Section 7 — Cytogenetics 



the number of chromatid-bridges accompanied 

 by acentric fragments of first divisions of 

 PMC's, and accordingly the number of tetrads 

 with micronuclei and persisting bridges, rises 

 considerably if B-chromosomes are present. 

 In 2B-plants B-chromosomes themselves form 

 bridges. 



From these findings it may be inferred that 

 B-chromosomes of C. capillaris enhance vari- 

 ability and therefore may well have an evolution- 

 ary effect. 



markers in linkage groups II, III, and VI has 

 given genetic evidence of nuclear passage ; coinci- 

 dent second division segregation for all markers 

 is observed in one type of ascus. No such coinci- 

 dent segregations were found in other types of 

 asci from this cross nor in asci from control 

 crosses. 



* Deceased. 



7.7.Frequency and Geographical Distribution of 

 Rye (Secale cereale) with Accessory Chromo- 

 somes in Korea. Woong-Jik Lee (Seoul, Korea). 



Frequency of accessory chromosomes in po- 

 pulation of rye from Korea was known to be 

 exceedingly high. Extensive present work for 

 frequency and geographical distribution of rye 

 with accessory chromosomes in Korea has been 

 carried out and the result has revealed that the 

 frequency is ranged from 39 to 53 per cent. This 

 figure, smaller than the result in previous report, 

 is still higher than any other region in the world. 

 Correlation between frequency of accessory 

 chromosomes and edaphic factor (type and 

 pH of soil) will be discussed. 



7.8. The Cytogenetic Effects of a Paracentric Inversion 

 in Neurospora crassa. Patricia St. Lawrence 

 and Jesse R. Singleton* (Berkeley and La- 

 fayette, U.S.A.). 



The mutant SI 325 carries a paracentric 

 inversion which is inseparable from a niacin 

 requirement and involves about 70 per cent of 

 the known genetic length of the right arm of 

 linkage group I. Recombination between the 

 nic-2 and lys-3 loci is much reduced in viable 

 progeny from crosses of SI 325 to normal stocks. 

 In immature asci from such crosses a conspicuous 

 bridge and fragment is seen at anaphase I. 

 Persistence of the bridge is occasionally 

 observed leading to fusion of the daughter 

 nuclei or to aberrant chromosome segregations 

 in subsequent divisions. Migration of the nuclei 

 is irregular and non-identical sister spores may 

 be formed. It has not been possible to determine 

 whether a breakage-fusion-bridge cycle is 

 operative. At maturity most of the asci are 

 completely abortive; the major class of asci 

 with ripe spores has one ripe pair in each half of 

 the ascus. Analysis of isolates from asci of 

 crosses of SI 325 to a stock carrying centromere 



7.9. Polyploidy and the Intel change-heterozygote. 



Keith Jones (Kew, Great Britain). 



The nature of chromosome pairing and its 

 genetic consequences are well understood in 

 diploid interchange-heterozygotes. Permanent 

 hybrids of the Oenothera type have maintained 

 heterozygosity by control of chromosome 

 segregation and the elimination of homozygotes. 

 Polyploidy is also an efficient means of maintain- 

 ing heterozygosity and its role in the evolution 

 of Anthoxanthum odoratum (2w = 20) is discussed. 



7.10. Genically Controlled Variability of Chromosome 

 Number in Pennisetum Hybrids. P. Gilden- 

 huys and K. Brix (Pietermaritzburg, South 

 Africa). 



Crosses between diploid Pennisetum typhoides 

 (2« = 14) and tetraploid P. purpureum (2« = 28) 

 yield sterile triploids (2«=21) with regular 

 mitotic behaviour. The colchicine induced 

 hexaploids (amphidiploids) are fertile but show 

 instability in somatic chromosome number, even 

 in cells of the same root, as do the C2 plants 

 (2» = 36 to 49 with 2«=42 occurring most 

 frequently). These hexaploids produce pollen 

 grains which contain from 12 to 21 chromo- 

 somes. When back-crossed to P. typhoides the 

 resultant progenies, as expected, have 2m = 21 to 

 2« = 28 chromosomes, but, in addition, the 

 crosses also yield offspring with chromosome 

 numbers in excess of 2n = 28. These arise from 

 unreduced egg cells in P. typhoides. 



Whilst CI and C2 plants all show similar 

 ranges of intraplant variability in somatic 

 chrosome number, this is not so in the back- 

 crosses, where the range of variation is highly 

 significantly different from plant to plant. For 

 example, in one plant numbers range from 

 18 to 29 (28 occurring most frequently), in 

 another from 32 to 38 (35 most frequent), where- 

 as in another plant the range is only 34 to 36 



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