370 



CHAPTER 40 



B 



FIGURE 40-1. The centromere and its granules 

 in corn. {Courtesy of A. Lima de Faria, ''Com- 

 pound Structure of the Kinetochore in Maize," 

 J. Hered., 49:299, 1958.) 



the DNA thread in the chromosome arms.) 

 The morphological similarity of centromere 

 and centrosome is paralleled by their simi- 

 larity in behavior. Centromeres are some- 

 times attracted to each other and to the cen- 

 trosomes. At anaphase, the centromeres 

 migrate toward the centrosomes. The centro- 

 some also has the capacity for movement, as 

 demonstrated by its preanaphase movement 

 and its behavior after a sperm has penetrated 

 an egg. 



From these facts some sort of kinship is 

 suggested ^ between centrosome and centro- 

 mere. This view is strikingly supported by 

 an additional piece of evidence. It has been 

 found,*' during the meiosis of certain mollusc 

 sperms, that certain chromosomes degener- 

 ate, as a result of which free, "naked" 

 centromeres are produced. These now-free 

 centromeres form a group with the centro- 



' Originally by C. D. Darlington, by F. Schrader, and 



by others. 



^ By A. W. Pollister and P. F. Pollister. 



some, and thereafter duplicate centrosomal 

 behavior and appearance exactly. In effect, 

 then, the freed centromeres become extra 

 centrosomes. The preceding evidences sug- 

 gest the hypothesis that the centromere and 

 centrosome represent alternative states of a 

 presently or previously existing episome. The 

 change from one episomal state to the other 

 would probably be influenced by the presence 

 of a nuclear membrane, by various other 

 genetic factors present in highly organized 

 cells, as well as by mutations which have 

 occurred in the episome when in its alternative 

 states. 



It is known that at the base of each cilium 

 or flagellum is a granular body, the kineto- 

 some, which is responsible for the motion of 

 the organelle. There is excellent evidence 

 that these kinetosomes are homologous to 

 centrioles (or centrosomes). This suggests 

 that kinetosomes are also episomes or episo- 

 mal derivatives. It has been suggested ' that 

 the episome F functions like a centromere. 

 Is it possible that the movements attributed 

 to F and the centromere and centrosome have 

 the same basis as the flagellar and ciliary 

 movement produced by kinetosomes? 



Let us speculate with regard to certain 

 matters which may not, at first, seem to 

 have any possible relation to episomes.^ 

 Heterochromatin (cf. p. 181) behaves cyto- 

 logically and physiologically (phenotypically) 

 less specifically than does euchromatin. 

 Moreover, because portions of heterochro- 

 matin can be added or subtracted from the 

 genotype without causing great phenotypic 

 modification, it is reasonable to believe that 

 the cistronic product of heterochromatin is 

 not only unspecific, but is duplicated in suc- 

 cessive heterochromatic segments. In view 

 of the likelihood that the cistrons of hetero- 

 chromatin produce an unspecific product, it 

 is hypothesized that each has a short nucleo- 



' Originally by R. C. Baumiller. 



« See I. H. Herskowitz (1961). The reading of the 



remainder of this Chapter is optional. 



