Genetic Systems II 187 



These supernumerary chromosomes in plants commonly are called 

 B chromosomes, and they are surprisingly frequent. In general, the 

 B chromosomes are smaller than the others, and they pair only 

 among themselves. In most instances they are heterochromatic. They 

 vary in number among individuals, they may be in odd or even 

 numbers or absent, and their presence usually cannot be detected 

 in the phenotype of the plant. B chromosomes may have physio- 

 logical eflFects, however, the evolutionary importance of which is 

 unknown. In Clarkia, supernumeraries probably have arisen as the 

 result of structural changes in the chromosomes. These plants often 

 have ring or chain arrangements of chromosomes in translocation 

 heterozygotes, and unequal separation may lead to the formation of 

 trisomies. For some reason, in this genus extra chromosomes do not 

 disturb the genetic balance or reproduction. 



Supernumerary chromosomes are found among many inverte- 

 brates. They appear to be largely heterochromatic. Some are mitot- 

 ically stable; others show nondisjunction. Perhaps related to these 

 supernumeraries are the so-called "limited" chromosomes of some 

 Diptera. These chromosomes are limited to the germ line, v/here 

 they often show bizarre cytological behavior. The fungus gnat Sciara 

 copropliila, for example, has seven chromosomes in somatic cells of 

 males and eight in somatic cells of females (Fig. 9.4). Cells in the 

 germ line that will produce gametes contain 10 (sometimes 9 or 11 ) 

 chromosomes; this group includes the four pairs of somatic cells plus 

 a pair of large chromosomes never found in cells other than the 

 germ line. These are the limited (L) chromosomes. In meiosis female 

 flies produce eggs with a full haploid set including an L. Meiosis in 

 males is unusual, in that synapsis does not occur and at the first 

 division a unipolar spindle is formed. The L's and one of each of 

 the other chromosomes proceed to the pole of the spindle, while 

 the others (which genetic evidence shows are all paternal chro- 

 mosomes) are eliminated as they go in the opposite direction and 

 are cut off in a small bud. During the second division a bipolar 

 spindle forms and the chromosomes behave normally except for the 

 X, which divides but sends both daughters to the same pole. Only 

 the spermatid that receives the X's matures; therefore only one 

 sperm is formed at each meiosis. It contains 2 L's, 2 X's, and one of 

 each autosome; except for the L's, these chromosomes come origi- 

 nally from the maternal parent. 



After fertilization, which results in a zygote with three pairs of 

 autosomes, 3 X's, and (usually) 3 L's, a series of cleavage divisions 

 takes place in which the L's and X's are selectively eliminated 

 from cells. Eventually the chromosome numbers and distribution 



