140 STRUCTURAL HYBRIDS 



of structural hybrid is that resulting from the union of gametes 

 which differ merely in one of them having two chromosomes 

 corresponding to two parts of one chromosome in the other. The 

 difference arises presumably from an unequal interchange followed 

 by loss of the smaller product, having the effect of fusion or followed 

 by reduplication of the smaller participant having the effect of 

 fragmentation (Fig. i6o). Such an organism may be described as a 

 fragmentation heterozygote and has been found in nature, and 

 produced by crossing different races, in Phragmatobia fuliginosa 

 (Seiler, 1925 ; cf. Ch. XIII). The hybrid has regular pairing at 

 metaphase of the two smaller chromosomes with ends of the large 

 one. As a rule they pass to the opposite pole from it at the first 

 division, but occasionally the smaller of the two fragments passes 

 to the same pole with the large chromosome. The association is 

 thus liable to the same kinds of variations of distribution resulting 

 from linear and convergent arrangement as are trivalents formed 

 from three identical chromosomes (Plate VI). 



The hybrid Victa sativa X V. amphicarpa (Sveshnikova, 1929, 

 a and h) behaves similarly ; two chromosomes of the first species 

 are evidently derived from one that is represented in the second. 

 They sometimes both pair at metaphase with the corresponding 

 parts of the larger homologue, and sometimes one of them 

 fails to pair. It is apparently too short to establish a chiasma 

 regularly. 



The pairing in a cross between the domesticated silkworm 

 (Bomhyx mori, n = 28) and its wild relative {B. mandarina, n = 27) 

 probably shows the same relationship, but one of the small chromo- 

 somes is sometimes paired by a lateral chiasma with the large one 

 (Kawaguchi, 1928). 



Fusion heterozygotes have been found by McClung (1905, 1917) 

 in Hesperotettix and Mermiria, and by Woolsey (1915) and Robertson 

 (1916) in Jamaicana subguttata. In the former a chromosome 

 which is found unpaired in the males (the sex chromosome) is fused 

 with one of the other chromosomes. This other chromosome pairs 

 normally with its mate, while the fused element, still attached, is as 

 usual unpaired (Fig. 46, c). 



A hybrid between species of Dicranura (Lepidoptera) with 21 and 



