Comparisons of performance and genetic variance among S1 lines and their testcrosses from the original Iowa Stiff Stalk Synthetic and three improved Stiff Stalk populations of maize

Iowa Stiff Stalk Synthetic (BSSS) was developed in the mid-1930s by intermating 16 inbred lines that were above average for stalk quality. BSSS has been involved in two long-term recurrent selection programs: reciprocal recurrent selection and half-sib followed by S2-progeny recurrent selection. Advanced populations from these programs are BSSS(R)C9 and BS13(S)C3, respectively;Objectives of this study were: (1) to compare the amount of genetic variation present among S1 lines from BSSSC0 (C0), two improved BSSS populations, BS13(S)C3 (C3) and BSSS(R)C9 (C9), and BS13(S)C3 x BSSS(R)C9 (C3 x C9), (2) to compare the amount of genetic variation present in testcrosses involving two testers of different heterotic background and the four sources, (3) to compare inbreeding depression in populations under different methods of recurrent selection, and (4) to correlate S1 line traits with testcross yield;Materials used in this study included: (1) 50 random S1 lines from C0, C3, C9, and C3 x C9 sources, (2) testcrosses of the S1 lines from each of each source with inbreds B73 and Mo17, and (3) the four populations per se at S0 and S1 generation levels plus the F1 of C3 x C9;The two recurrent selection programs used to improve BSSS produced significantly different plant and ear traits for C3 and C9. S1 lines from C3 x C9 showed significant high-parent heterosis for grain yield compared with lines from C3 and C9 per se. Genetic variance component estimates of S1 lines from C3 x C9 showed mid-parent values;Mean grain yield for Mo17 testcrosses was significantly greater than B73 testcrosses within each of the four sources. Testcrosses involving C3 and related inbred, B73, showed very small grain yield variance components. The high-parent heterosis observed for C3 x C9 S1 lines was not observed for C3 x C9 testcross means. Simple correlations between testcross grain yield and S1 line traits were low and would be of little predictive value in a selection program. Improved sources showed less inbreeding depression (S0 to S1) than did BSSSC0. Percentage of inbreeding for grain yield was slightly greater for C9 than for C3 while C3 x C9 showed inbreeding between parental values for most traits