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INTRODUCTION TO CYTOLOGY 



semisteriles, and that when standards and modifieds are crossed only 

 semisteriles result. When selfed, standards and modifieds give only 

 their own kind. All of these findings are consistent with the conclusion 

 that sterility here is due to abnormal assortments of chromosomal 

 materials. 



In addition to semisterile-2, described above, there are several other 

 instructive semisterile races in Zea. Semisterile-4 has a reciprocal 

 translocation involving chromosome II {B-lg linkage group) and chromo- 

 some V {Pr-Vi) (Rhoades, 19316). Semisterile-1 involves chromosomes I 

 and II and accordingly the P-hr and B-lg linkage groups (Brink and 

 Cooper, 1931), while semisterile-3 involves I and VII. When two such 



I 







N 



Fig. 188. — Diagram of reciprocal translocation in "semisterile-2" strain of Zea. a, 

 normal chromosomes IX (diagonal shading) and VIII (solid black). The clear portions 

 represent spindle-attachment regions. Arrows indicate location of interchange to produce 

 condition in h. c, synaptic complex in mid-prophase of meiosis after crossing a normal 

 plant with one carrying the interchange, d, the ring at diakinesis formed by opening out of 

 synaptic complex. N, larger normal chromosome (VIII) ; n, smaller normal chromosome 

 (IX) ; I, larger interchange chromosome (IX-VIII) ; i, smaller interchange chromosome 

 (VIII-IX). Cf. Figs. 187 and 189. (After McClintock, 1931a.) 



races having interchanges involving one chromosome in common are 

 crossed, a race showing a ring of six at meiosis may result (Fig. 190). 

 This occurs in crosses between semisteriles- 1 and -5 (Brink and Cooper, 

 1932) and between semisteriles-1 and -3. The members of the ring of six 

 are variously distributed in the meiotic divisions, with the result that 

 about 75 per cent of the pollen aborts. It may be calculated further that 

 with a ring of eight or a ring of ten the sterility would be still greater. 

 The useful term catenation has been suggested for such chain formation 

 by chromosomes (Gates, 19316). 



It is an interesting fact that in some cases in Datura and in CEnothera 

 the distribution of the catenated chromosomes is rather regularly alter- 

 nate, giving a high degree of fertility, and not so often adjacent as in 

 the semisterile strains of Zea mentioned above. In semisterile-2 the 



