DEPARTMENT OF GENETICS. Ill 



the abortion showed no relation to the arrangement in fours. In one 

 clone, however, which differs from the others in its flowers, no tetrad 

 was found in any flower in which two of the grains were not empty of 

 cytoplasm. 



COMPARATIVE STUDY OF THE CHROMOSOME GROUPS IN DIPTERA. 



This study is in charge of Dr. C. W. Metz, who has had associated 

 with him Dr. E. G. Anderson and Airs. Rebecca C. Lancefield and has 

 been assisted by Miss Mildred S. Moses, and, temporarily, by Mr. 

 S. H. Emerson. The work undertaken comprises the comparative 

 study of the genetical content of the germ-plasms of Drosophila 

 melanog aster, D. ivilUstoni, and D. virilis; the survey of the Diptera for 

 the most favorable genetical material; and the initiation of work on a 

 fly (rather distantly related to Drosophila), Leria pedinata. 



Drosophila willistoni. 



The combined genetical and cytological study of Drosophila irillis- 

 toni, in which Dr. Metz and Mrs. Lancefield cooperated, has yielded 

 surprising results. Dr. Metz states: 



"Drosophila wUlistoni resembles D. melanogaster sufficiently to suggest 

 that it is fairly closely related to this species, and the normal chromosome 

 groups of the two are superficially almost alike. These are shown in figure 2, 

 A and B. Owing to these resemblances it was supposed, at first, that the 

 pairs of chromosomes in one species corresponded respectively to the similar 

 pairs in the other (aside from the minute pair which seems to be absent in 

 willistoni). But a study of 'non-disjunction' flies (flies with an extra sex 

 chromosome) has shown that the sex chromosomes in the two species do not 

 correspond. In D. melanogaster the rod-like pair is the sex-chromosome 

 pair (Stevens, Bridges), whereas in D. willistoni we have found that one of 

 the large V-shaped pairs is the sex-chromosome pair. This relationship is 

 indicated in figures a and b, where the sex chromosomes are shown in solid 

 black. The results are in press. 



"Comparing the chromosomes of melanogaster and icillistoni (fig. 2, a 

 and b) it might be assumed that the corresponding pairs are essentially similar 

 in their genetic make-up. In this case it must be assumed that the sex- 

 differentiator (gene?) has been transferred from the rod-like pair {melano- 

 gaster) to one of the large V-shaped pairs (willistoni) or vice versa. If this 

 is the case, then it might be expected that the sex-linked group of mutant 

 characters in willistoni would correspond to one of the non-sex-linked groups 

 in melanogaster, providing it resembled any group in this species. 



"On the other hand, it might be imagined that the rod-like sex chromosome 

 of melanogaster is represented by one arm of the V-shaped sex chromosome of 

 wiUistoni and that the rem.ainder of the V is not present in melanogaster or 

 is included in one of its autosomes. In this case the sex-linked mutant char- 

 acters in icillistoni might correspond to the sex-linked ones in melanogaster 

 plus an additional series. 



"Other hypotheses are possible, among them the hj'pothesis of a radical 

 chromosomal reorganization such that the chromosomal resemblances in 

 the two species are purely incidental. It is hoped that the breeding work 

 will give a clue to the correct explanation. Up to the present there is little 



