Vol. XLIII December, 1922 No. 6 



BIOLOGICAL BULLETIN 



ASSOCIATION OF HOMOLOGOUS CHROMOSOMES IN 

 TETRAPLOID CELLS OF DIPTERA. 



C. W. METZ, 

 DEPARTMENT OF GENETICS, CARNEGIE INSTITUTION OF WASHINGTON. 



A characteristic feature of chromosome behavior in the Diptera 

 is the paired association of homologous chromosomes 1 in the 

 diploid cells, somatic as well as germinal. This association is par- 

 ticularly striking in the early prophase of each cell generation, 

 when the two homologous chromosomes are closely approxi- 

 mated so closely, indeed, as to resemble a single element in many 

 cases. 2 Like the association that occurs during synapsis in the 

 germ cells of most organisms, this is apparently the result of an 

 attraction between similar or identical elements, in the conjugating 

 chromosomes. 



In considering the nature of the attractive force involved here, 

 one of the first questions to arise is whether an equilibrium is 

 established when two homologous chromosomes associate (anal- 

 ogous to a magnetic attraction) or whether the attraction extends 

 to an indefinite number of members, so long as they are homolo- 

 gous. The observations on sporogenesis in certain triploid and 

 tetraploid plants by El. and Em. Marchal Cii), Digby ('12), 

 Osawa ('20), and Belling ('21) indicate that in the metaphase 

 following synapsis homologous chromosomes are frequently 

 grouped together, although not uniformly so. This indicates that 

 there is, sometimes at least, an attraction between homologous 

 chromosomes in numbers greater than two ; but it tells little about 

 the detailed nature of the association. 



The observations of Holt ('i/), of Metz ('16), and of Bridges 

 ('22) on multiple chromosome groups of somatic cells in certain 



1 First described by Stevens in 1907 and 1908. 



2 See Metz, '16, for a detailed account. 



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