Genetic and Molecular Analyses in Crosses of Race 2 and Race 7 of Albugo candida

Abstract

The inheritance of avirulence and polymorphic molecular markers in Albugo candida, the cause of white rust of crucifers, was studied in crosses of race 2 (Ac2), using isolates MiAc2-B1 or MiAc2-B5 (metalaxyl-insensitive and virulent to Brassica juncea cv. Burgonde) with race 7 (Ac7), using isolate MsAc7-A1 (metalaxyl-sensitive and virulent to B. rapa cv. Torch). Hybrids were obtained via co-inoculation onto a common susceptible host. Putative F1 progeny were selfed to produce F2 progeny. The parents and F1 progeny were examined for virulence on the differential cultivars B. juncea cv. Burgonde and B. rapa cv. Torch. Segregation of avirulence or virulence of F2 populations was analyzed on cv. Torch. Putative F1 hybrids were confirmed by random amplified polymorphic DNA markers specific for each parent. Avirulence or virulence of F 2 progeny to B. rapa cv. Torch suggested 3:1 in each of three populations, supporting the hypothesis of a single dominant avirulence gene. Amplified fragment length polymorphism markers also segregated in regular Mendelian fashion among F2 progeny derived from two F1 hybrids (Cr2-5 and Cr2-7) of Cross-2. This first putative avirulence gene in A. candida was designated AvrAc1. These results suggest that a single dominant gene controls avirulence in race Ac2 to B. rapa cv. Torch and provides further evidence for the gene-for-gene relationship in the Albugo-Brassica pathosystem.