Metz and Moses: Chromosomes 



203 



CHROMOSOME DIFFEREN 



Figure 5. The chromosomes of D. muUcri 

 are of the F type, those of rcplcta (right) 

 are of the I type, which differs from "F" 

 only in having the X-chromosomcs large and 

 V-shaped instead of rod-like. Externally the 

 two species look very much alike, and no 

 differences were noticed until it was found 

 that they did not hybridize. The close rela- 

 tionship of the two seems certain. 



was found that they differed in their 

 chromosomes and that they would not 

 hvhridize. Suhsequently they were dif- 

 ferentiated by several external charac- 

 ters also, but their close relationship 

 seems certain. One of these species 

 (mullcri) possesses chromosomes of 

 type F (one of the most common 

 types); the other (rcplcta) is a rep- 

 resentative of type I — which dif- 

 fers from F in that the X-chromo- 

 somes are long and V-shaped instead 

 of rod-like. These relations are shown 

 in Figure 5. Since the remainder of 

 the chromosoiTie group appears to be 

 identical in the two species it might 

 be inferred that at some time a de- 

 cided change has occurred in the sex- 

 chromosomes without visibly aft'ecting 

 the autosomes. If the change were 

 from type I to type F it would seem 

 to involve the loss of half of the 

 X-chromosome, without necessarily 

 affecting the Y. If it were in the 

 opposite direction it would seem to in- 

 volve the doubling of the X-chromo- 

 some. 



It is of interest to note that the 

 latter type of change has actually been 

 observed in a mutant race of Dro- 

 sopJiila mclanogastcr (by L. V. ^lor- 

 gan)." Here one rod-like X-chromo- 

 some has doubled leaving the two 

 daughter elements connected end to end. 

 This case may not have any signifi- 

 cance in the present connection, for 



F 9 



ja^ 



CES IN SIMILAR SPECIES 



Figure 6. D. obscura has chromosomes of 

 type J, while a similar European species has 

 chromosomes of type F. Here the differ- 

 ences involve not only the sex chromosomes, 

 but at least one pair of the others as well, 

 for there are only five pairs in type J, and 

 six in type F. These species do not hybridize 

 in spite of their similarity. 



both sexes are asymmetrical in respect 

 to their sex-chromosomes and half of 

 the offspring die ; but it at least demon- 

 strates that chromosomes may become 

 doubled and remain so. 



A third example of closely related 

 species is furnished by an American 

 race of Drosophila obscura and Euro- 

 pean material of a very similar spe- 

 cies which fails to hybridize with it.* 

 Drosophila obscura, as noted above, has 

 chromosomes of type J, while the 

 European form falls under type F. 

 Its chromosomes have not heretofore 

 been described, hence we include two 

 camera lucida drawings of sperma- 

 togonial groups in Figure 3 (Nos. 1 

 and 2). In both of these forms numer- 

 ous good chromosome figures have 

 been obtained and we feel confident of 

 the accuracy of the observations. The 

 dift'erence here again involves the sex- 

 chromosomes, but it also includes one 

 pair of autosomes, as shown in Figure 

 6, for in F there are five pairs of 

 autosomes and in J only four. The 

 four are duplicated in F, but an addi- 

 tional rod-like pair is left over. When 

 only the female groups are considered 

 it might be imagined that the two X's 

 of type J represent the two X's of 

 F with a pair of autosomes attached 

 at their ends ; but when the male fig- 

 ures are reckoned with, the matter is 

 more complicated and involves either 

 the loss (actual or apparent) or the 



* We are indebted to Dr. A. H. Sturtevant for a stock of the latter and for information 

 about its behavior. 



