Changes Involving Unbroken Chromosomes 



153 



ogous chromosomes. However, this is not 

 always the case. If chromosome doubling — 

 naturally or artificially induced — occurs at 

 an early stage, a normal diploid (and homo- 

 zygous) embryo may be produced; for ex- 

 ample, chromosome doubling has produced 

 parthenogenetic salamanders and (female) 

 rabbits. In these instances, at least, abnor- 

 mal development as a haploid must have its 

 basis in quite a different factor — probably 

 one involving the surface-volume relation- 

 ships within the nucleus and between the 

 nucleus and the cytosome. These relation- 

 ships are changed when cells that are 

 adapted to be diploid are haploid. A sim- 

 ilar explanation can be offered for the ob- 

 servation that development of triploid and 

 tetraploid mouse zygotes ceases after a few 

 days, even though initially they have a nor- 

 mal mitotic rate. 



Ploidy changes also occur during gameto- 

 genesis and fertilization. These and certain 

 other examples of ploidy change already dis- 

 cussed are normal in various organisms. (A 

 ploidy change should be considered muta- 

 tional only when it is novel.) Autopoly- 

 ploidy can occur as a normal process in a 

 portion of a multicellular organism; for ex- 

 ample, it occurs normally in certain somatic 

 tissues in man such as liver cells. Many 

 of the examples of autopolyploidy mentioned 

 involve an increase in ploidy which is accom- 

 plished by endoreplication; that is, the ge- 

 nomic contents replicate and remain in one 

 nucleus. In these cases, the daughter chro- 

 mosome strands separate to produce an in- 

 creased number of separate chromosomes, 

 each chromosome in the nucleus proceed- 

 ing independently to mitotic metaphase. In 

 another consequence of endoreplication, all 

 daughter chromosome strands remain syn- 

 apsed, so the number of separate chromo- 

 somes is not increased. Let us consider an 

 example of this condition as found in the 

 giant salivary gland cells of Drosophila 

 larvae. 



2. Poly ne my 



Recall that the metaphase chromosome in 

 the usual cell of Drosophila is rod-shaped 

 (see Figure 7-5) and contains chromatids 

 each of which is coiled tightly in a series of 

 spirals like those in a lamp filament, and 

 that during interphase the chromatids un- 

 wind. The chromatids in the chromosomes 

 of the salivary gland cell nucleus are also in 

 an unwound state, perhaps even more so 

 than in ordinary interphase, and undergo 

 three special changes: 



1. Each chromosome present endorepli- 

 cates synchronously a number of times in 

 succession, so that one chromosome pro- 

 duces two, two produce four, four produce 

 eight, and so on. Endoreplication can occur 

 at least nine times, so each chromosome can 

 produce 512 daughters. 



2. All daughter strands, instead of sep- 

 arating, remain in contact with the homol- 

 ogous loci apposed, giving the appearance 

 of a many-threaded — polynemic or polytenic 

 — cable. 



3. The original members of a pair of ho- 

 mologous chromosomes are paired at homol- 

 ogous loci, demonstrating what is called 

 somatic synapsis. Accordingly, a double 

 cable is formed which can contain as many 

 as 1024 chromosomes. 



When seen under the microscope (Figures 

 1 1-4 through 6), these double cables have 

 a cross-banded appearance due to differ- 

 ences in density along the length of the un- 

 wound chromosomes. A band is formed by 

 the synapsis of the same dense regions in all 

 the strands; in this case, an interband region 

 is also formed by the synapsis of correspond- 

 ing regions of lesser density (Figure 11-5). 

 The pattern of bands is so constant and 

 characteristic that it is possible to identify 

 not only each chromosome but different re- 

 gions within a chromosome (Figure 11-6). 

 The giant size of salivary chromosomes, very 

 long because they are unwound and thick 



