Inheritance of greenbug, Schizaphis graminum (Rondani), virulence to Gb2 and Gb3 resistance genes in wheat

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Abstract
The inheritance of greenbug, Schizaphis graminum (Rondani), virulence to wheat, Triticum aestivum L., was investigated. Clones of greenbug biotypes C, E, and F were induced into the sexual cycle, reciprocally crossed, and inbred. The resulting progeny were cloned via parthenogenetic reproduction, so their virulence to resistance genes Gb2 ('Amigo') and Gb3 ('Largo') could be established using diagnostic feeding lesions. The data for both resistance sources fit a duplicate gene – modifier gene inheritance model where avirulence was dominant and virulence was recessive. Virulence to genes Gb2 and Gb3 was conditioned by duplicate genes and a dominant modifier gene epistatic to one of the duplicate genes. Linkage was definite among the genes conditioning virulence to Gb2 and Gb3 when heterozygous males were used in crosses, as a result of achiasmic spermatogenesis. When homozygous males were used in reciprocal crosses, 50% recombination occurred. This unique linkage affinity suggests that the multiple genes conditioning virulence to Gb2 and Gb3 reside on the same chromosomes but are ≥50 map units apart. Specific aphid–host genetic interactions did not fully conform to the gene-for-gene inheritance hypothesis normally associated with host–parasite relationships. Nevertheless, polygenic inheritance of greenbug virulence in wheat could easily be regarded as a gene-for-gene relationship because the duplicate gene – modifier gene mode of inheritance still influences a single gene product in the aphid.Key words: Schizaphis graminum, Triticum aestivum, insect resistance, insect virulence, insect biotypes.