Section 7 — Cytogenetics 



(6.0 : 1/8.0 : 1) and total (5.5 : 1/5.3 : 1). These 

 ratios lead to the following conclusions: 



(a) Transmission is almost the same through 

 male and female gametes when all seven 

 chromosomes are considered together. 



(b) In spite of differences in transmission these 

 seven chromosomes fall into five groups: 

 (1), I; (2), II and V; (3) III; (4) IV; and 

 (5), VI and VII. 



(c) Groups (1), (3) and (4) consist of single 

 chromosomes, and show almost the same 

 transmission through male and female 

 gametes. 



(d) In groups (2) and (5), each consists of two 

 chromosomes, and the transmission ratio 

 of one through the male gamete is the 

 same as that of the other through the 

 female gamete. 



From the above transmission data of rye 

 chromosomes it is tempting to infer that the rye 

 progenitor may have had 5 and not 7 pairs of 

 chromosomes and that the evolution of genera 

 and species in the Triticeae has been from an 

 ancestral type with 5 pairs of chromosomes: 

 undoubtedly through alterations in the physical 

 constitution of certain chromosomes only. 



7.19. The Nature of a Spontaneous Transfer of Hairy 

 Neck fiom Rye to Wheat. C. J. Driscoll and 

 E. R. Sears (Columbia, U.S.A.). 



A series of backcrosses transferred the rye 

 character "hairy neck" (pubescent peduncle) to 

 Chinese Spring wheat from the Cornell wheat 

 selection 82al-2-4-7, a derivative of a wheat-rye 

 hybrid. This dominant character exhibited mono- 

 genic inheritance. By means of monosomic 

 analysis it was located on chromosome 4A (IV). 



When heterozygous plants, with one entire 

 and one telocentric chromosome 4A, were selfed, 

 non-hairy offspring were obtained that possessed 

 an entire 4A. Since the hairy-neck gene was 

 carried by the entire chromosome in the parent 

 plant, this result demonstrated that the gene is 

 located on the arm homologous to the telocentric. 

 In the plants heterozygous for hairy neck and the 

 telocentric, pairing (to form a heteromorphic 

 bivalent) was observed in only about 30 per cent 

 of pollen mother cells. When unpaired, the two 

 chromosomes often exhibited co-orientation, 

 suggesting that synapsis had occurred but that 

 crossing over and chiasma formation had not 

 ensued. As regular pairing occurs between 

 normal 4A and this telocentric, it may be as- 

 sumed that the hairy-neck gene is located in a 

 rye segment of substantial length in which 

 crossing over with the telocentric does not occur. 



Heterozygotes with the heteromorphic bivalent 

 were backcrossed as males to Chinese for deter- 

 mination of the distance of the hairy-neck gene 

 (or rye segment) from the centromere. As desyn- 

 apsis leads to fewer functioning non-cross over 

 gametes, a correction factor, calculated as 

 0.56, will presumably need to be applied to the 

 obtained value. 



7.20. Cytogenetic Studies Bearing on the Nature of 

 the Centromere. L. M. Steinitz-Sears (Colum- 

 bia, U.S.A.). 



Telocentric chromosomes resulting from mis- 

 division were studied in hexaploid wheat to 

 determine whether their somatic instability might 

 be due to deficient centromere regions. Among 

 3134 offspring of monosomic 3B, which carried 

 the hemizygous-ineffective gene v (virescent) 

 as a marker of the short arm, 28 telocentrics and 

 7 isochromosomes for the short arm of 3B were 

 found. Eighty additional short-arm telocentrics 

 were presumably present in the virescent 

 seedling but were lost before meiosis. A few of 

 these plants, virescent as seedlings though green 

 and monosomic at the time of meiosis, had 41 

 normal plus 1 telocentric chromosomes in their 

 root tips. 



In order to prevent meiotic loss, a telocentric 

 for the long arm of chromosome 3B, which 

 carries a gene for synapsis, was combined with 

 the different telocentrics for the short arm. Thus 

 lines were established possessing 20 normal and 

 2 telocentric pairs, in which the very stable 

 pair for the long arm of 3B was identical but the 

 pairs for the short arm differed in origin. 

 Observations of leaf sectors suggest differences 

 in the degree of stability of the short-arm 

 telocentrics. 



In the meiosis of individuals with only one of 

 each of the telocentrics, varying amounts of 

 co-orientation were found at metaphase I. The 

 most stable short-arm telocentric was co-oriented 

 with the long-arm telocentric in approximately 

 half of the analysable cells. Co-orientation is 

 interpreted to be a consequence of an overlap of 

 centromeric regions of the two different stable 

 telocentrics. Centromeric regions are believed 

 to be complex and specific for each chromosome. 



7.21. Gene Evolution in Polyploid Wheat. E. R. Sears 

 (Columbia, U.S.A.). 



Where genes are duplicated, as in polyploids, 

 mutations to a different level of activity-even 



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