6O A. H. STURTKVAXT AND C. R. Pl.rXKKTT. 



limb of the third chromosome (in terms of map distance) has 

 been inverted in one or the other species since there is no clue 

 as to which is the original sequence. The left-hand limit of the 

 inverted section lies between scarlet and peach of tnelanogaster, 

 which is presumably very close to the spindle-fiber attachment, 

 and is certainly in a region where the mutant genes are mapped 

 very close together and where coincidence of crossing over is very 

 high (Bridges and Morgan, 1923). It will be interesting, as more 

 mutant genes are studied in simulans, to see how this same 

 material behaves when it lies further from the middle of the 

 chromosome. 



In the earlier account (Sturtevant, 1921^) it was suggested 

 that the rearrangement of parts demonstrated for the melano- 

 gaster and simulans third chromosomes might be explained as 

 resulting either from a duplication or from an inversion of a 

 section. The new data here reported speak strongly for the 

 inversion interpretation. 



One might surmise that this inversion of a piece of a chromo- 

 some is the fundamental factor in the specific difference between 

 melanogaster and simulans and the sterility of their hybrids. 

 But it has been shown (Sturtevant, 1920) that at least some of 

 the factors concerned in the inviability of certain classes of the 

 hybrids are in the X chromosomes, where there has certainly 

 been no such inversion. 



This case shows that identity of sequence is not a necessary 

 condition for correspondence of genes in different species. It 

 also suggests that we need not be surprised should such an in- 

 version be found within a species. 



BIBLIOGRAPHY. 



Bridges, C. B., and T. H. Morgan. 



'23 The Third-chromosome Group of Mutant Characters of Drosophila melano- 

 gaster. Carnegie Inst. Wash., publ. 327. 251 + 10 pp.. 3 plates. 

 Sturtevant, A. H. 



'20 Genetic Studies on Drosophila simulans. I. Introduction. Hybrids with 



D. melanogaslcr. Genetics, 5: 488-500. 

 '2ia Genetic Studies on Drosophila simulans. II. Sex-linked Group of Genes. 



Genetics, 6: 43-64. 

 '216 Genetic Studies on Drosophila simulans. III. Autosomal Genes. General 



discussion. Genetics, 6: 179-207. 

 *2ic A Case of Rearrangement of Genes in Drosophila. Proc. Xat. Acad. Sci., 7: 



235-237. 



