THE CYTOGENETICS OF HYBRIDS 367 



of homology as to conditions affecting the relative rate of development of 

 the various chromosomes in the prophase or to one of the modifying 

 influences mentioned at page 273. 



Amphidiploid Hybrids. — Primula floribunda (n = 9) was crossed 

 with P. verticillata {n = 9), producing a sterile Fi hybrid with 18 somatic 

 chromosomes. From this plant there arose a bud sport known as P. 

 Kewensis; this was found to be tetraploid (36 chromosomes) and fertile. 

 Since the tetraploid condition arose through some form of somatic 

 chromosome doubling in the Fi, P. Kewensis has 

 two florihunda and two verticillata chromosome 

 sets. It is said to be amphidiploid, since it is 

 diploid for each kind of set present. It is also 

 said to be allotetraploid, because it has four sets c^ 

 of nine which are in part unlike (see p. 340). 

 Such forms are called "sexual nuclear chimeras" 

 by Tschermak. In some such cases the original Fig. 209. — Somatic 

 parental sets do not have the same number of o^r-TaSdipS c"el!i-* 

 chromosomes (Fig. 209). For example, Nicotiana capHiaris x c. tectomm 

 glulinom (n = 12) X N. tabacum (« = 24) gave ^jts"to^* ::Srtr.lZ 

 in F2 the fertile amphidiploid N. digluta with 72 species. Satellites on tec- 



1 rpi • ,1 n J. u I, u -J torum D-chromosomes do 



chromosomes. This was the first such hybrid ^^^ g^ow at metaphase in 

 whose cytological constitution was understood, this hybrid. {After hoI- 

 Amphidiploidy is one of the most significant of ^^^^ ^^ ' 

 all cytological phenomena in hybrids because of its evident relation to 

 the origin of new fertile types. Several such fertile amphidiploids are 

 now known, ^^ among which the following are of much interest. 



Raphanus sativus (radish; n = 9) was crossed with Brassica oleracea 

 (cabbage; n = 9). In some of the sporocytes of the Fi hybrids the 18 

 chromosomes showed deficient synapsis and other irregularities, giving 

 various numbers in the spores (Fig. 210). In some of these were all 

 18 chromosomes, and from the union of such gametes there were obtained 

 F2 plants with 36. These amphidiploid plants, with two radish and two 

 cabbage sets, were fertile, and they differed in character from their 



^'^ Primula Kewensis (Digby, 1912; Newton and Pellew, 1929); Raphanus X 

 Brassica (Karpechenko, 1924a6, 1927o6; Karpechenko and Shchavinskaia, 1930); 

 Triticum X Mgilops (Tschermak and Bleier, 1926; Kagawa, 1928, 1929; Tschermak, 

 1929, 1930; Taylor and Leighty, 1931); Fragaria (Ichijima, 1926); Nicotiana (Clausen 

 and Groodspeed, 1925; Rybin, 1927, 1929); Rosa Wilsoni (Blackburn and Harrison, 

 19246); Brassica (Frandsen and Winge, 1932); Digitalis (Buxton and Newton, 1928; 

 Buxton and Darlington, 1931); Aquilegia (Skalinska, 1932); Spartina (Huskins, 1931c). 

 Amphidiploids which are largely or completely sterile are known in Nicotiana (Lam- 

 merts, 1931, 1932), Solanum (j0rgensen, 1928), Triticum X Secale (Lewitsky and 

 Benetzkaja, 1930, 1931), Crepis (Babcock and Nawaschin, 1930; Holhngshead, 1930a; 

 Poole, 1931, 1932), and Saxifraga (Marsden-Jones and Turrill, 1930). Winge (1932) 

 r^\'i:;ws most of the known cases. 



