DEPARTMENT OF GENETICS. 101 



" In Drosophila virilis, whose study has likewise been continued during the 

 present year, 44 mutant characters are now known, of which 11 are dominant 

 characters. Present evidence indicates that 6 groups of linked characters 

 have now been identified in this species, corresponding to its six pairs of chro- 

 mosomes. The "chromosome map lengths" of these groups, based upon 

 percentages of crossing over, range from approximately 110 units in the case of 

 the X-chromosome to zero in the case of one group of three non-sex-linked 

 characters. It is possible that the latter group represents the small, dot-like 

 pair of chromosomes, although the evidence is not yet clear, partly because of 

 the fact that the sixth group has not yet been tested for crossing over. 



"Evidence from the sex-linked characters in Drosophila virilis tends more 

 and more to indicate a genetic as well as morphological correspondence 

 between the X-chromosomes of this species and those of Drosophila melano- 

 gaster. The non-sex-linked characters have not appeared in sufficient numbers 

 to make possible a comparison of the non-sex-linked groups. 



"With the accumulation of mutant types for comparison, both in these 

 species and in species studied by other workers, significant results are obtained 

 more and more rapidly. Furthermore, the element of speculation, which 

 loomed large when the study of Drosophila virilis was first undertaken, has 

 now been dispelled to a great extent by the appearance of parallel characters 

 in this and other species. Such characters agree in different species to such 

 an extent as to make the probability of homology very great; and this in turn 

 allows a comparison of the genetic make-up of chromosomes in different species 

 to be made with some assurance. 



"If the present evidence is reliable, even in part, it indicates a considerable 

 degree of stability in chromosomal organization over relatively long periods of 

 time during the evolution of the species involved." 



Work on Leria pectinata, on which a beginning was made by Dr. E. G. 

 Anderson (Year Book, 1921, p. 112), has progressed in spite of various inter- 

 ruptions. 



Dr. Anderson has also carried out the experimental study of crossing over 

 in triploid flies, as a first step in which it has been necessary to make to order 

 all of the flies to be used in the experiments. Each of the initial stocks to be 

 used contains four or more mutant characters. The required compound 

 stocks have been synthesized. 



Cytological Work. 



It is believed that progress will be most rapid when the cytological and the 

 genetic methods of attack on the problem of chromosome organization and 

 behavior are combined. The studies on spermatogenesis and oogenesis in 

 various Diptera have been continued by Dr. Metz. Attention has been 

 devoted especially to the exceptional conditions found in one of the robber 

 flies, Lasiopogon bivittatus Lw., specimens of which were taken and prepared 

 in Wyoming. In these specimens the spermatocyte nuclei at certain stages 

 exhibit such peculiar relations between nuclear wall and chromosomes as to 

 suggest that the latter are surrounded by thick, transparent envelopes of a 

 gelatinous nature, which serve, among other things, to hold the chromosomes 

 away from one another and from the nuclear wall. 



The study of what appears to be incomplete synapsis of certain chromo- 

 somes during spermatogenesis in Dasyllis, reported previously, has been com- 

 pleted by Dr. Metz, together with a partial study of oogenesis in the same 

 forms. 



