STRUCTURAL HYBRIDITY 



interchanges in Campanula persicifolia with i6 chromosomes, to 

 build up multiple hybrids with rings of twelve. 



Inversions, though less obvious than interchanges, are a com- 

 moner and more important source of structural hybridity. They are 

 found as heterozygotes in a proportion of individuals probably in 

 all cross-breeding species. And in the homozygous condition they 



Ml 





MELANOGASTER 



SIMULANS 



se St p ap AH 



1 ^ t 



ca 



J06 



J34 



se 



St HA ap 



P 



ca 



Fig. 31. — The right arm of chromosome III of Drosophila sitmilans has a. segment 

 which is inverted relative to the corresponding chromosome arm in D. melanogaster. 

 The effect of this inversion may be seen cytologically in the pairing of the chromo- 

 some arm in saUvary gland nuclei of the species hybrid (above) or genetically in 

 the order of homologous genes when mapped by the crossing-over percentages 

 occurring within each of the species (below). It will be observed also that crossing- 

 over is freer in chromosome III of simulatis than mdanogaster, the interval se-ca 

 (which includes the whole of the inversion) being 127 units as compared with 80 

 (after Patau, 1935). 



distinguish different species (Fig. 31). A true inversion, one which 

 does not include the centromere, leads to a pairing of the two 

 relatively changed segments the wrong way round with respect to 

 the centromere. The inverted segments pair in a loop (Fig. 30). Any 

 single crossing-over within this loop gives two new chromatids, 

 one with two centromeres attached to it, a dicentric, the other with 

 none, an acentric. The consequences of this structure are seen at 

 anaphase: the acentric chromatid is left helpless on the equator and 



131 



