HETEROCHROMOSOMES IN MOSQUITOES. 1 19 



In Metapodius Wilson ('09) traces the origin of an unpaired 

 heterochromosome and also of the supernumeraries to an irregular 

 mitosis in which both idiochromosomes go to one daughter cell. 

 In the Diabroticas the supernumeraries probably owe their origin 

 to an irregular division of the unpaired heterochromosome. 



But in no case are we able to say when or how or why certain 

 spermatogonial chromosomes became specially differentiated 

 as heterochromosomes. 



We have associated the X and Y chromosomes of the male 

 with sex-determination, but possibly they have some other 

 meaning and are only so correlated with the sex-determining 

 mechanism that they have the same distribution as the sex 

 characters. The case of the mosquitoes certainly indicates 

 that we must study the heterochromosomes, apart from the idea 

 of sex-determination, more intensively, in order to determine 

 if possible why we have such chromosomes differentiated in 

 some forms and not in others. 



In such cases as Anopheles the sex characters may be supposed 

 to be located in the unequal heterochromosomes or in the larger 

 equal pair with which they are connected. This may also be 

 true of the composite heterochromosomes of Drosophila am- 

 pelophila and Hesperotettix (McClung, '05). In the ordinary 

 cases of an unpaired heterochromosome or an unequal pair, we 

 may suppose the sex characters to be located in the heterochro- 

 mosomes, but to be independent of the special differentiation, 

 which may be entirely absent as in Culex and Theobaldia. 



If we apply Morgan's new sex formula 



FmFm = 9 , Fmfm = cf , 



to the cases under discussion, m may be located in any equal 

 pair but F must be either in the heterochromosomes or in chro- 

 mosomes correlated with them. Sex-limited inheritance in 

 the male (Morgan, '10) would then seem to be possible only 

 where unpaired or unequally paired heterochromosomes are 

 present, and not in such cases as, Anisoloba, the Lepidoptera, 

 Culex and Theobaldia. 



It occurs to me as possible that heterochromosome differenti- 

 ation may be directly related to sex-limited inheritance of certain 



