ISOLATING MECHANISMS AND SPECIES FORMATION 



Meur*^, 



i^^^ 



SlM. 



aie5 



Al P4-5- 



42 E 2-3- 



a=z>. 



_42C2-3 siM. 

 - A1 VA-'r 



5lM.- 





\ 



ctiRoivioeeNTea. - 





SIM, 



3IM^ 



i 5 





62 P, 



63 6 



SIM. 



9»5 





8 



Figure 103. Failure of Synapsis Because of Chromosomal Rearrangements in a 

 Hybrid Between Drosophila melanogaster and D. simulans. In each figure, the species 

 from which each strand is derived is marked. Numbers identify the bands with respect 

 to the standard map of the sahvary gland chromosomes of D. melanogaster. (From 

 Horton, Genetics, V. 24, 1939. ) 



something on the order of 250 bands ) . The small inversions were too short 

 to permit the formation of typical inversion loops, but they did suppress 

 synapsis not only within their own length but also for a variable distance 

 beyond the ends of the inverted sections. Occasionally, a pair of homolo- 

 gous bands did synapse in very short inversions, showing that identical 

 orientation of the bands is not essential for synapsis. As Horton pointed 

 out, this raises the possibility that single band inversions may be unde- 

 tectable by any means now available, yet they could have a mutational 

 effect. The other four identified rearrangements were at the ends of the 

 chromosomes, and were more difficult to analyze; however, the evidence 

 indicates that they were based upon a series of small translocations. In 

 addition, there were fourteen regions in which suppression of synapsis 

 was not associated with definitely identifiable rearrangements, but Horton 

 believes that small, cytologically undetectable rearrangements are most 



281 



