Crosses of Zea diploperennis with corn

The purpose of the Argentine plantings (see item from Chapel Hill) was to complement the plantings in the United States and to take advantage of the fact that in the southern hemisphere, mid-summer corresponds to mid-winter in the northern. The plantings in both countries were designed to test the hypothesis of Garrison Wilkes, who postulated that several of the races of annual teosinte, which some students of corn's ancestry regard as the progenitors of cultivated corn, might instead be the progeny of the hybridization of Zea diploperennis with a cultivated corn in the early stages of domestication.

The plantings in Argentina, like those in the United States, began with crossed seed produced by Mangelsdorf. In a small garden in Chapel Hill, N.C., completely isolated from all other Maydeae, he crossed plants of Zea diploperennis, grown from seed obtained from Hugh Iltis, with a primitive Mexican popcorn race, Palomero Toluqueno. This popcorn race was chosen as the corn parent of the cross because it, or its precursor, is the race most likely to have been grown in Jalisco in the area where Z. perennis was discovered by Iltis et al. It was chosen also because it has small pointed kernels, like those of teosinte, so that relatively little segregation for kernel size and shape would be expected in later generations of the cross.

The F1 hybrids: F1 plants were grown at two localities: Buenos Aires, at latitude 34o35' and altitude 25 meters, and Tilcara in the province of Jujuy, at latitude 23o35' and altitude 2460 meters. The plants at Buenos Aires first flowered on February 15 from seed sown on September 13, an interval of 155 days. The planting at Tilcara first flowered on February 25, from a sowing made on September 25, an interval of 153 days. Since the average day lengths at Tilcara are not much shorter than those at Buenos Aires during this period, but the mean temperatures were very much lower, the fact that the interval from sowing to flowering was about the same in the two localities, 153 and 155 days respectively, suggests that differences in mean temperatures had no marked effect. In vegetative growth, however, the plants grown at Tilcara grew much less luxuriantly than those grown at Buenos Aires.

Annual growth habit dominant: None of the F1 plants in either planting had fully developed rhizomes like those of their teosinte parent, and in this respect habit of the corn parent may be considered as dominant. However, several plants had small strongly condensed rhizome-like structures, similar to those that in some species of perennial plants are called "bulbils."

Most of the F1 plants also had profuse and robust adventitious roots, perhaps one expression of perennialism. After a period of dormancy, several of the F1 plants at both Buenos Aires and Tilcara resumed growth. A dissection of the root system of one of these in the Tilcara planting showed that the new growth arose from a tiny bulbil. It can be said, however, that in the majority of F1 plants the annual growth habit of the corn parent was dominant.

Pollen of an F1 plant: The pollen of an F1 plant grown at Buenos Aires comprised well-filled grains approximately normal in size and smaller grains, some well-filled and others only partially so. The pollen does not resemble the semisterility characteristic of plants that are heterozygous for chromosome translocations or long inversions.

The F2 population: A population of 107 F2 plants was grown at Buenos Aires. Of these it was possible to classify 84 plants for the four characteristics under study: distichous vs. polystichous pistillate spikes, solitary vs. paired pistillate spikelets, fragile vs. tenacious rachis segments, and perennial vs. annual growth habits.

Although the population was too small to be reliable in revealing Mendelian ratios for all of the characteristics under study, the F2 population, like the F1, shows the annual growth habit to be dominant to the perennial. The numbers of annual and perennial plants were respectively 62:22, a close approximation to a 3:1 Mendelian ratio.

With respect to the remaining characteristics, the ratios are not clear-cut: polystichous vs. distichous, 41:43; paired vs. solitary, 68:16; tenacious vs. fragile, 37:47.

There are indications of linkage between several of the teosinte characteristics but the numbers are too small to establish these clearly. There is no indication that the inflorescence characteristics of teosinte are linked with the perennial growth habit.

Somewhat surprising is the relatively high frequency of phenotypes combining all of the parental characteristics. Of 84 plants, 23 had all of the botanical characteristics of the corn parent; 5 all of those of the teosinte parent. The results, so far as they go, suggest that not more than four major gene pairs distinguish the two parents of the cross in their principal botanical characteristics.

Perhaps even more surprising is the relatively high frequency of the recombinant phenotypes, annual teosintes and perennial corns. The numbers are respectively 10 and 5.

Unexpected characteristics in the F2: Two characteristics not apparent in either parent, red pericarp and pod corn, appeared in several of the F2 plants. These could not be attributed to contamination since the F1 plants were grown in a small botanical garden situated on the Faculty of Agronomy campus, far removed from any other corn plantings.

The red pericarp color may represent the interaction of the genes for brown pericarp color of the teosinte parent with the cloudy pericarp color, known as "dingy," of the popcorn parent.

A possible explanation of the pod corn is that the popcorn parent of the cross carries a lower allele of the Tu locus that is not expressed because it also carries the tunicate inhibiting gene, Ti. Recombination in the F1 allows the tunicate allele to be expressed.

A second F2 population: From the seeds of the same F1 plants, a second and much larger F2 population was grown at Calilegua, a locality in a subtropical, sugarcane growing area. The data from this planting have not yet been completely analyzed but a preliminary examination shows the results to be quite similar to those obtained from the first F2 population. Annual-type plants outnumbered perennial-type plants by a ratio of approximately 3:1, as they did in the Buenos Aires plantings. Apparently the inheritance of perennialism in Zea is far more simple than had earlier been generally supposed.

Also, in the Calilegua planting, the recombinant phenotypes, annual teosinte and perennial corn, occurred in about the same frequencies as in the Buenos Aires F2 populations.

A small backcross population: From a planting of 40 seeds of a backcross of the diploid perennial x F1, 32 plants were obtained. Of these, 10 were classified as annual and 22 as probably perennial. This is not a significant deviation from the 1:1 ratio expected if the perennialism of teosinte is recessive to the annual growth habit of corn as it seemed to be in the two F2 populations described above.

In this population no perennial-corn phenotypes were to be expected and none occurred, but the frequency of annual-teosinte phenotypes might be much higher than that occurring in F2 populations. Apparently it is. The pistillate spikes of all of the annual-type plants have not yet been examined but several of those that have resemble the spikes of known races of annual teosinte such as Central Plateau and Nobogame. The spikes of several of the plants classified as "probably perennial" resemble the annual teosinte race, Huehuetenango, in having trapezoidal instead of triangular rachis segments.

There appears to be no difficulty in producing annual teosintes from backcrosses of the diploid perennial parent by the F1. This may have been the manner in which annual teosinte races arose in Mexico.

Results support the Wilkes hypothesis: Our results from both F2 populations and one backcross population are consistent with the Wilkes hypothesis that annual teosintes could have been created by the hybridization of Zea diploperennis and a cultivated corn in the early stages of domestication.

They are also consistent with the suggestion of Iltis et al. that perennial corns might be produced by similar hybridizations.

Julian Camara-Hernandez and Paul C. Mangelsdorf


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