RECIPROCAL TRANSLOCATION 331 



members of the ring were responsible for the sterihty and that the 

 ring itself was a result of reciprocal translocation.^ The chromosomes 

 concerned were number VIII (carrying the j-wss linkage group) and 

 number IX (carrying the C-sh-wx group), and these could be distinguished 

 on the basis of length, position and size of attachment region, and a 

 terminal knob on IX. The semisterile plant contained (in addition to 

 eight normal chromosome pairs) one normal VIII, one normal IX, one 

 interchange chromosome composed of portions of VIII and IX, and one 

 interchange chromosome composed of the remaining portions of IX and 

 VIII. The portions interchanged were unequal in length and were 

 such that each of the modified chromosomes had an attachment region. 



In the meiotic prophase the synapsis of homologous portions in such 

 semisterile plants gives rise to a cross-shaped group of four, the center 

 of the cross marking the points of interchange in two of the members. 

 Later in the prophase the four open out, remain attached at their ends, 

 and shorten to form the conspicuous ring seen at diakinesis and meta- 

 phase /. In anaphase / the four members pass two by two to opposite 

 poles. In about half of the sporocytes adjacent members go to one pole, 

 while in the other sporocytes alternate ones go together. When distribu- 

 tion is "adjacent," all of the chromosome sets resulting are abnormal, 

 since they have a certain portion of one chromosome in excess and lack 

 a portion of the other chromosome concerned. The pollen grains 

 containing such sets abort: hence the sterility observed. On the other 

 hand, when distribution is "alternate," all of the resulting pollen grains 

 contain complete sets and are viable, although in one-half of them the 

 chromosomal materials have a new arrangement. 



When a semisterile-2 plant is selfed or crossed to another of its kind 

 the progeny includes plants of three different chromosomal types in the 

 ratio of 1 : 2 :1 : plants with the standard chromosome complement, semister- 

 ile plants again forming a ring of four in meiosis, and plants with two modi- 

 fied chromosome pairs but fully fertile because the complement is sym- 

 metrical and balanced. Had the "modified" type been discovered first, 

 it might have been regarded as the "standard." Thus in Datura stra- 

 monium it appears that the standard line lA, which has been so exten- 

 sively studied, is actually less prevalent in nature than the 5-race, these 

 two types evidently having been differentiated by reciprocal transloca- 

 tion. The discovery of such related chromosomal types may be expected 

 in other genera also, especially among self-pollinated species. 



The diagram for Zea (Fig. 187) shows further that in this case stand- 

 ards give about 50 per cent semisteriles when crossed with semisteriles, 

 that modifieds give about 50 per cent semisteriles when crossed with 



* This explanation of semisterility was first suggested by Belling (1925c) for 

 SHzolobium. It also applies to certain forms of Pisum sativum (Pellew and E. San- 

 some, 1931). 



