Effects of Mass Selection and Irradiation in Corn Measured by Random S1 Lines and Their Testcrosses

Two populations of corn (Zea mays L.) improved by nine cycles of mass selection for yield and the unselected parent variety, ‘Hays Golden,’ were evaluated in 1969 and 1970 using random S₁ lines and their testcrosses to related and unrelated single-cross testers. One selected population received thermal neutron seed treatment prior to the first and third selection cycles. S₁ lines of the selected populations, per se and in testcrosses, demonstrated significant superiority in mean yield over lines of the parent variety in each year. Like progenies of the two selected populations produced similar yields. Selection evidently eliminated radiation-induced deleterious mutants from the “irradiated” population while increasing frequencies of favorable yield genes common to both selected populations. This apparently produced similar germ plasm reservoirs much more suitable for the extraction of superior inbred lines than the parent variety. Indications are that selection also reduced genetic variability in yield and combining ability. Prolificacy resulting from selection for yield was evident in S₁ lines but not in the testcrosses. Since inbreeding did not reduce prolificacy in the selected populations, results are interpreted to mean that tester genes masked recessive genes which induce prolific potential when homozygous. All phenotypic correlations between S₁ and testcross yields were too low to be of much predictive value; but in general, lines of the selected populations exhibited the least correlation with their testcrosses. Genetic correlations were somewhat higher, indicating sizeable environmental fluctuations in individual experiments. We postulate that non-prolific testers by inhibiting prolificacy in turn limit the expression of genes controlling yield in these populations. Use of a prolific tester might have provided greater yield differentiation, more precise estimates of line genotypes and higher correlations between S₁ and testcross progenies.