ORIGIN OF THE FREE THRESHING CHARACTER IN HEXAPLOID WHEAT

All 15 hexaploid wheats (2n = 42 = AABBDD) synthesized from various combinations of nine tetraploid wheats (2n = 28 = AABB) and seven forms of Aegilops squarrosa L. (2n = 14 = DD) were non-free-threshing, regardless of the presence or absence of the Q factor. Monosomic and telosomic analysis of synthetic hexaploids RL 5404 and RL 5406, produced from crosses of Tetra Canthatch (the AABB component extracted from the common wheat cultivar Canthatch) with two forms of Ae. squarrosa, revealed the presence of a partially dominant gene for tenacious glumes, Tg, on chromosome 2Dα. This gene, derived from the squarrosa parent, inhibited the expression of Q located on chromosome 5A. The recessive allele tg as well as Q must be present for the expression of the free-threshing character in hexaploid wheat. On the assumption that Ae. squarrosa of the past possessed Tg, as apparently do all extant forms, it is hypothesized that the primitive hexaploid progenitor of free-threshing hexaploid wheat also carried this gene and, therefore, was non-free-threshing. The mutation from Tg to tg is presumed to have occurred at the hexaploid level.