ISOLATING MECHANISMS AND SPECIES FORMATION 



translocated chromosomes, while the other half have only translocated 

 chromosomes. Theoretically, the limit to the number of chromosomes 

 which may be joined together in such a translocation complex is set only 

 by the total number of chromosomes which a plant possesses, and this 

 limit is actually reached in some species of Oenothera. O. hookeri has 

 seven pairs of chromosomes, all of which behave independently. The other 

 species all show translocation ring formation of some degree, and in some, 

 all fourteen chromosomes form a single large ring at the metaphase of 

 the first meiotic division. 



The genetic peculiarities referred to above all follow from the behavior 

 of translocation heterozygotes if one more special feature be added. Be- 

 cause of the alternate distribution of chromosomes of these rings, only 

 two types of gamete are formed. This should permit the formation of 

 three types of zygote, the two homozygous types, and the heterozygous 

 type. But only the heterozygous type ever appears in the progeny of selfed 

 plants. This is because each chromosome complex (a complex being a 

 group of chromosomes inherited as a unit because of alternate disjunction) 

 contains a recessive lethal gene, but it is a different gene in each complex. 

 The result is that neither complex can become homozygous. 



Thus O. lamorckiana, a species studied by De Vries, is a heterozygous, 

 self-fertilized species. But self-fertilization yields only parental type plants, 

 and only 50 per cent of the seed is viable. When crossed to O. hookeri, 

 twin hybrids are formed, and this makes possible the demonstration of 

 genetic difiFerences between the two chromosome complexes of lamarcki- 

 ana. The two complexes present in this species are called gaudens and 

 velans. Gaudens carries the lethal gene Ii while velans carries lo- Each 

 carries the normal allele of the lethal carried by the other. As a result, 

 g'g and V'V zygotes die, while g*o zygotes survive. At synapsis, the chro- 

 mosomes of O. lamarckiana form a ring of twelve chromosomes and a pair. 

 This does not mean, however, that all of the chromosomes of one set are 

 normal, and six of the chromosomes of the other as translocated. If O. 

 hookeri be regarded as the standard, then translocated chromosomes are 

 present in both the gaudens and velans complexes. If the hookeri chromo- 

 somes be named a-a', b*b', cc', d*d', e*e', f'F, and g'g^ then the chromo- 

 somes of other Oenothera species can be described in terms of transloca- 

 tions of this standard set. The velans complex has the constitution a-a', 

 b'b', c'd', d'c', e*e', f'f, and g*g'. Available data will permit two possible 

 interpretations of the structure of the gaudens complex, as follows: 



a*a', b-f, c'c', d'f, e*b', d'-g', g*e' or 

 a-a', b'f, c*c', d'g, e*g', e'*b', f'd'. 



Further data are necessary in order to determine which of these structures 

 is correct. 



The 50 per cent fertility which characterizes most Oenothera species is, 

 then, a result of the balanced lethals which the several chromosome com- 

 plexes carry. The twin hybrids are formed when any species is crossed to 

 O. hookeri because each complex of the heterozygous parent ( hookeri is 

 largely homozygous) is radically different in genetic content from the 



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