CHROMOSOMAL ABERRA TIONS 



199 



near its periphery the types with larger rings are more numerous. It 

 seems hkclv that such translocation has been a factor in the differentiation 



i 



Fig. 147. — a, Kings and cliains-of-4 in microsporocyte o£ Tradescantia. (Some of the 

 chromosomes have been pressed out of the coll in making the preparation.) b, ring-of-12 

 in microsporocyte of Rhoco. Each chromosome shows its two arms. {Courtesy of K. Sax.) 



of races and species, although its importance in this respect cannot yet 

 be measured. It is held to be a major factor in the 

 genus Crepis. In Datura it has been found in investi- 

 gations extending over many years that the same 

 fundamental genome of 12 chromosomes has its parts 

 arranged in numerous ways in the various species from 

 different parts of the world. They appear to be homo- 

 zygous translocants, or modifieds, derived from one or 

 more main chromosomal types. Within the species 

 stramonium there are several such natural races, or prime 

 types. The arrangement of chromosome parts in the 

 genome of a given race is determined by observing the 

 meiotic configurations (rings, etc.) in crosses between 

 this race and one or more others in w^hich the arrange- 

 ment is know-n in terms of an arbitrarily chosen stand- 

 ard. Since prime tj^pes in the same species are so 

 closely similar morphologically, it seems best to regard 

 reciprocal translocation as supplementary to mutation, 

 hybridization, and isolation as a factor in speciation in 

 such genera. 



The genus Oenothera is of unusual interest in this con- 

 nection because of its relation to the mutation theory 

 propounded many years ago by Hugo de Vries, the great 

 Dutch botanist. Oenothera lamarckiana, a truebreeding 

 type, was observed to produce occasional offspring unlike 

 itself. This was interpreted as the production of new 

 species by sudden large steps, or mutations, from a pure 

 parent species. Since that time cytogeneticists have developed another 

 interpretation of the phenomenon. 



Fig. 148.- 

 Catenated chro- 

 mosomes in an 

 evening prim- 

 rose, Oenothera 

 franciscana sul- 

 phur ea. a, late 

 meiotic prophase, 

 with a ring-of-12 

 and one free pair. 

 b, first meiotic 

 anaphase, show- 

 ing alternate dis- 

 tribution of 

 members of ring. 

 (After R. E . 

 Cleland.) 



