Linkage Analysis between Gametophytic Restorer Rf 2 Gene and Genetic Markers in Cotton

In heterozygous fertility restored F 1 plants of the CMS‐D8‐ Rf 2 gametophytic restoration system of cotton ( Gossypium hirsutum L.), only the pollen grains with the Rf 2 allele are functional; consequently, all F 2 plants are fertile. Our objective was to develop a statistical method to estimate the recombination fraction (r) between Rf 2 and genetic markers linked in repulsion or coupling phases in fertile F 2 populations. Alleles linked to Rf 2 are preferentially transmitted through pollen, whereas the transmission of alleles linked to rf 2 is reduced. Thus, linked genes give distorted segregation ratios depending on the linkage strength. Genes independent of Rf show normal segregation. The traditional 3:1 or other appropriate ratios can be used to test the linkage between a marker and the Rf locus in an F 2 population. Examples are given for two crosses: (D8R × T586) F 2 for repulsion linkage and (D8R × T582) F 2 for coupling phase linkage. The morphological data confirmed that Rf 2 is not linked to nine dominant genes in T586 or to five recessive genes in T582. However, a RAPD marker, UBC188 500 , present in D8R and absent in nonrestoring lines, exhibited extremely skewed segregation in the D8R × T586 F 2 population with only two plants without UBC188 500 in a population of 76 plants. The recombination frequency between Rf 2 and this marker is 5.26%, which agrees with our previous estimate from a testcross, (D8R × T586) × H1330. This indicates that the proposed method is a valid alternative for mapping gametophytic Rf 2 Its advantages and limitations are discussed.