Genetic characterization of the partial restorer of fertility gene, Rf8, in T cytoplasm maize

Texas (T) cytoplasm maize (Zea mays L.) is a model system for studying cytoplasmic male sterility and fertility restoration. The mitochondrial gene responsible for male sterility is T-urf13. Fertility is restored to T cytoplasm maize by the combined action of two nuclear genes, Rf1 and Rf2a. Rf1 is associated with the accumulation of additional 1.6/0.6 kb T-urf13 transcripts while Rf2a is an aldehyde dehydrogenase and not involved in transcript accumulation. There are also partial restorers of fertility for T cytoplasm. Rf8 and Rf*, with the combined action of Rf2a, restore partial fertility and are associated with the accumulation of additional 1.42/0.42- and 1.4/0.4-kb T- urf13 transcripts, respectively. Rf8 was positioned on the long arm of chromosome 2 near white pollen ( whp1) using a segregating backcross population. PCR markers were developed to map Rf8. Flanking PCR markers that span a physical distance of 4.56 Mb in the B73 maize genome sequence correspond to a genetic distance of 8.28 +/- 3.25 cM. This region includes seven pentatricopeptide repeat genes. Partially male-fertile plants segregated for the presence or absence of the Rf8-associated transcripts, indicating that presence of the 1.42/0.42-kb transcript is not necessary for anther exertion. Fertility phenotypes decreased with successive generations of backcrossing to (N) W64A, where as re-introducing the (N) wx1-m8 background increased fertility. The day after planting to the first flowering was observed for partially fertile and mostly fertile plants. Mostly male-fertile plants flowered 2.7 to 3.4 days earlier than partially male-fertile plants. These results demonstrate the flanking region rf8 is located in and the possibility that male fertility is under the control of more than one nuclear locus.