Meiosis and Chromosomal Segregation 



23 



will contain different paternal segments along 

 their lengths, and the monads with the pa- 

 ternal centromeres will have the comple- 

 mentary maternal sections along their lengths. 

 Accordingly, segregation of maternally-de- 

 rived chromatid material from paternally- 

 derived chromatid material occurs for cen- 

 tromeric and some other regions of the chro- 

 matids at anaphase I and is accomplished 

 for the remaining regions of the chromatids 

 at anaphase II. Since bivalents line up at 

 metaphase I independently, meiosis will nor- 

 mally result in the segregation of homologous 

 chromosomal regions and independent segre- 

 gation of chromosomal segments located on 

 nonhomologous chromosomes. 



Life Cycles of Multicellular Organisms 



Since the diploid number of chromosomes is 

 maintained generation after generation in 

 sexually reproducing forms, it is not sur- 

 prising that meiosis always occurs at some 

 time in the life cycle of each sexually repro- 

 ducing individual. In most animals meiosis 

 comprises the last two nuclear divisions be- 

 fore the mature sperm or egg is produced. 

 Meiosis occurs at different times in the life 

 history of different plants, but rarely just 

 before the formation of gametes. Different 

 species show minor variations in the details 

 with which meiosis is carried out. Let us 

 consider the life cycles of certain multi- 

 cellular organisms which have proven to be 

 especially interesting both for cytological and 

 genetic investigations — that is, in cytoge- 

 netics. 



1 . Drosophila melanogaster 3 



The adult stage of D. melanogaster, com- 

 monly called the fruit, banana, or vinegar 

 fly, is shown in Figure 2-6. Although size 

 depends upon nutritional and other environ- 



; See the Appendix to this chapter, on p. 29. for 

 references dealing with this and other species of 

 Drosophila. 



^^5 



I I I I 1 I I II | 



-lnrr: - 



& ; . 



i>-<vt"^- 



lip 



figure 2-6. Normal (wild-type) Drosophila 

 melanogaster male (A) and female (B). 

 (Drawn by E. M. Wallace.) 



mental factors, the adult is usually 2-3 mm. 

 long, and females are larger than males 

 raised under comparable conditions. As 

 found in nature, the wild-type fly has a grey 

 body color and dull-red compound eyes. 

 Males are readily distinguished from females 

 by having sex combs on the anterior pair of 

 legs and an abdomen which terminates dor- 

 sally in a single broad black band (instead 

 of a series of bands) and which ends ven- 

 trally in a penis and claspers (instead of an 

 ovipositor). 



