464 Allopolyploids 



ence of duplicate genes is also fairly strong evidence of a remote 

 polyploid origin. In some genera, a series of species may be 

 found with chromosome numbers in multiples, and this also indi- 

 cates that the higher forms are probably allopolyploids. 



It is interesting to note that two of the classic examples of 

 duplicate genes, Nilsson-Ehle's genes for seed color in wheat 

 and Shull's triangular capsules in Capsella, are found in genera 

 where there is more than one diploid chromosome number. For 

 example, Capsella rubella, C. Viguieri, C. grandifiora, and C. 

 tuscaloosae are all diploids in which the haploid chromosome 

 number is eight. Other species, however, such as C. biirsa-pas- 

 toris, C. Heegeri, C. occidentalis, C. orientalis, C. djurdjurae, and 

 C. penarthae, have sixteen as the haploid number and are very 

 probably allotetraplotds. It is highly interesting to note that the 

 duplicate genes which have been found in this genus are only in 

 the second group of species. In the known amphidiploid, Gale- 

 opsis Tetrahit, Miintzing has recorded several characters con- 

 trolled by duplicate genes, and the possible connection between 

 allopolyploidy and duplicate genes has been suggested by a num- 

 ber of investigators. 



A complicated and interesting situation is found in wheat. 

 Species of the genus Triticum can be divided up into three chro- 

 mosome groups. The Einkorn group includes T. monococcum 

 and is diploid, with n = 7. This species is of little importance 

 economically but is generally more resistant to disease than the 

 other species. The Emmer group includes T. dicoccum, T. po- 

 lonicum, T. persicum, T. dicoccoides, T. durum, and T. turgidum. 

 They are tetraploids {n = 14) and are of little value except for 

 the last two, but they are more resistant to disease than 

 the next group. The Vulgare group includes T. vulgare, T. com- 

 pactu7n, and T. spelta. They are hexaploids (n = 21) and, with 

 the exception of the last species, are of greatest economic im- 

 portance and widest distribution. They are the bread wheats but 

 unfortunately they are considerably more susceptible to disease 

 than the other groups. 



In all these species, meiosis is very regular, and all the chro- 

 mosomal configurations are bivalents. The series of chromosome 

 numbers in multiples of seven indicates that the species with 

 higher numbers developed by polyploidy, and this view is 

 strengthened by the presence of duplicate and triplicate genes 



