Speed breeding accelerates the conversion of waxy maize inbreds to C-type cytoplasmic male sterility for hybrid seed production

Hybrid maize seed production requires male-sterile maternal lines, yet converting elite lines to cytoplasmic male sterility (CMS) by conventional field backcrossing takes years. Our goal was to developed a speed breeding (SB) protocol adapted for maize and apply it to the rapid conversion of maternal lines onto CMS-C cytoplasm (Charrua) through successive backcrosses with parallel phenotypic evaluation of tassel fertility. The work was carried out with the maternal waxy lines V158wx and WF-9wx, which are the parental components of the heterotic hybrids “Rustark AP”, “Volya” and “Waterloo” (LLC “Agroplazma”). Crossing of the CMS-C donor and the maternal lines was performed under field conditions; for line V158wx, the first backcross was also conducted in the field. Subsequent crosses were carried out in SB chambers with the additional application of embryo culture. Five backcross cycles in maize were completed in 13 months. On average, under SB conditions, maize tassels flowered at 52.0–57.1 days depending on the genotype, and silks emerged 2.0–2.3 days later. The full generation averaged 79.0–81.2 days across five cycles. Performing hybridization in SB proved highly effective despite difficulties such as synchronizing flowering of the maternal and paternal lines. On average, across the five backcrosses performed, 176.3 and 241.4 kernels per ear were produced for V158wx-C and WF-9wx-C, respectively. Assessment of tassel and pollen fertility under SB conditions in BC 4 F 1 WF-9wx-C and BC 5 F 1 V158wx-C plants showed that their anthers were shriveled, not exserted from the floral glumes, and contain non-viable pollen. GBS genotyping confirmed the progressive increase in similarity to the recurrent parent from 50.3% in F 1 to 96.5% in BC 5 F 1 for WF-9wx-C, and from 86.7% in BC 1 F 1 to 98.3% in BC 5 F 1 for V158wx-C. After five backcrosses, the created CMS analogues were sown under field conditions, where they demonstrated male sterility and were phenotypically similar to the recurrent parents. Both sterile analogues (V158wx-C and WF-9wx-C) developed under SB will be incorporated into the industrial seed production of heterotic hybrids. To our knowledge, this is the first practical demonstration of coupling SB with backcross conversion of CMS cytoplasm in maize material, yielding the CMS versions of the parental lines of commercial hybrids. The approach is broadly accessible to conventional breeding programs, not only for maize but also for other crops such as sunflower, rapeseed, and rice.