Spike Density, Speltoidy, and Compactoidy in Hexaploid Wheat

F2 progeny from F1 plants monosomic for chromosome IX of Hymar obtained from the cross of chromosome IX deficient Chinese Spring × Hymar, a variety of club wheat, were studied for spike type segregation and chromosomal constitution. Spikes from plants with two members of chromosome IX from Hymar were club, intermediate, or common-type. Spikes from plants monosomic for chromosome IX were likewise club, intermediate, or common type but all had typical speltoid characteristics. When the spike type segregates were combined, there resulted an almost perfect fit to a 1:2:1 ratio of club, intermediate, and common spike type plants, respectively, indicating that chromosome IX was not associated with inheritance of spike type. It is suggested that subcompactoidy and compactoidy resulting from an increase in the dosage of chromosome IX is caused by an increase in the dosage of the gene for square-headedness carried by this chromosome. The shortening of plants having higher dosages of chromosome IX is probably caused by abnormal chromosomal constitution resulting in genetic unbalance.