DEPARTMENT OF GENETICS. 



97 



the 



series 



Wl 



D. 

 listoni 



D. 



Obscura 



I 

 I 

 I 



vinlis 



D. melano- 

 ma step 

 I- Scute ar 

 Yellow 



-Crossveinless 



.Yellow 

 Scute 



Scute 

 -YePow 



Crossveinless 

 Vermilion 



-F(=Singed) 



-Singed 



-Vermilion 



.Crossveinless 

 "Ve'rmilion 



-Singed 



Scute 

 ■Yellow 



-Vermilion 



-Singed 



series scute-yellotD-crossveinless-vermilion-forked. In D. virUis 

 yellow, crossveinless, vermilion, and singed has been noted in previous 

 reports. To this we have now added the parallel scute. In this case the order 

 seems to be yellow-scute-crossveinless-vermilion-singed. (Forked in the 

 willistoni series resembles singed in the other series.) 



The sequence of yellow and scute, in relation to the 

 other three characters, appears to be just the oppo- 

 site in these two species. The relations are shown 

 graphically in the accompanying diagrams or chro- 

 mosome maps, in which are included also the 

 X-chromosome maps of Drosophila melanogaster 

 (after Bridges) and D. ohscura (after Lancefield). 

 Only the loci considered here are included on the 

 maps (Fig. 3). 



"The characters scute and yel- 

 low are emphasized because they 

 are very closely linked in all four 

 species. This parallelism, both in 

 appearance and in degree of link- 

 age of the characters, makes it 

 seem probable that we are dealing 

 with actual homologues. It there- 

 fore becomes of particular interest 

 to compare their linkage relations 

 with those of the other parallels in 

 the species concerned. 



"In D. melanogaster (according 

 to Doctors Sturtevant and Bridges) 

 scute and yellow are so closely 

 linked that not enough crossovers 

 have been obtained to establish 

 their sequence; hence they are 

 given the same locus. In D. viri- 

 lis our evidence, just obtained, is 

 based on experiments involving 

 yellow, scute, and vermilion, in 

 which three crossovers were se- 

 cured, all of the same type and 

 indicating the order given here. 



This number of crossovers is not large, but it is believed to be reliable in 

 indicating the sequence. 



"The two maps on the left in the accompanying figure are from species 

 possessing rod-like X-chromosomes ; the other two represent long V-shaped 

 chromosomes. It will be noted that the sequence of scute and yellow agrees 

 in the two species having V-shaped X-chromosomes, but is reversed in virilis, 

 which has the rod-like X. 



"The presence of so many 'parallels' indicated that a homology exists 

 between the X-chromosomes of the four species, but the altered sequence 

 suggests that a rearrangement of genes may have occurred. This is also sug- 

 gested by the map position of vermilion in D. melanogaster, which differs 

 from that of vermilion in the other three species. It agrees, too, with the 

 demonstrated case of map-displacement noted by Sturtevant in the third 

 chromosome of D. simulans as compared with D. melanogaster. 



"In both D. virilis and D. willistoni the number of groups of linked genes 

 now equals that of the haploid number of chromosomes (6 in virilis and 3 in 



I 



Fig. 3. — Comparison of sex chromosome. 

 in 4 species of Drosophila. 



Maps 



