HETEROSIS IN A NEW POPULATION 



439 



As sliown in Table 27.9 some outstanding hybrids have been attained 

 through a recombination of lines from Celaya. The Celaya lines used in the 

 hybrids of Table 27.9 were isolated by topcrossing with a variety of the 

 race Conico Norteno and with the variety Jal. 35. The two hybrids, H-309 

 and H-307 were made with Si lines, while H-310 was made with S2 lines. 

 Both H-309 and H-307 gave slightly better all around performance in yield 

 and in disease resistance than did H-310, but H-309 was the best of the three 

 in yield, disease resistance, and general agronomic characters. Its yield, as 



TABLE 27.9 



YIELDS OF DOUBLE CROSSES MADE FROM Si AND S2 LINES 



COMPARED WITH THE AVERAGE YIELD 



OF CELAYA, H-301, AND H-306* 



* An average of two years' data from two locations. 



shown in Table 27.9, was 23 per cent higher than an average of the three 

 varieties used as checks. The checks, in addition to Celaya, included two 

 double topcross hybrids (H-301 and H-306) which brought the level of 

 yield of the checks up to 107 per cent of Celaya. The actual difference be- 

 tween H-309 and Celaya, therefore, would be somewhat greater than 23 

 per cent. 



Hybrid H-309 should also make a good synthetic because: (1) the average 

 yield of the six single crosses possible between the four inbred lines is very 

 nearly the same as the yield of the double cross H-309, and (2) because the 

 four Si lines were among the most vigorous obtained from Celaya. The actual 

 yield relationship between Fi, F2, and F3 progenies, where available for a 

 series of double crosses in a randomized block experiment with eight replica- 

 tions, is given in Table 27.10. 



All yields in Table 27.10 are expressed in per cent of Celaya. All double 

 cross hybrids with the exception of No. 1, involve Si lines from varieties of 

 the race Celaya. Hybrid No. 1 was made with S2 lines from varieties of the 

 race Celaya. Hybrid No. 2 contains the same lines as H-309 in Table 27.9, 

 but in a slightly different combination. This combination has resulted in 



