The efficiency of induced mutagenesis is much higher in self-pollinated crops than in cross-fertilized ones (A. Micke, Genetika, USSR, 2 (1):166-167,1976). The latter accumulates spontaneous mutations. Therefore, a particular interest of corn breeders to this method could be explained by identical spectra of spontaneous and induced mutations. In addition, because of a too low rate of adaptive valuable induced mutations the great expenditures needed to detect them were not compensated.
H. Stubbe (Genetika, USSR, 11:9-30, 1966) reported the evidence of a lack of new factors in mutation process in self-pollinated crops. We tested it in corn. In 9 inbreds, 1106 visible mutations induced with NMU were found. The average frequency of mutation occurrence in all variants of the test was more than 100 times higher than the control (µ = 0.389 ± 0.009) and (µ = 0.003 ± 0.016). The mutation frequency was evaluated in 100 oligogenes. All loci varying in their mutability were divided into 5 groups:
1. Highly mutable genotype-independent loci (µ = 0.005 - 0.010): Su1, Sh2, Wc;
2. Highly mutable genotype-dependent loci: Ae (µ = 0.011) in W64A, Ys1 (µ = 0.010),Sh1 (µ = 0.008), Ts2 (µL = 0.005) in A344;
3. Mutable loci (µ = 0.002 - 0.004): Su2, Wx, Bt1, Br1, Du, D1, Na1, Sh4, Bt2, O1, O5, Mn1, G1, Cr, Pl1, Ffr1, Ffr4, Ba1;
4. Low mutable loci (µ = 0.001):
a. Well-known loci: Et, Ts1, Bm3, Ba2, Ra1, Po, ts5, Pi, Ra2, Id, An, Si, Mn2, Ds, Py, Sk, La, Zn, Nl, Fl2, Y, Sl, Ad1, Ffr2, Ffr3, Fsh, Fmd, Fww, Sm;
b. New loci: Sup-W70o2, Meg-Gb334, Tts - Gb834, Air-W23, Sin-A344, Dul-W64A, Tea-W64A, Prf-A344, Prf2, Afd-W23, PamA344, Pam2, Dsy-A344, Dsy2, Ms43, Ms28, mei-025-W64A;
5. Stable loci (µ = 0): O2, O2 +, Bm1, Bm2, Bm4, Lg1, Lg2, lg3, cg, tp1, tp2, tu, Br2, Gt, Gs, Pn, Td, Tb, ts6, ts3, Ts4, vg, D2, D3, G2, Rd, Br3, Bv, pt.
New mutations in corn were induced with NAU at a frequency of 0.005. To identify new mutations, we used only mutants of kernels and plants with a high level of viability. Seven high-lysine corn mutants with floury endosperms were not included in the group (A.S. Mashnenkov, MGCNL, 53:111-112, 1973). They were tested for allelism with a limited number of identified genes. The brief characteristics of all new mutants are given in the Transactions of our Institute (A.S. Mashnenkov Sb. trudow KNIISH, USSR, Krasnodar, 27:127-139, 1984). The information on the meiotic mutations was reported elsewhere (A.S. Mashnenkov, I.N. Golubovskaya, Genetika, USSR, 16 (9):1632-1640, 1980).
Five mutations: Sup-W70o2, Sin-A344 (short internodes), tts-Gb834 (a tillering plant with tassel silk development), meg-Gb834 (miniature endosperm and germ), prf-A344 (prolific) could be used in corn breeding programs for a qualitative change of a reaction norm. [Ed. note: the symbol for prolific, prf, is suggested and used here in place of pro, because of prior use of pro for proline].
In diallel crosses, some indirect evidence of great importance, of new favourable induced mutations - polygenes controlling a high level of heterosis for kernel yield - was obtained (A.S. Mashnenkov Sb. trudow KNIISH, USSR, Krasnodar, 27:127-139,1984).
A tendency to an increase in a number of new inbreds with high combining ability resulting from hybrid corn seed treatment with the mutagens was observed. However, we did not manage to evaluate true efficiency of induced mutagenesis as a method of improving corn productivity. The true efficiency of the method could only be evaluated from the number of commercial cultivars released. It could be realized in conjunction with breeders in comparative tests.
A. S. Mashnenkov
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