Section 7 — Cytogenetics 



such duplications occur (there is some indepen- 

 dent genetic evidence for this) it is argued that 

 during meiosis in the hypoploid individual, the 

 chromosome arm that is in haplo condition 

 should synapse occasionally with segments of 

 other chromosomes representing duplications of 

 chromatin in the haplo arm. Crossing over in 

 such "illegitimately" paired regions should yield 

 gametes carrying reciprocal translocations; the 

 identification and analysis of interchanges origi- 

 nating in this way might be expected to reveal 

 the nature and extent of naturally occurring 

 duplications. 



In our scheme, hypoploid individuals among 

 the progeny of pollen parents carrying an A-B 

 translocation, are used as pollen (or egg) 

 parents in crosses with vigorous single-crosses. 

 The progeny of such crosses are grown on a large 

 scale and at harvest are searched for the occur- 

 rence of scatter-grain ears, an indication of 

 heterozygosity for a reciprocal translocation. 

 These are routinely analyzed to confirm the 

 presence of an aberration and to identify it 

 cytologically. 



Preliminary results based on an analysis of 

 progeny of TB-9a and TB-9b hypoploids indicate 

 the occurrence of reciprocal translocations which 

 appear to involve preferentially the haplo arm 

 of the hypoploid as one member of the inter- 

 change. On the other hand, the haplo arm is not 

 always involved in the identified interchange, but 

 even in these cases it appears that certain arms 

 may be involved more frequently than others. 

 It may be that the haplo arm in the hypoploid 

 plant leads to secondary "illegitimate" associa- 

 tions, or that chromosomes are more prone to 

 break in hypoploids and that in this respect some 

 regions are more unstable than others. Prelimi- 

 nary results indicate that certain hypoploids 

 yield higher frequencies of interchanged progeny 

 than do others. 



7.14. Inversion Polymorphisms in Teosinte. Y. C. 



Ting (Chestnut Hill, U.S.A.). 



During last few years, five varieties of Guate- 

 malan teosinte and eight varieties of Mexican 

 teosinte have been cytologically investigated. 

 Chromosome inversions were identified in most 

 of these varieties. In9 in the short arm of chromo- 

 some 9 was present in the Guatemalan varieties, 

 Florida, Jutiapa, El Valle and Lake retene; but 

 not in the variety Huixta of northern Guatemala. 

 Measurements at pachytene showed the length of 

 this inversion to be about 60 per cent of the 

 length of the short arm. Bridges and fragments 

 were found at both anaphase I and anaphase II 



of the microsporocyte divisions. Among Mexican 

 teosinte, the varieties, Chalco, Xochimilco, 

 Durango, Nobogame and perennial teosinte 

 were observed to have the same In9. In addition, 

 In8 was also present in these Mexican varieties; 

 but not in perennial teosinte. The average 

 length of this In8 at pachytene occupied about 

 50 per cent of the total length of the short arm 

 of chromosome 8. It was further found that the 

 variety Xochimilco had, in addition to In8 and 

 In9, an In3 in the long arm of chromosome 3. 

 This In3 being equivalent to about half of the 

 length of the long arm, manifested inter- 

 chromosomal effect on the frequency of crossing 

 over within the inverted segments of both ln8 

 and In9. The convincing evidence of this effect 

 was that at pachytene the frequency of loop for- 

 mations of both In8 and In9 was higher than this 

 in in plants having only In8 and In9 but not In3. 

 (Detailed report of this study will be published 

 in a monograph under preparation.) 



7.15. An Evolutionary Significance of a Pericentric 

 Inversion in a Barley Trisomic for Chro- 

 mosome 6. Takumi Tsuchiya (Yokohama 

 Japan). 



The seven primary trisomic types have been 

 produced in Hordeum spontaneum var. transcas- 

 picum. A new trisomic type has been found in 

 the progeny of Purple, the trisomic for chromo- 

 some 6. The extra chromosome of the new 

 trisomic type had two constrictions located at 

 subterminal positions of opposite sides. The 

 study of mitotic chromosomes showed that the 

 one of the two constrictions was the kinetochore 

 and the other the secondary constriction of the 

 chromosome 6. Thus the changed chromosome 

 was assumed to be derived from the chromosome 

 6. The length of the changed chromosome was 

 about 18 per cent shorter than that of the normal 

 chromosome 6. The tiivalent chromosomes 

 associated with the nucleolus at diakinesis. 

 Thereby it has been confirmed that the changed 

 extra chromosome might be derived from 

 chromosome 6. Meiotic behaviour was almost 

 normal resembling that of the primary Purple 

 trisomies. From the somatic chromosome mor- 

 phology and the meiotic behaviour it has been 

 assumed that the structural change in the new 

 Purple trisomic would be a pericentric inversion 

 accompanied by a deletion. The new Purple 

 trisomicd showed vigorous growth with inter- 

 mediate characteristics between normal diploid 

 and primary Purple trisomies and almost normal 

 fertility. Tetrasomic and some other chromoso- 

 mal types have been obtained in the sibling and/ 



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