Genetics of Branching in Sunflower.

Several morphological and seed traits are pleiotropically affected by branching in sunflower (Helianthus annuus L.).  Two branching loci have been identified, the apical branching locus (B) and the basal branching locus (Bbr).  While hybrids between branched and unbranched elite inbred lines only segregate for B, hybrids between unbranched elite lines and branched wild ecotypes segregate for two or more branching loci which confound the effects of B and Bbr.  The number, intra- and interlocus genetic effects, and genomic locations of branching loci other than B and Bbr are unknown.  The present study focused on identifying and more finely mapping B, Bbr, and other branching loci and gaining a deeper understanding of the genetics of branching in elite x exotic and elite x wild populations segregating for diverse branching phenotypes.  Bbr mapped distal to B on linkage group 10 in an advanced backcross population (BC₂) developed from a H. annuus x H. argophyllus hybrid (NMS-P21 x ARG1805).  For finely mapping B and other branching loci, 380 advanced-intercross recombinant inbred lines (AI-RILs) were developed by four generations of intermating and seven generations of single-seed descent from a hybrid between NMS373, an elite branched oilseed inbred line, and Hopi, an unbranched Native American landrace and one of the most primitive cultivars known.  Branching loci segregated and are currently being finely mapped in the NMS373 x Hopi population.  Finally, to identify branching loci segregating in elite x wild populations, 178 RILs were developed from a hybrid between an unbranched elite inbred line (CMS-HA89) and a branched self-compatible wild ecotype (ANN1238).  Three small-effect branching QTL were identified and masked the effect of the B locus.  Hence, several wild alleles had to be purged to develop single-headed cultivars, a critical turning point in sunflower domestication.