CHROMOSOME STUDIES ON THE DIPTERA. IV. 



INCOMPLETE SYNAPSIS OF CHROMOSOMES 



IN DASYLLIS GROSSA FABR. 



C. W. METZ, 

 CARNEGIE INSTITUTION OF WASHINGTON, DEPARTMENT OF GENETICS. 



INTRODUCTION. 



In the robber fly, Dasyllis grossa, synapsis of chromosomes in 

 spermatogenesis (and possibly in oogenesis) appears to be par- 

 tially inhibited, in such a manner that three of the pairs of chromo- 

 somes undergo synaptic association for only a portion of their 

 length. The behavior is relatively uniform and constant in these 

 particular chromosomes and indicates that different regions of a 

 chromosome behave differently as regards synapsis. The present 

 account deals primarily with this feature of chromosome behavior 

 and with its possible bearing on cases of abnormal cross-over 

 values in genetical experiments with Drosophila. 



The description of spermatogenesis is taken largely from one 

 specimen of D. grossa Fabr., kindly identified by Mr. C. W. John- 

 son ; but it applies in a general way to all of the Dasyllis material 

 I have studied (a dozen or more specimens, representing D. grossa 

 Fabr., D. thoracica Fabr., and probably two or three more species). 

 I am not certain that all of these agree in detail, but each one 

 shows indications of incomplete synapsis in one or more pairs of 

 chromosomes. 



Dasyllis provides particularly favorable material for a study of 

 chromosome behavior during spermatogenesis, because of the small 

 number and relatively large size of the chromosomes; the large 

 size of the nuclei ; the relatively enormous number of cells present, 

 representing all stages from early spermatogonia to spermatozoa; 

 and particularly the serial orientation of successive stages through- 

 out the tubular testes, making it easy to trace the chromosome 

 behavior step by step through the growth period. In addition, it 

 should be mentioned that there is no contraction or synizesis stage, 

 and no true " diffuse " stage when the chromatin can not be seen. 



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