444 



E. J. WELLHAUSEN 



then is another case where a hybrid between Si lines of two different races 

 under one set of conditions is no better than the better of the two parents, 

 but, under a different set of conditions, is superior to both. 



Double cross hybrids made from Si lines of Celaya in combination with 

 Si lines of the race Conico Norteiio have in general given good results in 

 Zone 2, with yields ranging from 20 to 25 per cent higher on the average 

 than the native varieties commonly grown in the area. 



TABLE 27.12 



YIELD OF CELITA AND PERCENTAGE DRY MAT- 

 TER AT HARVEST COMPARED TO CELAYA AT 

 FOUR DIFFERENT LOCATIONS, 4 REPLICATIONS 

 EACH 



In the tropical areas (Zones 4 and 5) hybrids were under test for the first 

 time in 1950. These involved principally combinations of Si and S3 inbred 

 lines from the races Tuxpeno and Vandeno (Fig. 27.4). 



Lines Selfed Once versus Lines Selfed More Than Once 

 in Hybrid Formation 



The use of Si lines in the early stages of a breeding program has many 

 advantages. It means that testing for combining ability can begin in the 

 first generation of selling. It can, in fact, begin with selected open-pollinated 

 plants which may be simultaneously selfed and crossed. Lines thus isolated 

 in a breeding program where uniformity is not of prime importance can be 

 utilized immediately in the formation of hybrids and synthetics. Since Si 

 lines are more vigorous than advanced generation selfed lines, they also have 

 a definite advantage in the formation of synthetics. It has never been defi- 

 nitely determined whether high yielding hybrids can be obtained more 

 readily with homozygous lines than with heterozygous lines. Jenkins (1935) 

 has shown that crosses of lines selfed only once are on the average as pro- 

 ductive as crosses involving the same lines selfed six to eight generations. 

 This may indicate, as Jenkins suggests, that the effects of selection are al- 

 most exactly balanced by the loss of good genes through the rapid attain- 

 ment of homozygosity. 



Some data have been accumulated to date in the Mexican program 



