Inheritance and Genetic Relationships of the D8 and D2‐2 Restorer Genes for Cotton Cytoplasmic Male Sterility

In cotton, cytoplasmic male sterility conditioned by the D 8 alloplasm (CMS‐D8) is independently restored to fertility by its specific D 8 restorer (D8R) and by the D 2 restorer (D2R) that was developed for the D 2 cytoplasmic male sterile alloplasm (CMS‐D2). However, knowledge of the relationship between the two restorer genes is lacking. The objectives of these studies were to determine the inheritance and genetic relationship between D2R and D8R. A series of intercrosses and test crosses were used to determine the inheritance and relationships. CMS‐D2 lines crossed with D8R resulted in all sterile F 1 plants, confirming that the D8 restorer gene does not suppress D 2 cytoplasmic dysfunction. A segregation ratio of one fertile to one sterile in three testcross combinations revealed that D2R contained one restorer gene, Rf , for both CMS‐D8 and CMS‐D2. The Rf gene functioned sporophytically, whereas the D8R gene is known to function gametophytically. However, Rf pollen with the D 8 or D 2 cytoplasm was preferentially effective in fertilization based on the low number of sterile plants obtained when alloplasmic F 1 plants were self‐pollinated or used as the pollen‐parent in crosses with CMS‐D8. In tests for allelism only 8 sterile plants out of 1921 were obtained in two testcrosses of (D8R × D2R)F 1 × normal cotton. Thus, the two restorer loci are not allelic but are tightly linked with an average genetic distance of 0.93 cM. The D 2 gene Rf is redesignated Rf 1 , and Rf 2 is assigned to the D 8 restorer gene.