| We have studied the inter-relationships among the hexaploid Triticum
species in two ways. First, a set of diallel crosses among these species
was made and F1, F2 and F3 progenies were
studied both cytologically and genetically. Secondly, all the species were
treated with mutagens and mutations of phylogenetic interest occurring in
the M2 and subsequent generations were studied. The data, in
addition to revealing what is already known, have provided the following
information. (1) T. sphaerococcum could not have arisen through a deletion from T. aestivum as suggested by Ellerton since aestivum mutations readily occur in sphaerococcum. The sphaerococcum locus, which tends to behave as one unit in recombination, can be broken up by irradiation resulting in phenotypes lacking the compact growth habit and rigidity of leaves but possessing hemispherical glumes and spherical grains. (2) The compactum locus C behaves in every way like S in mutation experiments. (3) The gene controlling vavilovoid expression (i.e., elongation of rachillae) is also situated on the long arm of chromosome 5A between q and the awn inhibitor B1. Vavilovoid expression is suppressed by Q ; as a result, a vavilovoid mutant can occur in a free-threshing wheat (either tetraploid or hexaploid) only in conjunction with speltoidy, In crosses between T. aestivum and T. vavilovi, segregation would hence occur both for Q and V (gene for vavilovoid expression). This explains the 15:1 segregation found by Singh, Anderson and Pal (Agronomy J. 49, 4-11, 1957) in the crosses T. aestivum x T. vavilovi and T. sphaerococcum x T. vavilovi. (4) As suggested by Kihara (Proc. First Int. Wheat Genetics Symposium, pages 243-248, 1959), T. macha has genes for both compactness of the spike and for spelting. The compacting gene of macha seems to be homologous with the C gene of compactum. q and c can be independently removed from macha resulting respectively in dense eared and lax-eared types. (5) The basic morphological frame of hexaploid wheats consists of two types: (a) the aestivum sensu stricto type and (b) the spelta type. From the former, compactum and sphaerococcum have developed through macromutations in chromosomes 2 D and 3 D and from the latter vavilovi has developed through a recessive mutation in chromosome 5A. T. macha and T. zhukovskyi appear to have genes from both these groups. In view of the fact that the key characteristics separating the 42-chromosome Triticum species are controlled only by 1 or 2 genes, all of them can be considered as subspecies of T. aestivum L., as suggested by Mac Key. A similar situation probably prevails in the tetraploid group. The important role that macromutations appear to have played in the differentiation of the tetraploid and hexaploid species of Triticum suggests that this means of variation may be a potent factor in the diversification of polyploids. |
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