222 



FUNDAMENTALS OF CYTOLOGY 



One of the earliest known amphidiploid hybrids appearing in culture 

 was the fertile Primula kewensis, with 36 somatic chromosomes. A 

 cross between P. florihunda (2n =18) and P. verticillata (2n = 18) had 

 yielded the sterile diploid P. kewensis (2n = 18) with one genome from 

 each parent species. From a lateral bud on this plant there arose spon- 

 taneously a tetraploid shoot with two genomes from each pai'ent, and this 

 proved to be fertile. The numerical changes may be represented as 

 follows: (9 + 9) X 2 = 36. 



An example of doubling which evidently occurred after sj^ngamy is 

 afforded by a fairly fertile amphidiploid columbine that appeared after a 

 cross of the two diploid species Aquilegia chrysantha and A. flahellafa nana: 

 (7 + 7) X 2 = 28. A general formula for such 

 cases would be (n + 7i)2 = 4n. Among com- 

 mercially important plants evidently ha\'ing such 

 a constitution are the pink-flowered ornamental 

 tree, Aesculus carnea, and tobacco, Nicotiana 

 fahacum. The former arose in cultivation as a 

 hybrid between the horse chestnut, .4. hippo- 

 castanum, and A. pavia: (20 + 20) X 2 = 80. 

 The latter has been shown by a long series of 

 studies to represent in all probability an am- 

 phidiploid hybrid derived from a cross of N. 

 sylvcstris and another species which now appears 

 to have been A^. otophora: (12 -^ 12) X 2 = 48. 



Some plants spoken of as amphidiploids have 

 arisen from crosses of species differing in chromo- 

 some number. The chromosome complement of 

 such an amphidiploid Crepis plant is shown in 

 Fig. 165. Nicotiana " digluta'' arose ivom a, cross 

 of N. gliitinosa (24 somatic chromosomes) and N. tahacum (48 somatic 

 chromosomes): (12 + 24) X 2 = 72. If, as indicated in the preceding 

 paragraph, A^. tahacum is tetraploid with 2 genomes from each of 

 two species, N. ^'digluta" in terms of the basic number for the 

 genus, 12, would be allohexaploid with 4 genomes from one species 

 and 2 from the other. The general formula for such cases would 

 be (n + 2n)2 = 6n. Whether such plants are regarded as amphi- 

 diploid because they carry the combined somatic complements of the 

 parents or as allohexaploid because of their number of basic genomes, 

 they have the degree of fertility exhibited by other hybrids in which 

 each genome has a duplicate with which to pair at meiosis. Further 

 examples of this type of hybrid are the domestic plum, (8 -f 16) X 2 

 = 48; certain hyln-id mints, (48 -|- 12) X 2 = 120; and certain hybrid 



Fig. 165. — Somatic 

 chromosome complement 

 in an amphidiploid hybrid 

 between Crepis capillaris 

 and C tectorum. It com- 

 prises two genomes of four 

 members each from tec- 

 torum (7') and two gen- 

 omes of three members 

 each from capillaris (C). 

 Satellites on tectorum D- 

 chromosomes do not show 

 at metaphase in this hy- 

 brid. {After L. Hollings- 

 head.) 



