Vermilion. 69 



X. THE CASE OF VERMILION, AND THE POSSIBILITY 

 OF A REARRANGEMENT OF GENES. 



The resemblance between virilis, willistoni, and ohscura in respect 

 to the location of vermilion, and their consistent difference from 

 melanogaster in this feature, have already been pointed out. They 

 suggest that the vermilion of melanogaster is not homologous to any 

 of the others. This view receives some support from the obser- 

 vations of Lancefield (1922, p. 377) on ohscura, showing that the 

 double-recessive eosin vermilion differs from the eosin vermilion of 

 melanogaster. As Dr. Sturtevant has suggested to us, however, the 

 fact that vermihon is a frequently appearing mutant — at least in 

 some species — and that only one such character is known in the 

 sex-linked group of any of the above species supports the opposite 

 view, i. e., that the same gene is involved throughout. This argu- 

 ment may be questioned, perhaps, on the ground that vermilion 

 (in ohscura and melanogaster at least) resembles the autosomal char- 

 acter scarlet in ohscura, melanogaster, and simulans (cf. D. E. Lance- 

 field, 1922, p. 377), but such an objection is not serious enough to 

 rule out the hypothesis. There are other cases also (e. g., sepia, 

 magenta, wax, vesiculated, etc.), as noted above, involving similar 

 characters but a different map order. And more striking examples 

 are known in other species. Sturtevant (192 Id), for instance, has 

 presented evidence that indicates a rearranged map order in the 

 third chromosome of D. simulans (as compared with melanogaster) , 

 where the homology of genes has been tested by hybridization. 

 Lancefield (1922) has observed a similar case in D. ohscura, although 

 here no hybridization tests are possible. Both of these cases have 

 been discussed by the authors mentioned and the details need not 

 be given here. The most probable interpretation of the results, 

 at least in the case of D. simulans, would seem to be on the assumption 

 of a rearrangement of genes, brought about by a transposition of 

 part of a chromosome. And this is the interpretation given by 

 Sturtevant (1. c). 



Recent evidence obtained by Dr. H. J. IMuller, however, brings 

 up the possibility of another interpretation for cases of this kind. 

 In a paper presented before the American Naturalists at the Toronto 

 meeting (1921),^ Muller described the results of experiments with a 

 mutant race of Drosophila melanogaster in which crossing-over was 

 greatly reduced by a "cross-over modifier." One of the pecularities 

 of these results was that when treated in the ordinary fashion for con- 

 structing a map they gave a reversed order of loci for certain well- 

 known characters. This was apparently due to an excess of double 



1 "A lethal gene which changes the order of the loci in the chromosome map." The title of 

 this paper, without abstract, is printed in the Anatomical Record, Jan. 1022, 23:129. The 

 present reference is made with Dr. Muller's permission. 



