The Origin of Crepis and Related Genera 29 



species). All the 8-chromosome species have one pair each o£ 

 types A, B, C, D. The two 6-chromosome species have one pair 

 each of A, B or C, and D. All species thus far analyzed with 

 numbers higher than 10, have been found to have genoms re- 

 sulting either from duplication of certain of the types present in 

 the basic genom or from complete doubling of the whole comple- 

 ment. Thus, in the 12-chromosome species, there may be dupli- 

 cation of A, B, C, D, or E and sometimes one type is absent; 

 whereas the 14-chromosome species always have the basic genom 

 plus extra chromosomes of C, D, or E types. This indicates 

 hybrid origin for both the 12- and the 14-chromosome species. 

 Again, all the i6-chromosome species have only A, B, C, and D 

 types and are polyploids derived from 4-paired ancestors. Analy- 

 sis of distribution of chromosome types in the species with 

 higher numbers has not been completed, but in some of the 

 American species having 22 chromosomes in their somatic cells, 

 the two pairs of D chromosomes are unlike. This strongly indi- 

 cates that these species originated through interspecific hybrid- 

 ization followed by doubling of both specific complements 

 (amphidiploidy) . Similar evidence has been found in two Old 

 World species having 40 chromosomes. Figure 6 illustrates the 

 basic genom of three primitive species, one from each subgenus; 

 and derived genoms with smaller chromosomes and with less 

 and more than 5 are shown. 



Chromosome number and morphology are taxonomic criteria 

 of great value in the genus Crepis, especially when used in con- 

 nection with other available criteria. It is certain that morpho- 

 logically similar species have similar chromosomes and, con- 

 versely, close similarity in the chromosomes has often proved to 

 be a helpful clue to a closer degree of relationship than was at 

 first surmised from superficial gross morphology. Thus it has 



