Genetics of horizontal resistance: measurement of the effects of B, P-WR, Plp and Krn on corn earworm resistance and yield

In MNL 58:38-45, linkages of resistance to corn earworm (Heliothis zea) with B, P-WR, Plp and Krn were shown using the standard wx set of translocations in IAC Maya crossed to Purple Husk Cateto (PHC), and back-crossed to Maya wx. To preserve heterotic patterns we transferred these factors to the IAC-1 maize cultivar which is about 50% Cateto and Colombian flint and 50% Tuxpeno. IAC Maya is nearly all Tuxpeno and to it we tried to transfer Zapalote Chico resistance factors. In a modified ear-to-row scheme with reps of IAC-1, in the detasseling plot 25 female ear to row half-sib families were planted with the original cross and two backcrosses to IAC-1. PHC is B, P-WR, Plp and IAC-1 B-W, P-WW, pl. In the generation PHC x IAC - 1(3) a reading was done for unholed husks, husks with the earworm exit hole, for the anthocyanin factors above, and for kernel row number. Our B gives purple color to the outer husks, usually shaded not a solid color, and our B-W gives no husk color. P-WR, corn cob color is an inadequate name, really it gives a reddish color to the inner soft bracts (lemma and palea), and Plp gives purple color to the outer hard bracts, the glumes. The data are tabulated in Table 1. For the total of the 25 families, a X2 was applied contrasting unholed minus holed for all genetic combinations, including 16 and more kernel row numbers, 14 rowed, and 12 are less rowed number, forming a 2x2x2x3 factorial design. The X2 are positively significant for +++-, and +-+-0, and negatively for the -+-- combinations at P = 0.05. Quite unexpectedly P-WW from the recurrent male was better than P-WR in combinations with B, and Plp was better than the original combination from the "resistant" non-recurrent source. To clarify the interactions, a further analysis was done for the 8 families in which there was the treble simultaneous linkage of resistance with B, P-WW, Plp. So filtered the results get crystal clear. The best combinations concentrated all in the +-+ cell with all row numbers, with two significant deviations. Furthermore, correcting within it the X2 for family size n, (n * n) x X2 they turn to 9.03 for 16 rows, 3.78 for 14 rows and 2.95 for 12 rows. To quantify the effects a standard analysis was done as a 2x2x2 fractional factorial eliminating from the analysis -+ + and -- + cells and disregarding Krn, Table 2. The coefficients of the variables were adjusted to a sample size of 100 so the effects are presented in percentage. Note that for the group of 8 selected for earworm resistance if the treatment is P-WW (-), all effects including the third order interaction turn to positive, summing up to + 11. 1 over the mean. The numbers within a Krn class are insufficient to get a meaningful analysis, they should reach 22 or 23 family size to get good results, getting a binomial distribution. Pooling kernel row numbers in each of the 2x2x2 cells the ears were also weighted in grams, and a mean ear weight was obtained. With the mean weight, again a fractional factorial 2x2x2, B x P-WR x Plp was gotten and analysed. This fractioning increases enormously the error of estimation of B, Plp and B Plp coefficients. The best combinations for earworm resistance tended also to be the best yielders, 201 g for the unholed and 188 g for the holed for the + - + combination, confirming that greater earworm resistance also brought greater yield. As this is also a breeding program we are continuing the work with the eight families, and eliminated all P-WR individuals. Next breeding season we expect to do new readings so the available coefficients can be more accurately measured.

As our recurrent theme is horizontal resistance to pests, remember that as far as is known, Plp only appears if B is present. B spreads color (and resistance?) also to the coleoptile, roots, and the husk, the critical beginning and end of the plant cvcle. The standard PI allele includes the glumes and permits expression of all B effects in the dark. Yes indeed, as for an environmental resistance system Ite1, which doesn't include pests, corn has evolved an horizontal resistance to pests which completes the first. The preceding article explains much of why these effects are quite measurable and parallel.

Table 1.

Luiz Torres de Miranda, Luiz Eugenio Coelho de Miranda and Nelson Cembranelli Schmidt

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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