

RECIPROCAL TRANSLOCATION 



PARACENTRIC DELETIONS 



figure 17-14. Formation of two rod-shaped 

 chromosomes from a V-shaped chromosome 

 and a Y chromosome. 



division and the number of genes lost is 

 small enough, the absence of these parts 

 may be tolerated physiologically by the or- 

 ganism. 



The reverse process, the formation of two 

 rods from a V, necessitates the contribu- 

 tion of a centromere from some other chro- 

 mosome. In Drosophila, this second chro- 

 mosome may be the Y (Figure 17-14). 

 Suppose the V is broken near its centromere 

 and the Y is broken anywhere. Should a 

 eucentric reciprocal translocation follow, two 

 chromosomes would be produced, each hav- 

 ing one arm derived predominantly from the 

 Y. If subsequent paracentric deletions oc- 

 cur in these Y-containing arms, rod shapes 

 will result, thereby completing the change 

 from a V to two rods. Note that almost 

 every part but the centromere of the Y chro- 



CHAPTER I 7 



mosome is eventually lost in this process. 

 But this loss may have little or do disadvan- 

 tage to the Drosophila. since the Y carries 

 relatively few loci and is primarily concerned 

 with sperm motility. For example, this 

 series of mutations may be initiated in the 

 male germ line, producing two chromosomes 

 — each containing part of the Y. Deletion 

 of Y parts can occur without detriment if 

 these chromosomes happen to enter the fe- 

 male germ line; they may stay in the male 

 germ line provided that a regular Y chro- 

 mosome is included in the genotype in due 

 time. The small IV chromosome in melano- 

 gaster, whose monosomy is tolerated in either 

 sex, may also contribute a centromere in 

 the process of changing a V to two rods by 

 an identical or similar series of mutational 

 events. 



Karyotype comparisons of Drosophila 

 confirm the expectation (Chapter 12), that 

 whole arm translocations are able to survive 

 in natural populations. Such rearrange- 

 ments and pericentric inversions are ex- 

 tremely useful in helping us establish evolu- 

 tionary relationships among different species. 

 But it should be emphasized that this kind 

 of information by itself does not prove that: 



1 . The formation of different species is a 

 primary consequence of the occurrence 

 of these rearrangements 



2. These rearrangements are of secondary 

 importance in species formation 



3. These mutational events occur after 

 species formation is complete. 



As exemplified by Oenothera and Dro- 

 sophila, we have seen that gross chromo- 

 somal rearrangements of various types have 

 persisted in the evolutionary course of differ- 

 ent groups of organisms. For this reason 

 it would perhaps be wise at this point to 

 retrain from predicting — except generally as 

 in Chapter 12 — which, if any, structural 

 changes might be associated with the evolu- 

 tion of other particular groups of organisms. 



