1. Tetraploid maize-Tripsacum hybrids. In 1942 the excised embryo technic was utilized to obtain two hybrids of tetraploid corn and tetraploid Tripsacum. Since these hybrids received two sets of chromosomes from each parent it was anticipated they would be fertile if the chromosomes comprising these sets synapsed to form bivalents. But these two hybrid plants proved to be completely sterile. They not only produced no functional pollen but when used as the seed parent in backcrosses to their parents no viable seed was obtained from them. A variable number of bivalents were formed and in addition there were always present from one to several multivalent complexes that could not be fully analyzed.
Compared with the elaborate technic of Mangelsdorf and Reeves a relatively simple procedure was employed to obtain these hybrids. The husks of the earshoots were opened sufficiently to permit a mixture of Tripsacum and corn pollen to be sifted in about the bases of the silks, the husks were then replaced about the earshoot and held in position by the glassine earshoot bag reinforced with rubber bands. Approximately three weeks after pollination the embryos of the partly developed kernels were excised and cultured in two ounce screw cap bottles on the sterile nutrient medium employed by Randolph and Cox for the culture of iris embryos (Proc. Amer. Soc. Hort. Sci. Vol. 43, 1943). As soon as a root system and seedling leaves were formed the seedlings were transferred to soil.
The two hybrids produced in 1942 resulted from tho pollination of 14 earshoots of a synthetic tetraploid corn hybrid involving 5 different yellow dent lines (Stock A in accompanying table) with a mixture of tetraploid Tripsacum and tetraploid corn pollen carrying a full complement of genes for colored aleurone. Corn pollen was included with the Tripsacum pollen because Mangelsdorf and Reeves found that the presence of a certain number of normally developing corn grains on the ears aided the development of any rare hybrid kernels that might result from the functioning of Tripsacum pollen. Colored aleurone was involved to facilitate tho separation of hybrid from the non-hybrid seeds.
In 1943 a further attempt was made to obtain additional hybrids for a more adequate study of their characteristics. Four vigorous tetraploid hybrids of commercial lines of yellow dent corn were selected as the seed parents. From a total of 88 pollinations 68 immature embryos or embryo-like structures were cultured. Most of these were inviable and the eight seedlings obtained from them proved to be non-hybrid corn seedlings.
The stocks used in 1944 to repeat the cross differed from those used in the preceding two years. These are listed as stocks B-F in the following table which summarizes the results obtained in 1942 and 1944. Stock B was a multiple recessive tetraploid combination of one or more recessive genes in each of the ten chromosomes (Pv-bm2, b-lg, A-cr, su, pr, y-pl, in, j, c-wx, Rg-g). Stocks C, D and E were, with respect to most of these recessives, duplex heterozygotes, the recessive stock having been crossed with an aB Pl lg type to produce C, and AB Pl R2 type to produce D and with the inbred 187-2 to produce E. Stock F was an F1 hybrid of two commercial yellow dent lines, one of which was 187-2.
Perhaps the most interesting conclusion to be drawn from the results of these attempts in 3 different years to obtain hybrids between tetraploid corn and tetraploid Tripsacum is that hybrids may be obtained much more readily from certain stocks than from others. Gene differences affecting crossability may be involved, or, if the suggestion of Mangelsdorf and Reeves that corn carries segments of Tripsacum chromatin is to be taken seriously the possibility that such segments were present in the stocks which crossed most readily should be considered. However, there were no pronounced differences in knob frequency in the Stocks A-F; all had relatively few knobs.
The hybrids obtained in 1944 have not yet reached the sporocyte stage. One of the hybrids obtained in 1942 produced abundant tillers and has been maintained by vegetative propagation without difficulty; the other 1942 hybrid was less vigorous, produced few tillers and could not be kept alive by vegetative propagation. Extreme differences in the vigor of the 11 hybrids obtained from the 1944 crosses suggest that they may differ appreciably with respect to their chromosomal configurations.