporidial cul- 

 tures. I believe that if one analyzed the data on the assumption 

 that this variability was due to mutation and that third division re- 

 duction does not occur, a linkage map could readily be developed, 

 in spite of the fact that progress to date has been disappointingly 

 slow. 



The cytological evidence presented in support of the third di- 

 vision segregation is quite irrelevant. Four chromosomes have 

 been reported on the metaphase plate and an occasional third di- 

 vision has been observed in which two chromosomes were reported 

 going to the poles. Since the haploid number of chromosomes al- 

 ways appears at the metaphase plate of MeiosesI and n (due to 

 synapsis), there are at least four chromosomes in the haplophase 

 and the supposed observation of two chromosomes at a later divi- 

 sion can only be due to inability to resolve small closely approxi- 

 mated bodies. That this must occur frequently is shown by the 

 failure to find four chromosomes always going to the poles in 

 Meioses I and n. 



The fact that parasitism on the host only occurs when two 

 sporidia of opposite or complementary mating type copulate and 

 produce a dicaryon suggests that complementary genes linked to 

 the mating type genes are essential for production of infective 

 hyphae. The complementary genes must exchange infrequently, 

 a condition which could be achieved by the presence of an inverted 

 section (in which crossing over was inhibited) adjacent to the mat- 

 ing type alleles. Complementary pathogenicity genes would be 

 expected to accumulate in such an inverted section. The fact that 

 the dicaryon is an obligate parasite and, therefore, at least one 

 growth cycle of a small grain is required to complete the smut 

 life cycle is an obvious and serious disadvantage of these organ- 

 isms as compared to yeasts for genetical study. 



RUSTS 



The rust fungi comprise 3500 known species attacking a great 

 variety of plants. At least 200 distinct strains of wheat rust are 

 now known, and it seems probable that an equal variety may exist 

 for each of the 3500 species. Because of the great economic im- 



