1. With further reference to our hypothesis that (1) maize originated from a wild form of pod-corn, (2) that teosinte is the product of natural hybridization between maize and Tripsacum, and (3) that most North American varieties of maize are contaminated with Tripsacum, we have spent a good share of the past year in reviewing the archaeological and historical evidence which has a bearing on this problem. We have found nothing seriously in conflict with the hypothesis and a great deal of evidence in support of it.
In the last News Letter we made the suggestion that the knobs on the chromosomes of maize may have come originally from Tripsacum, in which case pure South American varieties might be found in which the chromosomes were knobless. This has proved to be the case. Of 17 lots received from Peru, all but two had knobless chromosomes. Collections from other parts of South America, however, all had knobbed chromosomes, the average numbers being as follows:
If the knobs on maize chromosomes have come originally from Tripsacum, it is evident that Tripsacum-infected varieties have replaced pure maize varieties in all parts of North and South America except the Andean region, which we regard as the primary center of domestication. Bolivian varieties have not yet been studied from the standpoint of chromosome knobs, but we anticipate that the majority of them will be found to be knobless.
The objection most frequently raised to the hypothesis that maize originated from pod-corn is that pod-corn is sterile in the homozygous condition and a sterile form could scarcely have served as a progenitor. We have attributed pod-corn's sterility to the fact that it has been maintained in a heterozygous condition for so many generations it is now a monstrosity when homozygous. We have suspected, however, that a fertile, homozygous form might still be developed by selection since there is great variation in the expression of the glumes and other characteristics of pod-corn. During the past season we have found that the Ts5 gene apparently is a strong modifier of fertility of TuTu plants. Homozygous tunicate plants carrying the Ts5 gene are highly fertile on the pistillate side and exsert a few good anthers. Self-pollination is impossible because the silks are dried up before anthesis occurs. Sib-pollinations can be made, however, and we expect to have true-breeding stocks of pod-corn available in the near future.
P. C. Mangelsdorf and R. G. Reeves