HETEROSIS IN A NEW POPULATION 



437 



complex from the mountains in southern Jalisco. The breeding work for 

 Zone 3 was concentrated in the Bajio and had the following objectives: 



1. The development of hybrids and synthetics liigher yielding than 

 Celaya, with a high degree of root and ear-rot resistance, which have general 

 adaptation to Zone 3 under irrigation. 



2. The development of early drought resistant hybrids and synthetics 

 for Zone 3, which are adapted to the normal rainy season of the area, and 

 which have a high degree of root and ear-rot resistance. 



Since the variety Celaya already had considerable root and ear-rot re- 

 sistance, it was used as basic breeding material in the attempt to attain the 

 above objectives. 



One of the first attempts at the im[)rovement of Celaya involved the pro- 



TABLE 27.7 

 YIELDS OF TWO DOUBLE TOPCROSSES IN COMPARI- 

 SON TO THE OPEN-POLLINATED 

 VARIETY CELAYA 



* The variety of Celaya used as a check in this and in the following tables 

 is the one being maintained by the Agricultural Experiment Station in Leon, 

 Gto. 



duction of double topcrosses. In this method, two different high yielding 

 varieties adapted to Zone 3 were used as testers. One of these, designated as 

 Gto. 59A, was one of the better late varieties found within the race Celaya. 

 The other, designated as Jal. 35, was a high yielding variety obtained from 

 southern Jalisco. This latter variety apparently was derived from a highly 

 heterogeneous mixture of several races that came together in southern Jalis- 

 co: Tabloncillo, Celaya, and the Jaliscan mountain complex (Wellhausen et 

 al.). Inbred lines (Si) from Gto. 59A and other varieties of the race Celaya 

 were topcrossed with Jal. 35, and Si inbred lines from Jal. 3>S and similar 

 varieties were topcrossed on Gto. 59A. These topcrosses were then tested 

 in three locations in Zone 3 with Celaya as a check. As a result of these tests, 

 the ten best topcrosses with Gto. 59A were selected and each crossed with 

 each of the ten best topcrosses with Jal. 35. Subsequently, the resulting 

 double topcrosses were tested at two different locations in Zone 3. Finally, 

 two double topcrosses were selected and released for commercial produc- 

 tion. The yields of the two double topcrosses for commercial production to- 

 gether with the yields of Celaya are given in Table 27.7. These comparative 



