In Different Species. 63 



Comparison with Sex-Linked Characters in Drosoimiila melanogaster. 



The parallelism between yellow and forked in D. mrilis and 

 D. melanogaster was pointed out in an earlier paper (Metz, 1918). 

 This was extended to include crossveinless by Weinstein (1920); and 

 it may now be enlarged by the addition of singed, as suggested above. 

 When this is done and the X-ehromosome maps of the two species 

 are compared, the sequence and relative positions of the four loci 

 agree as closely as would be expected if the characters were known 

 to be homologues. These relations are indicated in figure 15. 

 The two outside maps in this figure are drawn to the same scale; 

 but since the mrilis map is considerably longer than that of mela- 

 nogaster (due presumably to a greater amount of crossing-over in 

 this species), the relative positions of the genes are shown better by 

 making the maps the same length. For this reason the melanogaster 

 map is represented twice, the right-hand one being drawn to a dif- 

 ferent scale, to equal that of virilis. 



Another case of close resemblance is found in the allelomorphs 

 glazed and wax compared with the allelomorphs lozenge and lozenge-2 

 in melanogaster, as discussed above under wax (p. 19). The map 

 order here is very different, however. The locus of glazed and wax 

 is near the end of the map (102), while that of lozenge is near the 

 middle (28). The third allelomorph, rugose, in virilis is not repre- 

 sented in inelanog aster. Perhaps the resemblance in the two species 

 is accidental, but the similarity in appearance and in the sterilit}', 

 or tendency toward sterility, of the females, suggests homology, 

 and this in turn suggests a rearrangement of genes. 



Of the other sex-linked characters, however, none parallels any 

 in melanogaster sufficiently to make homology very probable. Sepia 

 resembles prune in melanogaster, and its locus is near that of yellow, 

 as in melanogaster, but it is on the opposite side of yellow. If the 

 two are homologous they apparently indicate a rearrangement of 

 genes. The case of magenta in virilis and garnet in melanogaster 

 is similar. The characters are somewhat alike, but the map location 

 does not correspond very closely, although the order of genes is the 

 same. The great difficulty in both of these cases is that the charac- 

 ters are not sufficiently reliable for comparison. If they were 

 distinguished by a combination of features, instead of merely by 

 the possession of darker eyes than usual, the case would be more 

 plausible. This is especially true in view of the radical difference 

 between the "normal" eye-colors in the two species and the fact 

 that several dark-eyed mutants are known in each. 



Much the same argument may be used in the case of vermilion 

 virilis and vermilion melanogaster, vesiculated virilis and inflated 

 melanogaster and droop virilis and depressed melanogaster; hence it 

 seems unnecessary to give a detailed discussion of these. Their 



