366 INTRODUCTION TO CYTOLOGY 



suggest that the gametic complements of certain species are made up as 

 follows: cernua, AB; chinensis, A; Rapa, A; Napella, AC. The hybrids 

 tend to resemble the parent with the higher chromosome number. 



Very extensive studies of this kind have been made upon Triticum 

 and the closely related genus Mgilops.^ In Triticum the three main 

 groups of species are characterized by different chromosome numbers. 

 In the einkorn wheats {monococcum, villosum, cegilopoides) the gametic 

 number is 7; in the emmer wheats {dicoccum, polonicum, turgidum, 

 durum) the gametic number is 14; in the spelt wheats (vidgare, compactum, 

 spelta) the gametic number is 21. In many of the hybrids between 

 members of these groups, as well as in those between Triticum and 

 ^gilops, the chromosomes tend very strongly to form a characteristic 

 number of bivalents and univalents, as in Drosera and other cases men- 

 tioned on foregoing pages. There is, however, some variation in behavior 

 under different conditions. 



On the basis of the synaptic relations exhibited, the theory has been 

 developed'' that the chromosome complements of Triticum and /Egilops 

 combine in various ways as many as five different but related sets of 7 

 (A, B, C, D, E). In Thompson's recent tabulation these sets are pres- 

 ent as follows in the gametic complements: einkorns. A; emmers, AB; 

 spelts, ABC; /Egilops cylindrica, CD; M. ovata, DE; /E. crassa, C. Other 

 species of /Egilops have less certain combinations, and Secale (rye) has Z). 



Studies on the morphology of Triticum chromosomes suggest that the 

 species with the larger numbers of sets are not simply autopolyploid 

 derivatives of some ancestor with two similar somatic sets, for in four 

 species about 17 different types of chromosome are found (Kagawa). 

 Some types appear in only one of the species, while others appear in two, 

 three, or four. This suggests hybridization in the differentiation of 

 such species. On the other hand, the chromosomes in certain crosses 

 may undergo synapsis in spite of their differences in type, which points to 

 translocation and deletions as probable causes of some of the cytological 

 diversities in the members of this genus. That synaptic behavior is a 

 reliable measure of relationship in grains is questioned by Bleier, who 

 lays emphasis on the probable role of the spindle substance in determining 

 irregular meiosis in hybrids (1930c, 1931a). Furthermore, it is to be 

 borne in mind that failure to pair may often be due not so much to lack 



8 Sax (1922 et seq.), Watkins (1924 et seq.), Kihara (1919 et seq.), Gaines and Aase 

 (192G), Kagawa (1927afe, 1929ac), Bleier (1928a6, I930acd, I93ld), W. P. Thompson 

 (1926 et seq.), Melburn and Thompson (1927), Huskins (1928a6), Jenkins (1929), 

 Percival (1930), Tschermak and Bleier (1926), Tschermak (1929), Kihara and Nishi- 

 yama (1928, 1930a), Kihara and Katayama (1931). For convenient general accounts, 

 see Aase (1930), Watkins (1930), and Thompson (19316). See also footnote 13, p. 342. 



9 Gaines and Aase (1926), Sax (1928), Kihara (1928), Thompson (19316), Kihara 

 and LiUenfeld (1932). 



