178 I The Process of Evolution 



united, forming one great ring at meiosis (Fig. 9.1). Directed alter- 

 nate disjunction may occur in these specialized types. Since synapsis 

 is very precise, homology determines that each chromosome has its 

 own place in the ring which can be changed only by further struc- 

 tural changes. 



When more than one interchange has occurred, a new type of 

 chromosome segment is set apart. The homologous chromosome arms 

 are called pairing segments, and the portion of the chromosome be- 

 tween the point of interchange and the centromere is the interstitial 

 segment. The new area, which is associated with the central part of 

 a chromosome with which it is not paired terminally, is the diflFer- 

 ential segment which may or may not include the centromere. In 

 this area, crossing-over occurs rarely; when it does, the interchanges 

 are modified, and the arrangement of the chromosomes or the num- 

 ber of rings may change. The study of reciprocal translocations 

 requires careful and painstaking analysis of crosses between many 

 different individuals so that the homology of the pairing segments 

 can be determined. 



Floating translocations occur in a large proportion of diploid 

 plants (e.g.. Campanula, Paeonia), as well as in scorpions and cock- 

 roaches. In many invertebrate animals translocations are involved 

 in the sex chromosome mechanism. When an XX female and XO 

 male chromosome mechanism exists, one of the autosomal chromo- 

 somes may become translocated to an X. This leaves one of the 

 autosomes with its homologue as part of a modified sex chromo- 

 some (which may be called a neo-X). This autosome subsequently 

 behaves as do Y chromosomes or heterochromatic regions of chro- 

 mosomes in general. It is said to have become heterochromatized 

 and is called a neo-Y. Apparently this process has occurred in several 

 grasshoppers and mantids, as well as in other invertebrates. Pre- 

 sumably some readjustment of gene function takes place. The genes 

 in the autosome translocated to the X are now present in half their 

 previous number in the males. This is because heterochromatization 

 presumably results in their loss in the homologous autosome (the 

 neo-Y). Multiple sex-chromosome mechanisms (with several X's and 



Fig. 9.1 I ( see opposite page ) Chromosome behavior in two species of 

 Oenothera, a, seven bivalents in O. hookeri; h, ring of 14 in meiotic 

 prophase of O. biennis; c, ring of 14 in alternate disjunction at anaphase 

 I in O. biennis. {After Cleland, 1936, Bot. Rev. 2, and after Abrams, 

 1951, Illustrated Flora of the Pacific States, Stanford University Press.) 



