| Macro-mutations and sub-specific differentiation in
Triticum M. S. SWAMINATHAN and M. V. P. RAO Division of Botany, Indian Agricultural Research Institute, New Delhi, India 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(Frankel and Munday, WIS 11, 1-2, 1960) and serves as a suppressor of vavilovoid expression (Swaminathan and Rao, unpublished).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. In the Tritium group with 2n=42, the following seven species have been described.
From the work of Nilsson-Ehle, Ellerton, Sears, Unrau and Mac Key, it is now well known that T. compactum, T. sphaerococcum and T. spelta are each separated from T. aestivum by a single gene : C located on chromosome 2D, S on 3D and Q on 5A respectively. A whole set of characters are controlled by each of these loci with the result that though genetically they should all be regarded as members of one species (see Mac Key, Sv. Bot. Tidskr. 48, 579-590, 1954), taxonomists have described them as independent species. |
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