CYTOGENETICS AND EVOLUTION OF THE GRASS FAMILY 1 89 



in the hexaploids, plus 14 chromosomes derived from diploid species of the 

 sect. Bromopsis. They have probably originated as amphiploids from ancient 

 intersectional hybrids. This new cycle of amphiploidy met with great success, 

 since the species concerned are among the commonest grasses of western 



Chromosome 

 number 



2n = 84 



2n = 56 



2n = 42 



2n = 28 



2n = 14 



CERATOCHLOA 



A1A1B1B1B2B2LL 



B. carinatus 

 B. marginatus 

 B. pitensis 



112211 "2°2 3 3 4^4 



B. arizonicus 



BROMOPSIS 



LLLLLL 

 B. auleticus 

 B. uruguayensis 



LL 

 B. ciliatus 

 B. anomalus 

 B. Orcuttianus 



A, AjBj B J B2B2 



I y^o. catharticus 

 A^ B. haenkeanus=X 

 ''' B. stamineus 



k 

 A.A.B.B, I 





\ 



AiAj— X~BiBi B2B2 A2A2 



NEOBROMUS 

 A2A2B3B3B4B4 



= B. Trinii 



/ ^3^3^464 



^' i 



^3^3""^ B4B4 



Fig. 7. Chart showing the phylogenetic relationships among New World polyploid 

 species of Bromus. The broken Hnes indicate hypothetical connections between 

 extinct species; continuous double lines indicate connections between living species 

 which have been verified by experiments. 



North America. Finally the species of sect. Ccratochloa with 84 chromosomes 

 {B. arizonicids) has been shown to be the product of hybridization and 

 amphiploidy between a species of the rescue grass type and the sole surviving 

 species of another ancient polyploid complex, the hexaploid B. trinii of sect. 

 Neobromus. The success of B. arizonicus, which is a common weed in much 

 of its area of distribution, shows that two waning polyploid complexes can 

 receive new life through pooling their genie resources via an intersectional 

 hybrid and amphiploid. 



