The penetrance of the teosinte key traits, two-ranking and solitary female spikelets, in maize

The key traits separating teosinte from maize, specifically two-ranked ears and solitary female spikelets, fail to penetrate a background of modern maize and imperfectly penetrate even a primitive maize background (W.C. Galinat, MNL 45:99, 1971; MNL 46:108, 1972). Transfer of key segments of teosinte germplasm into Texas 4R-3 by P. C. Mangelsdorf was accomplished by selecting not for key traits but for such indicators of teosinte germplasm as cob induration, reductions in ear and kernel sizes and reduced kernel row number (P.C. Mangelsdorf, MNL 21:19-22, 1947). Development of a stable two-ranked, single-female-spikelet maize, designated "airplane corn", was accomplished by reconstructing a congruous background for these key traits by W.C. Galinat (MNL 52:60, 1978; Maydica 30:137-160, 1985).

The cryptic nature of these key trait genes in maize, the apparent reversal of dominance in their evolutionary sequence, and the nature of penetrance in a series of backgrounds is the subject of an ongoing investigation. The 4R-3 teosinte derivatives, later transferred into A158 (P.C. Mangelsdorf, Cold Spring Harbor Symposia on Quant. Biol. 23:409-421, 1958), and A158 derivatives of Central and South American "Tripsacoid" races of maize (P.C. Mangelsdorf, Boletin de la Sociedad Argentina de Botanica 12:180-187, 1968) were crossed with a single plant of the inbred "airplane corn" in the summer of 1983. The F1 hybrids of these seven A158 teosinte derivatives x airplane and ten A158 tripsacoid derivatives x airplane plus an A158 control x airplane were grown out for observation and selfing in 1984. Ear-to-row F2 segregations were grown this past season and are being evaluated now. The essential complex on chromosome 4 is most important in stabilizing the key traits of teosinte in a maize background (W.C. Galinat, Maydica 30:137-160, 1985). It is marked by the sugary (su) gene from the "airplane corn" parent. The F1 generation proved variable in penetrance of ranking and pairing both within and between lines. At one extreme, 73 percent of the F1 ears from the cross involving A158 containing segments of Florida teosinte chromosomes 3, 4, and 9 were two-ranked. Twelve lines, including the three whose parents contained one each of the above Florida teosinte segments, produced less than 20 percent two-ranked ears.

The distribution curve of the index for degree of pairing per ear was unimodal for all F1 lines except for the Honduras-Venezuela derivative hybrid whose pairing index distribution was bimodal. The means of the curves for the other hybrids were similar, clustering around 8.5 on a scale of 12, indicating a fairly strong degree of penetrance of this key trait.

The results suggest that a negative correlation exists between the penetrance of two-ranking and solitary female spikelets. Confirmation of this awaits evaluation of the F2 generation.

The superior penetrance of two-ranking in the A158 Fla 3, 4, 9 hybrid is compatible with the observation by W. C. Galinat (MNL:101-102, 1985) that these teosinte segments are responsible for the rind vascularization necessary for a congruous background for two-ranking.

Ann E. Kennedy and Walton C. Galinat
 
 


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