HETEROSIS IN A NEW POPULATION 433 



are not strikingly higher yielding is often so great that it is difficult to obtain 

 wide scale distribution and use. 



Considerable success was achieved in the formation of synthetic varieties 

 from double topcrosses in the early stages of the Mexican program, as de- 

 scribed by Wellhausen and Roberts (1949). Nevertheless, in consideration of 

 all factors, it seemed that the most logical procedure would be the formation 

 of good double cross hybrids which would also make good synthetics in ad- 

 vanced generations. In this way, the more progressive farmers could take ad- 

 vantage of the higher yield capacity of hybrids, while less progressive farmers 

 would still benefit by planting the advanced generation progeny. Also much 

 wider use of improved seed could be obtained more easily and rapidly since 

 hybrids always make a better showing than synthetics. With time, through 

 education, demonstration, and the formation of better hybrids, the demand 

 for hybrid seed could be increased gradually, and use of advanced generation 

 seed gradually would decrease. Some of the hybrids obtained for Zones 1 and 

 3 and their relative value as synthetics will be discussed below: 



Results in Zone 1 



In Zone 1, for March and April plantings under irrigation or under condi- 

 tions where subsoil moisture is sufficient for germination, the race Chal- 

 queno has become widely distributed. Some of the results obtained in the 

 attempt to improve this long season race are given in Table 27.5. 



The variety listed as Chalco in Table 27.5 represents an average variety 

 of the race Chalqueho, and its yield for the sake of comparison has been 

 taken as 100. Variety V-7 is one of the best varieties found in the race, and 

 has been widely distributed as an improvement of the common variety 

 Chalco. Its two-year average yield in comparison to Chalco was 118 per cent. 



Hybrid H-2 is a cross between two composites, one of which (Hgo. Comp. 

 1) was made up of a composite of five first generation selfed lines (Si) from 

 the variety V-7, that were good in topcrosses with the variety Urquiza, of 

 the race Conico Norteiio. The other (Urq. Comp. 1) was made up of a com- 

 posite of five Si lines from the variety Urquiza, selected on the bases of their 

 combining ability in topcrosses with a variety similar to Chalco. Urquiza 

 was the only variety of the different races tried in Chalqueho territory that 

 was fairly well adapted. Hybrids between two such composites are of in- 

 terest because the parents may be propagated as open-pollinated synthetic 

 varieties, thus eliminating the necessity of forming single crosses and the 

 maintenance of four inbred lines, as is the case in a double cross. 



The yield of hybrid H-2 was only slightly higher than the open-pollinated 

 variety V-7. Work is under way to find out how much this hybrid might be 

 imi)roved through reciprocal recurrent selection. 



The three-way hybrid H-1 was made by using a single cross involving an 



