534 Cytogenetics and Evolution 



circle formation in Datura was pointed out, and we showed 

 that numerous interchanges had also occurred in Oenothera. In 

 this latter genus numerous complexes are found. Each complex 

 consists of a haploid set of chromosomes whose ends are ar- 

 ranged in a definite way and which possesses a certain group 

 of alleles by which it can often be identified from purely genetic 

 evidence. Thus we pointed out that Oenothera Lamarckiana 

 consists of two complexes, velans and gaudens. Because of bal- 

 anced lethals, all velans ' velans and all gaudens 'gaudens plants 

 die at a very early stage. Some complexes have lost their lethal 

 or never had any lethals and can therefore exist in a homozy- 

 gous condition; this is true of the complex ^Hooheri (haplo- 

 Hookeri). By crossing it with various complex heterozygotes, 

 and by crossing them together as well, the interchanges that have 

 given rise to the various complexes can be determined and 

 identified. 



Figure 154, originally published by Cleland, shows the rela- 

 tionship between a number of the complexes. In this figure, 

 each circle is a complex. The complexes that are related ge- 

 netically are placed close together or overlap, whereas those that 

 differ in all or most of their known genes are placed far apart. 

 Complexes connected by a solid line are normally associated 

 together in one of the types that occur in nature; those that are 

 joined by a broken line have been found in plants that have 

 been produced in the experimental field. The number and size 

 of the circles formed by two complexes are indicated on the line 

 that joins them. The somatic chromosome number of each type 

 is 28. When the number of chromosomes in all the circles to- 

 gether in one plant is subtracted from 28, the remaining chro- 

 mosomes are found in pairs. 



Because of reciprocal translocations and balanced lethals, the 

 Oenotheras present some taxonomic problems not found in most 

 other genera. The genus is a far-ranging one in the Western 

 Hemisphere. Fifteen subgenera are in existence and all are found 

 in North America, but almost all the cytological studies have 

 been upon the subgenus Euoenothera. The taxonomic situation 

 has been worked out largely by Cleland and Munz. In Cali- 

 fornia, Northern Mexico, and adjacent territory reciprocal trans- 

 location has not occurred, except very rarely. Almost all plants 

 from this vast area show only paired chromosomes at meiosis 



