THE ORIGIN OF SPECIES 



wheat, being the only diploid species in the group, is no doubt the most 

 primitive, and may be represented by the formula AA. By mutation and 

 chromosomal rearrangements, this A genome has become sufficiently dif- 

 ferent, presumably in an extinct race, to be designated as a different but 

 related genome, B. Hybridization between AA and BB races resulted in 

 the formation of the allotetraploid species of the Emmer group, AABB. 

 Aegilops, as it is known today, is also an allotetraploid genus, CCDD, but 

 no doubt the genus has included in the past a diploid species, CC. Hy- 

 bridization between this primitive Aegilops and the Emmer wheats re- 

 sulted in the formation of the allohexaploid wheats of the Vulgare groups, 

 AABBCC. S S i ' 



Polyploidy in Bromus: a Complex Cose. An even more complicated 

 case has been analyzed by Stebbins and his collaborators in the genus 

 Bromus, a wide-spread complex of range grasses. Stebbins believes that 

 seven is the basic number of chromosomes in this group, but most of the 

 American species, of which B. carinatiis is typical, have 28 pairs of chro- 

 mosomes, or 56 chromosomes in the somatic tissues. Hence they are octo- 

 ploids. These chromosomes include 21 medium-sized and 7 large pairs. 

 A South American species, B. cathnrticus, however, has only 21 pairs of 

 chromosomes, and these are all medium-sized. Hence this species is hex- 

 aploid. A single American species, B. arizonicus has been found to have 

 84 chromosomes, all medium-sized. Hence this species is a duodecaploid 

 (12-ploid)! Crosses between these species have been made and the be- 

 havior of the chromosomes at meiosis in the hybrid studied. When B. 

 carinatus and B. cotharticiis are crossed, 21 pairs are formed by the me- 

 dium-sized chromosomes, while the 7 large chromosomes from B. cari- 

 natus behave as singletons. The three sets of medium-sized chromosomes 

 are called A, B, and C, while the set of large chromosomes is called L. On 

 this basis, the hexaploid species, B. cafharticus, has the formula AABBCC, 

 while the octoploid species, B. carinatus, has the formula AABBCCLL. 

 When B. carinatus is crossed to B. arizonicus, a very complex meiotic pat- 

 tern results. Such a hybrid will receive 42 medium-sized chromosomes 

 from B. arizonicus, and 21 medium together with 7 large chromosomes 

 from B. carinatus. At meiosis, the 7 L chromosomes of carinatus and 14 of 

 the chromosomes of arizonicus behave as singletons. Fourteen bivalents 

 (normal tetrads) are present, indicating that two sets of chromosomes, 

 arbiti-arily designated as the A and B sets, are held in common by the two 

 species. But in addition to this, as many as seven trivalcnts (complex asso- 

 ciations of three pairs of chromosomes) may be formed. Thus it appears 

 that one of the carinatus sets of chromosomes has considerable homology 

 with two sets in arizonicus. This set is arbitrarily designated Ci, while 

 the second C set in arizonicus is called C2. The two sets of chromo- 

 somes in arizonicus which are not represented at all in carinatus are 

 called D and E. Thus the hexaploid species, B. cafharticus, has the for- 

 mula AABBCiC^; the octoploid species, B. carinatus, has the formula 

 AABBCiCiLL; and the duodecaploid species, B. arizonicus, has the for- 

 mula AABBCiCiC.CoDDEE. 



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