Gametic Sampling from Selected Zygotes in Corn Breeding 1

T H E successful development of improved corn hybrids is dependent upon the improvement of the foundation lines involved in their production. The problem is primarily one o'f locating suitable sources of superior germplasm, desirable fractions of which can be incorporated into the line genotypes the breeder is seeking to improve. The most common source of new germp1as.m for improving widely used lines has been other estalblished lines. This approach rapidly increases the relationship among the better lines available, thereby restricting the use of the improved lines in hybrid combination. The continued use of open-pollinated varieties as a source of new ger,mplasm has 'been largely by-passed due to the ,extremely low frequency of superior genotypes in these populations. The need for further sampling of the varieties has 'been pointed out by Stadler ( 7 ) . He suggested the use of a scheme in which the gamet,e is the unit of selection, thereby greatly increasing the efficiency of sampling superior germplasm in these varieties. The technic can be illustrated as follows: If in a double-cross hybrid (A x B) x (C x D), line A is selected as needing improve,ment, it is first outcrossed to a varietal population. The resulting F, plants di@er only with regard to the gametes contributed from the varietal source. Individual F, plants designated as A/variety, are selfed and testcrossed to the C x D single cross against which ,the new lines may be ultimately used. Those F, plants having higher testcross yields than line A with the same tester are assumed to be combinations of A with gametes superior to A in yield genotype in this particular combination. Further inbreeding and selection within the superior A/variety plant progenies should permit the extraction of higher combining lines for use in this particular hybrid without having used up, as it were, other genotypes already isolated and identirhed. The type of improvement sought through the gamete sampling .method is, then, specific in nature. To the extent that specific combining ability is important in obtaining maximum yields, the suggested procedure should act as a guide in developing new lines in such a way that their combining ability in what is, in effect, a predeiermined combination should be enhanced. Pinnell et al. ( 5 ) report the use of the gamete sampling technic in the improvement of three hybrids. They sampled gametes from several open-pollinated varities as well as from several idbred lines. Almost 50% cif the gametes