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CROP BREEDING, GENETICS & CYTOLOGY

Linkage Analysis between Gametophytic Restorer Rf₂ Gene and Genetic Markers in Cotton

^a Dep. of Mathematical Sciences, Box 30001, New Mexico State Univ., Las Cruces, NM 88003
^b Dep. of Crop, Soil and Environmental Sciences, 115 Plant Science Building, Univ. of Arkansas, Fayetteville, AR72701

^* Corresponding author (jinzhang{at}nmsu.edu).

In heterozygous fertility restored F₁ plants of the CMS-D8-Rf₂ gametophytic restoration system of cotton (Gossypium hirsutum L.), only the pollen grains with the Rf₂ allele are functional; consequently, all F₂ plants are fertile. Our objective was to develop a statistical method to estimate the recombination fraction (r) between Rf₂ and genetic markers linked in repulsion or coupling phases in fertile F₂ populations. Alleles linked to Rf₂ are preferentially transmitted through pollen, whereas the transmission of alleles linked to rf₂ 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 F₂ population. Examples are given for two crosses: (D8R x T586) F₂ for repulsion linkage and (D8R x T582) F₂ for coupling phase linkage. The morphological data confirmed that Rf₂ is not linked to nine dominant genes in T586 or to five recessive genes in T582. However, a RAPD marker, UBC188₅₀₀, present in D8R and absent in nonrestoring lines, exhibited extremely skewed segregation in the D8R x T586 F₂ population with only two plants without UBC188₅₀₀ in a population of 76 plants. The recombination frequency between Rf₂ and this marker is 5.26%, which agrees with our previous estimate from a testcross, (D8R x T586) x H1330. This indicates that the proposed method is a valid alternative for mapping gametophytic Rf₂. Its advantages and limitations are discussed.

Abbreviations: CMS, cytoplasmic male sterility • ML, maximum likelihood • PCR, polymerase chain reaction • RAPD, random amplified polymorphic DNA • SD, segregation distorter