Regulation of Gene Action — Position Effect in Drosop/iila 



481 



that occurs within the A or B cistron of the 

 ill region of <£T4. The possibility exists, 

 however, that recombination does occur 

 within 5 as well as between functional gene 

 units in Drosophila, although mechanisms 

 other than crossing over may also be in- 

 volved. 



Consider the morphology of Drosophila 

 chromosome regions which contain pseudo- 

 alleles. The white series is associated with 

 a double band (doublet) in the salivary 

 gland chromosome; apr may be in one band, 

 w in the other. The vermilion series is as- 

 sociated with another doublet in the X chro- 

 mosome, whereas the bithorax series (com- 

 posed of five separate pseudoallele loci) is 

 connected with two doublets. (This last 

 fact demonstrates what is proved by other 

 data — that a band may contain more than 

 a single gene.) The great number of dou- 

 blets in salivary chromosomes suggests that 

 genes located in these regions are pseudo- 

 allele. 



The origin of adjacent loci with similar 

 types of action can be accounted for in sev- 

 eral ways. One explanation is that during 

 the course of evolution, adjacent genes pro- 

 ducing different effects mutated to alleles 

 which performed similar, presumably advan- 

 tageous, functions. A second explanation 

 might be that rearrangements brought to- 

 gether widely separated nonalleles with sim- 

 ilar functions. Though both of these ex- 

 planations may be sufficient for some of the 

 cases found, it seems more likely that most 

 adjacent and similar genes arose as duplica- 

 tions that occurred one or more times (as in 

 the bithorax case) in the ways described in 

 Chapter 12 (see also pp. 418-419). After 

 duplication linearly adjacent genes — origi- 

 nally identical — would have become some- 

 what different from each other functionally 

 by mutation. 



5 See W. J. Welshons and E. S. Von Halle ( 1962), 

 and A. Chovnick, A. Schalet. R. P. Kernaghan, 

 and M. Krauss (1964). 



What causes position effects? With re- 

 spect to gene action, genes that are linear 

 neighbors are perhaps more likely to be de- 

 pendent upon each other than upon their 

 alleles in a homologous chromosome (usu- 

 ally located a considerable distance away). 

 This kind of dependency might be subject 

 to position effect when the relative positions 

 of heterochromatin and euchromatin are 

 shifted by breakage. Position effects due 

 to structural changes might be particularly 

 common in species whose chromosomes or 

 chromosome parts take on special positions 

 in the nucleus relative to each other. Two 

 facts — that during nuclear division Dro- 

 sophila chromosomes show somatic syn- 

 apsis, and that somatic synapsis is found 

 in the giant interphase nuclei of salivary 

 gland and other cells — suggest that at the 

 time of gene action different chromosomes 

 and their parts are arranged so that the 

 products of gene action may be formed or 

 used in particular sequences. Due to the 

 presence of heterozygous reciprocal trans- 

 locations in Oenothera (Chapter 17), chro- 

 mosomal parts show a very orderly arrange- 

 ment in the circle of 14 chromosomes 

 formed during meiosis. Here also, a new 

 arrangement of chromosomal parts might 

 disturb functional sequences and produce 

 position effects. As a matter of fact, posi- 

 tion effect is known to occur in Oenothera. 



The molecular basis for position effects 

 in Drosophila is yet to be established. In 

 studying systems that control gene action 

 the results with bacteria and maize lead us 

 to hypothesize that position effects in Dro- 

 sophila can result if the production or the 

 translation of messenger RNA for a given 

 cistron is influenced by a change in its gene 

 neighbors. Since protein synthesis occurs 

 in the nucleus, the possibility also exists that 

 position effects can result from sequential 

 intranuclear reactions which are influenced 

 by the diffusion and, hence, the concentra- 

 tion of the protein products of gene action. 



