Rapid Isolation of Loss‐of‐Function Mutations for Dominant Traits: A Case Study Using Photoperiod Sensitivity Trait in Pima Cotton

In its natural state, cotton ( Gossypium spp.) is a perennial that flowers only under short‐day conditions. Early selection efforts eliminated this photoperiod requirement, allowing cotton cultivation to shift predominantly to annual growth under long summer daylengths throughout the world. Photoperiod sensitivity persists in many wild cottons and remains a major barrier to the introgression of beneficial alleles into elite breeding material. Identification of the variation and genetic factors underlying photoperiod response in cotton is needed to fully harness the genetic diversity of wild cotton and broaden the cultivated germplasm pool. Genetic variants, especially loss‐of‐function mutations in dominant genes conferring photoperiod sensitivity, would be highly valuable in this regard. After mutagenizing pima cotton ( G. barbadense L.) seeds heterozygous for the major photoperiod response gene Gb_Ppd1 with γ rays, we identified plants no longer sensitive to photoperiod. Genetic analysis of BC 1 F 1 plants derived from backcrossing the mutants with their photoperiod‐insensitive parent confirmed the loss‐of‐function mutations were allelic with Gb_Ppd1 . Characterization of the mutants with molecular markers confirmed that all but one were deletions of Gb_Ppd1 . Further, genotyping localized Gb_Ppd1 to a 12‐Mb region on chromosome D10. The newly identified genomic region and loss‐of‐function mutants may help assist with the identification and functional validation of candidate genes for photoperiod response in cotton, facilitating the valuable introgression of genetic diversity into cotton breeding programs.