THE CYTOGENETICS OF HYBRIDS 363 



and 42 chromosomes (hexaploid), there are regularly seven bivalents in 

 the microsporocytes, the other chromosomes (14, 21, and 28 in the three 

 types) remaining unpaired. Evidently in each case a set of seven 

 descended from one gamete pairs with a set of seven from the other 

 gamete, leaving the extra sets unpaired. Both types of synapsis must 

 occur in hexaploid Papaver hybrids with 42 chromosomes formed by 

 crossing P. nudicaule (7 in gamete) with P. striatocarpum (35 in gamete). 

 These hybrids show 21 bivalents in meiosis, indicating a synapsis of all 

 six sets irrespective of gametic derivation. On the theory that synapsis 

 is determined mainly by homology, the nudicaule set of seven must be 



u' 1^^ (Jar/ V ;^: 



ABC B 



Fig. 208. — The chromosomes of Triticum. A, metaphase of division / in T. mono- 

 coccum; 7 chromosomes. B, T. durum; 14 chromosomes. C, T. vulgare; 21 chromosomes. 

 D, division / in hybrid between members of emmer group (n = 14) and vulgare group 

 (n = 21); the 14 bivalents have disjoined and the 7 univalents are separating equationally. 

 (After Sax, 1922a.) 



homologous with at least one set in striatocarpum, and the four other 

 sets of the latter species must be of but one or two kinds. In certain 

 Digitalis hybrids with 72 chromosomes (48 lutea + 24 micraniha) , 36 

 bivalents appear at diakinesis, indicating the occurrence of autosynapsis.'' 

 The behavior of the chromosomes in meiosis when both bivalents and 

 univalents are present varies considerably in different cases. The 

 bivalents rather regularly disjoin in / and divide in II, but the univalents 

 behave more irregularly. In mitosis I they may wander at random as 

 wholes to the two poles {Drosera and some Triticum hybrids) ; or they may 

 split and occupy the equator, the daughter halves then separating pole- 

 ward (some Triticum hybrids and microsporocytes of Caninoc roses; 

 Figs. 207, 208); or some of them may distribute as wholes while the 

 others split and separate. This variable behavior seems to be associated 

 with the time at which the division of the univalent chromosome, partic- 

 ularly its spindle-attachment region, takes place. If the division is 

 sufficiently early, the univalent reacts by entering the equator and 

 separating; whereas, if the division is delayed, it fails to behave in such a 

 manner and tends to be carried toward the pole nearer which it happens 



^ Collins and Mann (1923) on Crepis; Tackholm (1920, 1922) on Rosa; Ljungdahl 

 (1924) on Papaver; Haase-Bessell (1921, 1932a) on Digitalis. 



