Numbers and Distribution of Chromosome Knobs in United States Maize.

HE chromosomes of maize possess a number of distinguishing features T used by cytologists to identify members of the complement. Among the more important of these are the dark staining enlargements known as knobs, which have been shown to occur a t certain points and only a t certain points on the chromosomes. Although chromosome knobs are distinguishable in mitotic prophases they are best studied a t the pachytene stage of meiosis a t which time they are highly pycnotic when stained with ordinary chromatin dyes. Knob number is constant for any individual plant and in United States maize may vary from 0 in some varieties to 14 in others. I t can therefore be used as one criterion in determining the relationships of various kinds of maize, a problem which is becoming increasingly important in modern maize breeding. Although this paper deals only with differences in total knob numbers there is some evidence to indicate that knob size and position might be equally useful as a tool in studying relationships in maize. LONGLEY (1938) studied knob number and position in a collection of maize from 33 Indian tribes of the United States. He found very few knobs on the chromosomes of most strains from the northern Indian tribes. A slightly higher number of knobs were found in the southeastern varieties and many knobs were observed on the chromosomes of most varieties from New Mexico and Arizona. On the basis of these findings, LONGLEY suggests that “the number of knobs on the chromosomes of a strain of Indian corn may give a clue to the geographical origin of the strain.” MANGELSDORF and REEVES (1939) have suggested that the presence of chromosome knobs in maize is a result of admixture with Tripsacum, a related genus in which high knob number has been established for several species. They suggested further that the hybridization of Tripsacum and Zea with repeated natural backcrossing to Zea resulted in the formation of a new genus, Euchlaena. These investigators point out that so-called “Andean” or uncontaminated maize of the South American highlands has few or no knobs while most types of maize which show morphological evidence of Tripsacum introgression (either directly or through teosinte) have knob numbers roughly in proportion to the amount of putative Tripsacum germ plasm present. Further evidence supporting this hypothesis was offered by REEVES (1944) who, in a study of knob numbers in maize from North, Central and South America, demonstrated a statistically significant relationship between numbers of knobs and proximity to Central America, a region in which in certain areas both Tripsacum and teosinte occur in abundance. However, GRANER and ADDISON