236 



( II MTI.R 17 



parently violating our concepts of pure lines 

 and independent segregation. More com- 

 plete analysis has shown, however, that 

 ( Oenothera's failure to behave as expected 

 was due to the operation of other, already 

 known, genetic events. Oenothera is an ex- 

 ception which should be treasured; for in 

 the exact correspondence between its atypical 

 genetics and its atypical cytology, it is an 

 outstanding example of the validity of the 

 chromosome theory of transmission genetics. 



Three aspects of the cytogenetic behavior 

 of Oenothera are disadvantageous under 

 many circumstances: reciprocal transloca- 

 tions; recessive lethals; and self-fertilization. 

 By combining all three of these disadvan- 

 tages in one plant, however, Oenothera's sur- 

 vival value is probably greater than it would 

 be without them. The self-fertilization 

 mechanism involves bringing the stigma 

 down to the level of the anther, so that a 

 much heavier pollination is attained than 

 would be likely were the plant pollinated by 

 insects. This self-fertilization mechanism 

 offsets the 50% mortality due to balanced 

 lethals. These lethals, together with the re- 

 ciprocal translocations and alternate segre- 

 gation, prevent the homozygosity usually 

 consequent to self-fertilization, enforce het- 

 erozygosity, and produce maximum hybrid 

 vigor. 



The great survival value of Oenothera is 

 demonstrated by the distribution of this 

 genus: it can be found from the southern 

 tip of South America to the far reaches of 

 Northern Canada and from the Atlantic 

 Ocean to the Pacific. It is interesting to 

 note that the most numerous sections of the 

 genus and those which have ranged the 

 farthest are the ones with large circles, bal- 

 anced lethals and self-pollination. 



Drosophila 



Although reciprocal translocations have 

 played an important role in the evolution 

 of Oenothera, it might be claimed that this 



X >x 



figure 17-12. Chromosome configurations in 

 several Drosophila species. 



genus is an unrepresentative example of the 

 importance of chromosomal rearrangements 

 in evolution because its cytogenetic behavior 

 is so unorthodox. Hundreds of different 

 species of Drosophila occur in nature. These 

 species can be compared ecologically, mor- 

 phologically, physiologically, and biochem- 

 ically. For those species able to interbreed, 

 recombinational genetic properties can also 

 be compared; banding patterns of salivary 

 gland chromosomes and the appearance of 

 chromosomes at metaphase of different spe- 

 cies are additional areas of comparison. 

 After all available information of this kind 

 is gathered, it is possible to arrange the chro- 

 mosomes of various species on a chart so 

 that those closest together are more nearly 

 related in descent — evolution — than are those 

 farther apart.- This arrangement is illus- 

 trated in Figure 17-12 which shows the 

 karyotype — the haploid set of chromosomes 



- Based upon work of C. W. Metz and others. 



