198 



FUNDAMENTALS OF CYTOLOOY 



somes and breed true. They differ, of course, in the linkage relations 

 of certain characters controlled by genes in the chromosomes involved 

 in the translocation. In Drosophila the "modifieds" are for some reason 

 sterile. Modified lines with all the chromosomes of the genome altered 

 by translocations have been established in Crepis tectorum by crossing 

 strains carrying different translocations, and these new lines are kept 

 distinct by the sterility of hybrids formed between them and the original 

 standard line. 



A second reciprocal translocation may follow a first in the same line, 

 or two translocations may be brought together from different lines. In 



Fig. 146. — Diagram illustrating formation of ring-of-6 by two reciprocal translocations. 

 In line A, chromosomes 1 and 2 are translocated. In line B, chromosomes 2 and 3 are 

 translocated. Union of gametes from these two lines may give a group of six chromosomes 

 forming a double-cross configuration at synapsis and a ring-of-6 at diakinesis. If the two 

 translocations had occurred at corresponding levels in the chromosomes, the synaptic com- 

 plex would have been a six-rayed star. Chromcsomes are marked with knobs to di.s- 

 tinguish ends. 



either manner, rings of more than four chromosomes may be built up 

 (Fig. 146). In extreme cases all the chromosomes of the complement are 

 combined in one large ring or chain in the meiotic prophase (Fig. 147, h). 

 Fertility in such plants tends to vary with the percentage of alternate 

 chromosome distribution in anaphase /. 



Reciprocal translocation and its effects have been found in a con- 

 siderable number of plant genera in nature. In a species of peony 

 (Paeonia calif ornica), which has 10 somatic chromosomes, there have 

 been found seven types showing respectively in sporocytes five inde- 

 pendent bivalent pairs, a ring-of-4 and 3 pairs, a ring-of-6 and 2 pairs, 

 a ring-of-8 and 1 pair, a ring-of-10, 2 rings-of-4 and 1 pair, and a ring-of-4 

 with a ring-of-6. Near the center of the range of the species in California 

 the types with free pairs and small rings are more abundant, whereas 



