468 Allopolyploids 



hybrid indicates that the genomes of the two parental species 

 could be designated AA BB CiCi LiLi for B. carinatus and AA 

 BB CiCi C2C2 DD EE for B. arizonicus. The Li sets from the 

 first species and the D and E sets of the second form the uni- 

 valents. The 14 bivalents result from pairing between the two 

 A sets and the two B sets, whereas pairing among the two Ci 

 sets and the one C2 set is the cause -of the 7 trivalents. 



One of the classi(;al examples of higher allopolyploidy is the 

 genus Rosa. Hurst, Blackburn, and others have shown that a 

 whole series of such polyploids exists. The basic chromosome 

 number in this genus is 7, and there are five diploid species each 

 of which may be considered a basic type from which all other 

 species and varieties have been derived. Some of the other types 

 are autotetraploids, hexaploids, and octoploids. There are, how- 

 ever, many varieties in which only some of the chromosomes pair 

 and the others are univalents. 



Secondary Association 



Normal chromosome pairing is the result of homologous seg- 

 ments in chromosomes that are partially or wholly homologous. 

 The chiasmata that arise in such paired segments hold the paired 



chromosomes together from zygotene 



• #f # •'•^ ^^ ^^^^ anaphase, and if two chro- 



*• % * #4 ^*lf • ^ • # mosomes do not pair and form chias- 



#* M^\ • • J • #* S mata during early prophase, they 



• ^* ••H* ^^^^ ^°^ ^^ joined in the same con- 



T^ .„^ „ , . - figuration at metaphase. This nor- 



FiG. 136. Polar views of ^ ... 



the first metaphase of Dahlia mal chromosome pairmg is some- 



variabilis {2n = 64) showing times referred to as primary associ- 



primary (multivalent) and ation to distinguish it from another 



secondary association of the ^^^^ ^^ chromosomal association 



chromosomes. (Redrawn from , . , . , , , , 



Lawrence in the Journal of ^'^^^^ ^"^es at prometaphase and 

 Genetics.) is called secondary association. If 



secondary association occurs, two 

 or more bivalents may lie close together on the metaphase 

 plate (Fig. 136). This type of association is believed to 

 indicate a remote homology between the bivalents. This ho- 

 mology is not sufficiently close to bring about normal chromo- 

 some pairing, but it is close enough to cause the slightly homolo- 

 gous chromosomes to be side by side. Secondary association has 



