P. R. DAY 



MORPHOLOGICAL MARKERS 



The most commonly used morphological markers are those with altered 

 spore pigmentation. The cytoplasm of the uredospores of P. graminis 

 normally contains an orange carotenoid pigment. The uredospore wall is 

 brown. The two colors superimposed produce the normal reddish brown color 

 of wild type uredospores. Newton and Johnson (1927) described two spon- 

 taneous pigment mutants: "orange" was due to colorless wall and "grayish- 

 brown" was due to colorless cytoplasm. Both characters were recessive 

 (Newton, Johnson, and Brown, 1930). Selfing an Fj between the two mutants 

 revealed a third or double mutant class with white spores in the F2 . 

 These spores had colorless walls and cytoplasm. 



Some other white mutants of P. graminis tritici and P. graminis 

 seoalis examined by Green (1964) behaved in a similar way. These white 

 mutants were observed by inoculating barberry with basidiospores produced 

 by telia collected in the field. Normal pycnial lesions are orange; the 

 mutant pycnia were white. The white pycnia were rather infertile and the 

 color of the few aecia which developed after fertilization depended on 

 the source of the fertilizing spermatia. If these came from white pycnia, 

 white aecia were formed. If the spermatia came from normal orange pycnia, 

 then orange aecia formed showing that "white", or colorless cytoplasm, 

 was recessive. White aecial spores from material presumed to be P. 

 graminis seoalis gave no infections on wheat, rye, or oats. White aecio- 

 spores of P. graminis tritici showed poor ability to infect wheat but the 

 uredia that were established from white aecia were grayish-brown in color. 



In P. graminis, Watson and Luig (1962b) found many mutations to 

 colorless wall and noted that all common Australian strains of stem rust 

 are available as orange uredospore stock cultures as well as wild type. 

 Baker and Teo (1966) treated uredospores of P. graminis avenae with the 

 chemical mutagen, ethyl methane sulphonate (EMS). They recovered yellow, 

 orange, and gray-brown mutants. The yellow mutant was reported as lacking 

 color in the uredospore wall but whether it was a double mutant or a wall 

 color mutant with a pleiotropic effect on the carotenoid of the cytoplasm 

 was not determined. 



In the same experiments Baker and Teo recovered two mutants in which 

 development of teleutospores was accelerated. At a day temperature of 

 27°C, uredospores of these mutants inoculated to susceptible oat seedlings 

 immediately gave rise to telia. Uredia were only formed at lower tempera- 

 tures. Wild type strains do not form telia on leaves of greenhouse grown 

 plants and in the field telia are restricted to mature plant stems. 

 Mutants of this kind could present certain genetic advantages. 



Apart from the lowered infectivity or avirulence of the white mutants 

 noted above, no gross changes in pathogenicity are associated with the pig- 

 ment mutants although the tests applied were not very sensitive. Anthraqui- 

 none pigment mutants of the tomato leaf mould disease fungus Cladosporium 

 fulvum show a marked disadvantage in competition with the parent, wild- 

 type strain which, however, is not so marked as the disadvantage imposed 

 by possessing unnecessary genes for virulence (Day, 1968) . 



Other morphological mutants in rusts are uncommon. The stringent 

 selection imposed by obligate parasitism presumably eliminates those 

 mutants which might be useful in genetic studies but which have the effect 

 of lowering fitness or reducing aggressiveness. Even so Flor (1965) has 

 recorded a recessive uredospore mutant with a smooth wall rather than the 



