HYBRIDIZATION IN THE EVOLUTION OF MAIZE 187 



with four varieties of teosinte, and by repeatedly backcrossing (three times 

 in most instances) the hybrids to the inbred strain, retaining various amounts 

 of teosinte germplasm through selection. The end result is a series of modified 

 inbred strains approximately like the original 4R-3 — all relatively isogenic 

 except that parts of one or more chromosome segments from teosinte have 

 been substituted for homologous parts from maize. 



That the substitution involves chromosome segments or blocks of genes 

 and not single genes is strongly indicated by the fact that the units have 

 multiple effects and that there is breakage within the units in some cases, 

 although in general they are transmitted intact. Their mode of inheritance 

 and their linkage relations can be determined as though they were single 

 genes. Yet each of the units affects many if not all of the characters in which 

 maize and teosinte differ. The block of genes on chromosome 3, for example, 

 although inherited intact as a single hereditary unit, affects number of ears, 

 size of ear, number of seeds, size of seeds, number of rows of grain, staminate 

 spikelets on the ear, and induration of the rachis. In addition it has a con- 

 cealed effect, discussed later, upon such characters as response to length of 

 day and the development of single spikelets. The block of genes on chromo- 

 some 4 has practically the same effects in somewhat greater degree, but this 

 block shows definite evidence of breakage or crossing over which is of the 

 order of 30 per cent. 



These blocks of genes are not random samples of teosinte germplasm, but 

 represent definite genie entities which are transmitted from teosinte to maize 

 in the process of repeated backcrossing. Different varieties of teosinte yield 

 comparable if not identical blocks of genes, and the same variety of teosinte 

 in different crosses does likewise. Regardless of the amount of introgression 

 of maize which teosinte has undergone in its past history, and regardless of 

 the differentiation which has occurred between varieties of teosinte, there are 

 still regions in all varieties of teosinte, perhaps near the centromeres, which 

 have remained "pure" for the original genes. 



Effects in Heterozygous Condition 

 When these blocks of genes are introduced into maize they have profound 

 effects which differ greatly in the heterozygous and homozygous condition. 

 Since maize and teosinte represent completely different morphological and 

 physiological systems (especially from the standpoint of their pistillate in- 

 florescences and their response to length of day), this substitution, of seg- 

 ments of chromatin from one species for homologous segments from the 

 other, represents a drastic interchange of parts comparable, perhaps, to in- 

 stalling a carburetor or other essential part from one make of car into an- 

 other. In the Fi hybrid of corn and teosinte where the blocks of genes are 

 heterozygous, there is no particular functional difficulty. Here the two com- 

 plete systems are operating simultaneously and the result is a vigorous hy- 



