The genus Triticum offers interesting examples of the role of macro-mutations which at one step lead to the origin of taxonomic categories higher than varieties. As is well known, the speltoid suppressor factor Q located on the long arm of chromosome 5A, which must have arisen as a mutation in a non-free threshing tetraploid wheat, has played a dominant role in the evolution of bread wheat. The Q factor not only inhibits spelting but also brittleness of rachis, a development of obvious importance in cultivated forms (Mac Key, Genetica Agraria 12, 210-30, 1960). In addition, it inhibits various classes of spikelet sterility (Frankel and Munday, WIS 11, 1-2, 1960) and serves as a suppressor of vavilovoid expression (Swaminathan and Rao, unpublished). Another remarkable mutation, the multivalent suppressor gene (see Riley, Heredity 15, 407-29, 1960), which along with Q has shaped the destiny of emmer and bread wheats, as interestingly enough taken place in the long arm of chromosome 5B.

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