Nuclear-cytoplasmic interactions in restoration of male fertility in the ‘9E’ and A4 CMS-inducing cytoplasms of sorghum

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The genetics of male-fertility restoration in sorghum in the “9E” and A4 CMS-inducing cytoplasms, was studied by crossing a number of fertility restorer lines of A1 cytoplasm to CMS lines [9E]T×398 and [A4]T×398 and the line [9E]Milo-10, which was obtained by backcrossing Milo-10 to [9E]T×398. It was revealed that both A4 and “9E” cytoplasms are characterized by a sporophytic mode of restoration of male fertility. Depending on the nuclear background of the male parents, fertility restoration was controlled by one or two dominant genes. Fertility-restorer genes of one of the tester lines, KVV-114, were effective in [9E]T×398 but could not restore [9E]Milo-10. A fertile line obtained from the fertile hybrid [9E]T×398/KVV-112, with “9E” cytoplasm, also failed to restore [9E]Milo-10. In a number of hybrid combinations with both A4 and “9E” cytoplasms a novel and unusual phenomenon of gradual restoration of male fertility in subsequent backcross generations was observed. Pollen from the fertile revertants did not transmit fertility restoration in progeny from crosses with the original CMS line and was poorly transmitted in sib-crosses. The appearance of fertile revertants and the different reactions of different CMS lines with the same cytoplasm in test-crosses may be caused by the action of recessive nuclear genes of the recurrent male parents that were accumulated during backcrossing; these may induce changes in cytoplasmic genes controlling CMS.