Characterization of a knock-out mutation at the Gc2 locus in wheat

Gametocidal (Gc) genes, introduced into common wheat from related Aegilops species, are selfish genetic elements that ensure their preferential transmission by inducing chromosomal breaks. Here we report the production and characterization of a knock-out mutation of the Gc2 gene transferred to wheat as a wheat-Aegilops sharonensis T4B-4S(sh)#1 translocation chromosome. In hemizygous Gc2/- condition, gametophytes lacking Gc2 suffer chromosomal fragmentation and produce non-functional gametes, which leads to sporophytic semisterility and exclusive transmission of the Gc2-carrier chromosome. We have identified one putative ethyl methylsulfonate (EMS)-induced Gc2 mutant that restores spike fertility and shows Mendelian segregation. Progeny screening mapped the mutation to the Gc2-carrier chromosome T4B-4S(sh)#1. C-banding and fluorescence in situ hybridization analyses showed that the loss of Gc2 function in the mutant is not due to a terminal deficiency. Analysis of first and second pollen mitoses in Gc2(mut) /- plants and C-banding analysis of testcross progenies showed that no chromosomal breakage occurs in the mutant. No gametophytic chromosomal breakage was observed in heterozygous Gc2(mut) /Gc2 plants, which had fully fertile spikes. These results suggest that Gc2 encodes two agents, one causing chromosomal breaks in gametophytes lacking Gc2 and another that protects the Gc2 carrier from breakage. The EMS-induced Gc2 mutant appears to be a knock-out of the gene encoding the "breaking" agent. These data are a first crucial step toward the molecular understanding of Gc2 action.