SEXUAL REPRODUCTIONS 12-12 



of complementary mating type. In Nature, cross -fertilization is 

 achieved by insects. 



4. Newton and Johnson perfected techniques for the growth of 

 the rust on barberry and artificial hybridization under controlled 

 conditions. They demonstrated that pathogenic characters are seg- 

 regated in a Mendelian manner, and'Johnson and Newton discovered 

 evidence of maternal inheritance of some characters. Occasional 

 reciprocal crosses produced different kinds of hybrids, depending 

 upon which parent contributed the sperm, indicating that some char- 

 acteristics cannot be transmitted through sperm. 



At present, the rusts are not promising material for the solu- 

 tion of fundamental problems in genetics, because the only char- 

 acters available are the teliospore color and pathogenicity of the 

 dicaryotic hybrids and tetrad analyses cannot be made. Therefore, 

 they are inferior to smuts which produce similar promycelia, but 

 whose sporidial cultures can be grown on artificial medium. The 

 most important problem in rust genetics is obviously the develop- 

 ment of an artificial medium for the growth of sporidial cultures, 

 and considering the present state of our knowledge of culture media, 

 this should not be a difficult task. Its solution would immediately 

 open up the field for tetrad analysis. 



NEUROSPORA 



Shear and Dodge named the genus, Neurospora, which contains 

 both 4-spored and 8-spored species. Dodge discovered that the 8- 

 spored species were heterothallic, while the 4-spored species pro- 

 duced homothallic spores, due to the inclusion of one nucleus of 

 each mating type in the spore. 



The perithecium of Neurospora crassa produces a large num- 

 ber of asci, each containing 8 spores. The spores fall into two 

 categories with regard to mating type. Each spore gives rise to 

 a thallus which produces a mycelium., on which male and female 

 sex organs, as well as asexual conidia, are developed. The fe- 

 male sex organ is called a bulbil and contains an oogonium and tri- 

 chogyne. The male sex organ produces sperm called spermatia. 

 The spermatia from an A type thallus are incapable of fertilizing 

 the oogonium developed on the thallus of the same type, but will 

 cross-fertilize an oogonium of a complementary mating type. The 

 super imposition of sex organs and mating type shows that mating 

 type genes are self-sterility alleles and bear no relation to either 

 sex organs or the evolution of sex. 



Single ascospore cultures are characterized either on morpho- 

 logical or biochemical characters and hybrids are made either by 

 transferring spermatia from a tube of one mating type to a tube of 

 the complementary mating type, or by inoculating a single tube with 

 conidia from each culture and allowing them to grow together. Lin- 



