SOMATIC MUTATIONS OF BRUCHUS. iy 



Bruchus that only elytral colors which are dominant to it have been 

 found, so it is essential to apply next the elimination mechanism 

 to the actual mosaics observed in this insect. 



The most common mosaic in this beetle is a red elytrum and a 

 black body originating from pure black stock. This condition is 

 accounted for by assuming a mutation to red in one autosome. 

 Let us now suppose in a normal female, homozygous for black, 

 that, previous to the formation of an elytrum, one autosome mu- 

 tated to red; the autosomes for the normal elytrum, therefore, 

 would be R b R b (pure black), while the mosaic elytrum autosomes 

 R b R (heterozygous for black and red). Since red is dominant to 

 black, the one R on the one side of this insect is sufficient to make 

 this elytrum red ; therefore, a mosaic results as was observed for 

 Bruchus. (This is true for the thirty-one mosaics found.) Now, 

 let us apply autosomal elimination for this mosaic; if the autosome 

 R b (black) was eliminated, the proper mosaic would result; but, 

 on the other hand, if the autosome carrying R (red) was elimi- 

 nated, the insect would not be a mosaic, but a normal, homozygous, 

 black individual. 



Again, if the mutation occurred in two autosomes (a very im- 

 probable assumption), it could cause the following mosaic from a 

 homozygous, black insect. Supposing the autosomes on the right 

 side of its body are normal, R b R b (black right elytrum), but on 

 the left side a mutation to red occurred in each autosome, then the 

 autosomes for its right elytrum would be R R (pure red). This 

 insect would then have a black right elytrum and a red left one; in 

 this case elimination could not change the mosaic. 



The first interpretation with reference to dominant autosome 

 mutations appears correct for these mosaics of Bruchus. It seems, 

 therefore, that to account for these mosaics autosomal elimination 

 is not essential, unless a mosaic is visible which is recessive to the 

 normal ; in that case autosome elimination would be necessary. 



The most plausible explanation, then, is to regard these thirty- 

 one dominant, elytral mosaics of Bruchus as somatic mutations that 

 originate in an autosome on one side of the body some time during 

 its ontogeny. The results as described in this paper for Bruchus 

 agree with Morgan and Bridges ('19) relative to dominant muta- 

 tions in somatic tissue. " The general evidence from mutations in 



