336 INTRODUCTION TO CYTOLOGY 



adequacy was demonstrated in an extensive series of crosses which gave 

 the chromosome configurations predicted on the basis of the theory.^ 

 Accordingly, the forms of (Enothera with catenated chromosomes are 

 now regarded as "structural hybrids," i.e., the two sets of chromosomes 

 in the diploid complement are unlike in the arrangement of the elements 

 composing them. This, together with the action of factors which are 

 lethal in certain combinations, goes far to explain the peculiar genetical 

 and cytological behavior observed in the genus. 



The general situation may be illustrated by the case of (E. Lamarckiana. 

 Some years ago Renner (1915 et seq.) advanced the hypothesis that in 

 Oenothera there are several different genetic factor complexes which are 

 combined in pairs in the various species and that these complexes segre- 

 gate as wholes in meiosis, each gamete carrying one or the other. More- 

 over, certain complexes are lethal in gametes, while some combinations 

 are lethal in zygotes. The two complexes in (E. Lamarckiana are called 

 gaudens and velans. Hence gaudens- and ye/aws-carrying (spores and) 

 gametes are formed; and by their unions three different combinations 

 are possible in the offspring (Fig. 192). But both gaudens and velans 

 are lethal when in the homozygous condition in the zygote, hence the 

 only surviving plants are Lamarckiana. In other words, these plants 

 breed true, not because they represent a pure species as supposed by 

 de Vries, but rather because they are "complex heterozygotes" whose 

 homozygous offspring do not survive. 



It was suggested at once that the two genetic complexes must be 

 carried by different gametes produced after meiosis,^ but only after 

 the theory of reciprocal translocation had been developed was it fully 

 apparent why the sets should segregate as wholes. The make-up of the 

 gametic sets of seven carrying the gaudens and velans complexes, respec- 

 tively, is such that, in meiosis, one free pair is formed while the remaining 

 chromosomes form a ring of 12 with the members of the two sets alternate 

 in fixed positions (Fig. 192). Since alternate chromosomes of the ring 

 usually pass to the same pole in anaphase 7, the members of each set 

 remain together when reproductive cells are formed. The single free 

 pair disjoins at random; but its two members are similar as to their shares 

 of the complexes concerned, so that their freedom does not alter the 

 complexes. Hence the different sets of seven chromosomes, like the 

 genetic complexes, may be treated as characteristically stable units. 



The way in which the various genetic complexes are combined in the 

 species of Q^yiothera occurring in nature has been extensively studied 

 genetically, particularly by Renner. In addition, several combinations 

 not before known in nature have been produced by appropriate crosses. 



8 Blakeslee and Cleland (1930), Cleland and Blakeslee (1930, 1931), Emerson and 

 Sturtevant (1931), Cleland (1932, 1933). 



9 Bartlett (1915, 1916), Cobb and Bartlett (1919), Renner (1917). 



