Graduate Thesis Or Dissertation

 

A comparison of two methods of divergent mass selection for heading date in two winter wheat (Triticum aestivum L. em Thell) crosses Public Deposited

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  • Responses to two cycles of mass selection for heading date, followed by selfing or intermating, were studied in two diverse winter wheat (Triticum aestivum L. em. Thell) crosses. Selection was bidirectional, resulting in early and late populations for each mating system, cycle, and cross. The influence of mass selection for heading date on the expressions of eight other agronomic traits was also studied. Concurrent with selection for heading date, generation means and variances within crosses were used to obtain genetic information about each of the nine traits. Additive effects of genes were an important source of variation among Cross I generation means for most traits. Predicted response to selection for heading date was 7.1 and 9.1 days/cycle for 1981 and 1982, respectively. Most traits by which the parents differed in Cross II generations were primarily affected by non-additive genetic effects. Progress from selection for heading date was predicted to be 6.9 days/cycle in 1981, and 1.4 day/cycle in 1982. Observed response to selection for heading date in Cross I showed gains of -4.8 and 5.5 days/cycle with selfing and -4.2 and 5.1 days/cycle with intermating. Further progress from selection for heading date could be achieved through additional cycles of selection and intermating in this cross. Response to selection for heading date with intermating in Cross II was superior to selfing in the early direction (6.0 vs. 5.0 days/cycle) and inferior to selfing in the late direction (0.9 vs. 1.9 days/cycle). These results were ascribed to an accumulation of minor genes via intermating and rapid fixation of recessive genes via selfing, for early and late selection, respectively. Few large correlated responses in the eight unselected traits with selection for heading date were noted in either cross. In Cross I, intermating was more successful than selfing in retaining 'genetic variation' in most of the unselected traits. In Cross II, intermating showed a slight advantage in maintaining 'genetic variation' in unselected traits, probably due to the low levels of additive genetic variation present in this cross.
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