Higher Allopolyploids 465 



in the hexaploid species. In crosses between T. monococcum and 

 T. turgidum, the hybrids have twenty-one chromosomes but form 

 seven bivalents and seven univalents. Apparently one genome 

 of T. turgidum is sufficiently like the genome of the other species 

 that it pairs with the Einkorn genome, leaving the other turgidum 

 genome unpaired. These seven chromosomes behave as univa- 

 lents and usually pass to one pole or the other without dividing, 

 but sometimes they divide equationally at the first division. The 

 results are similar in hybrids between the Emmer and Vulgare 

 species. The fourteen Emmer chromosomes pair with fourteen 

 of the Vulgare chromosomes whereas the other seven Vulgare 

 chromosomes behave as univalents. From the point of view of 

 wheat improvement, it appears to be unfortunately true that 

 most of the genes that make for the most desirable bread char- 

 acteristics are located in the seven Vulgare chromosomes that 

 do not pair and are generally eliminated from future generations 

 in Emmer X Vulgare crosses because they are unpaired. 



Crosses between the Emmer and Vulgare wheats and species 

 of the genus Aegilops, a grass native to the Mediterranean 

 region, throw considerable light on the nature of the chromo- 

 somes in these two groups of wheats. When Aegilops cylindrica 

 {n = 14) is crossed with the Vulgare wheats, seven Aegilops and 

 seven Triticum chromosomes pair, leaving the other chromosomes 

 of the two genera unpaired. This would indicate that each genus 

 has one seven-chromosome genome in common. 



Sax has proposed the following scheme for evolution in these 

 genera. The Einkorn wheats are the most primitive wheats and 

 have one genome ; they are AA. The Emmer group evolved from 

 it in part by a modification of this genome into B and are AA BB. 

 In some way Aegilops cylindrica evolved with the genomic con- 

 stitution CC DD. The C and D genomes are completely dif- 

 ferent from the ones in the Emmer wheats. A cross between 

 Aegilops cylindrica and a wheat of the Emmer group, followed 

 by a doubling of the chromosomes, produced the Vulgare wheats 

 whose chromosome sets are AA BB CC. The relationship of 

 these plants and the method of pairing to be expected in the 

 various hybrids between them is indicated in Fig. 135. These 

 studies show conclusively that although the Vulgare wheats 

 behave as diploids they are actually allohexaploids. 



