Section 7 — Cytogenetics 



valents was found to be 0.30. Random pairing 

 of homologues was ascertained through the use 

 of plants having two homologues with long 

 satellites and two homologues with short 

 satellites. Quadrivalent formation was found to 

 be of a specialized type: only the long arm of 

 chromosome no. 2 participates. For example, 

 in a ring quadrivalent each long arm must have at 

 least two chiasmata and one partner exchange. 

 Due to the space requirements the proximal 

 chiasma always lies near the centromere. 

 Consequently, all genetic markers on chromo- 

 some no. 2 will segregate independently of the 

 centromere. 



The predicted frequency of recessive zygotes 

 is 1/29.8 for markers separated by one chiasma 

 from the centromere when the frequency of 

 quadrivalents is 0.3. 



7.4. A Cytogenetic Study of the Tumour-bearing 

 Hybrid Nicotiana glauca X N. langsdorffii. 



M. R. Ahuja (Philadelphia, U.S.A.). 



It is well known that the cross N. glauca X 

 N. langsdorffii gives 100 per cent tumors in the 

 Fi progeny. The triploid and the tetraploid 

 combinations also yield an entirely tumorous 

 progeny. Kehr and Smith (Brookhaven Symp. 

 Biol. 6, 55-78, 1954) and Smith and Stevenson 

 (Z. Vererb. 92, 100-118, 1961) have shown that 

 tumor formation depends on the interaction of 

 many genes in the hybrid. Furthermore they 

 found that with one or several glauca chromo- 

 somes on diploid langsdorffii, tumors do not 

 form. On a different hybrid between An N. deb- 

 neyi tabacum x N. longiflora (Ahuja, Genetics, 

 47, 865-880, 1962) tumor production was found 

 to be a function of an alien longiflora chromosome 

 or a fragment of it on the debneyi-tabacum 

 background. Since Kehr and Smith did not 

 examine the cytology in the glauca-langsdorffii 

 hybrid combination it was considered worthwhile 

 to check the cytology in the triploid generation 

 (having two langsdorffii and one glauca genomes). 

 A study of meiosis revealed that in 15 plants not 

 all plants in the triploid generation carried a full 

 complement of 30 chromosomes (9n + 12i). 

 Three plants were found with 29 chromosomes 

 (9n + Hi). Somatic elimination of a chromo- 

 some is ruled out as buds on different branches of 

 the same plant had the same chromosome 

 number. All the three deficient plants are tumor- 

 ous indicating that tumor production in this 

 hybrid may not be function of the complete 

 genomes of the parental species. This opens 

 the way to a more complete analysis of the role 

 of chromosomes in tumor forming process. 



This work was supported by Grant DRG 622 

 from the Damon Runyon Memorial Fund for 

 Cancer Research. 



7.5. A Pseudo-supernumerary Chromosomein Col- 

 linsia heterophylla. E. D. Garber (Chicago, 

 U.S.A.). 



Progeny of plants of C. heterophylla (n = l) 

 but not of six other species in the genus treated 

 with colchicine included individuals either with 

 an extra chromosome (trisomy) or with a hetero- 

 zygous reciprocal translocation. Thirteen inde- 

 pendently obtained trisomies had the same extra 

 chromosome which behaved cytogenetically as 

 did supernumerary chromosomes in two other 

 species. Primary trisomes from an autotriploid 

 plant of C. heterophylla, however, were orthodox 

 in their cytogenetic behavior. Although a trisome 

 for one of the seven chromosomes in this species 

 simulates a supernumerary chromosome, this 

 chromosome cannot be lost (monosomy) from 

 the complement. The pseudo-supernumerary 

 chromosome presumably responds to colchicine 

 by undergoing non-disjunction during mitosis 

 and may be responsible for the unexpected 

 chromosome breakage induced in this species by 

 colchicine. 



Research supported by the National Science 

 Foundation. 



7.6. Genetics of B-chromosomes. A. Rutishauser 

 (Zurich, Switserland). 



Most accessory or B-chromosomes of higher 

 plants show some kind of boosting-mechanisms : 

 the B-chromosomes are preferentially included 

 in the generative or sperm nucleus (directed non- 

 disjunction in pollen grain-mitosis) or in the 

 functioning megaspore (preferential segregation). 

 In Crepis capillaris, where plants with one B- 

 chromosome rarely occur in nature, a new type 

 of boosting-mechanism has been found: the 

 number of B's is doubled in the inflorescences. 



The B-chromosomes of Crepis capillaris are 

 by no means genetically inactive. Although 

 partly heterochromatic they exert a variety of 

 effects on the carrier plants. The flowering time 

 is retarded, the shape of inflorescences changed, 

 the pollen fertility lowered and, depending on 

 the number of B's, seed setting reduced to \ 

 (IB) or 1/4 (2B's). Even more striking is their 

 effect upon bivalents of the A-chromosomes: 



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