Polygenic inheritance of aggressiveness in Ustilago hordei

Summary

It has been suggested previously that polygenes play an important role in host-parasite relationships, especially in controlling horizontal resistance of the host and aggressiveness of the parasite. Also, quantitative characters in many fungi were found to be under polygenic control. In the fungus Ustilago hordei, causing the disease covered smut in barley, the ordered tetrads of two teliospores homozygous for the recessive virulence allele Uh v-1 were selfed and crossed in all compatible combinations and the resulting 16 dikaryons were tested on the barley variety Vantage. Aggressiveness (i.e. degree of infection) was found to be a continuous character genetically controlled by polygenes which modify the expression of the recessive virulence allele Uh v-1. The analysis of variance showed a large amount of variability in aggressiveness both between and within the teliospores. It also showed that the dikaryons resulting from crossing were significantly more aggressive than those resulting from selfing, suggesting that the alleles for higher levels of aggressiveness are dominant and indicating the existence of heterosis. The results showed the importance of the genetic control of aggressiveness; about 65 per cent of the variability was due to genetic causes while 35 per cent was due to environmental ones. They also showed that the genetic control of the polygenes was not simply additive, there was an amount of gene interaction, which could be due to dominance, epistasis or both.