P. R. DAY 



The opportunities for genetic studies in any organism are dictated 

 by such aspects of its biology as life cycle, generation time, experi- 

 mental convenience and, increasingly nowadays, its economic significance. 

 In all respects except the last, the rusts present obstacles. But their 

 destructive effects on man's food and fiber crops far outweigh the other 

 considerations and rusts have in fact been the subject of more intensive 

 genetic study than any other group of fungal pathogens. This paper does 

 not review the considerable literature on rust genetics but is rather an 

 introduction to the basic features of theory and practice with some dis- 

 cussion of recent work and some speculation about the future of rust 

 genetics . 



RUST LIFE CYCLES 



In common with other basidiomycetes rusts have two phases, a homo- 

 karyon made up of haploid uninucleate cells and a dikaryon made up of 

 binucleate cells. The dikaryon in some respects is equivalent to a 

 diploid since in each cell of the mycelium the same two nuclei are 

 present. Five well defined stages, denoted - IV, characterize the 

 development of the most complex or "long-cycle" rusts. 



0. PYCNIA (SPERMOGONIA) AND SPERMATIA (PYCNIOSPORES)/ 



The flask-shaped pycnia develop as a result of infection by a haploid 

 basidiospore. In the cavity of the pycnium, uninucleate spermatia are 

 formed which act as fertilizing elements. They are carried in the drop 

 of sugary fluid which exudes from the opening of the pycnium. Receptive 

 hyphae produced by the pycnium extend into the drop of fluid. Here they 

 fuse only with compatible spermatia from a different pycnium carried there 

 by insects or rain splashes. Fusion between spermatia and receptive hyphae 

 is governed by a simple bipolar he.terothallism. The two mating types are 

 denoted + and -. Spermatia are unable to initiate new infections so this 

 phase of the rust, although it may grow perennially, is unable to repeat 

 itself by spore dispersal. Clonal propagation can be achieved by transfer 

 of infected tissue in laboratory studies (Hermansen, 1959; Garrett, 1960; 

 Patton, 1962). 



I. AECIA AND AECIOSPORES 



These are produced as a result of the sexual fusion described above, 

 The nuclei of the spermatia migrate through the hyphae of the pycnium, 

 passing from cell to cell through the septal pores until they reach an 

 aecial rudiment. The resulting dikaryon produces an aecium or pustule 

 consisting of chains of binucleate aeciospores which eventually burst 

 through the host epidermis. The aeciospores are released usually by a 

 discharge mechanism and may infect an appropriate host. 



II. UREDIA AND UREDOSPORES 



The uredia result from infection by aeciospores or uredospores. The 

 infecting mycelium is a dikaryon and the uredospores are stalked 

 binucleate cells. When released they can repeat this cycle of infection 

 for as long as conditions remain favorable. 



