Section 7 — Cytogenetics 



(35 most frequent) whilst in yet another the 

 number is constant at 2« = 35. It is suggested 

 that intraplant aneuploidy, known to exist in 

 other intergeneric and interspecific hybrids, is 

 in this case under genetic control, probably 

 with similar gene action to that which we have 

 demonstrated in the amphidiploid of the cross 

 between P. typhoides and the mitotically unstable 

 P. dubium. 



To be published in the South African Journal 

 of Agricultural Science. 



7.11. Colchicine-induced Somatic Chromosome Re- 

 duction in Sorghum. J. G. Ross, C. H. Chen 

 and G. M. Simantel (Brookings, U.S.A.). 



As a result of colchicine treatment of diploid 

 sorghum seedlings of certain genotypes, true- 

 breeding and segregating diploid mutants have 

 been observed. A laboratory method by which 

 light, temperature and humidity are controlled 

 was developed to ensure the appearance of the 

 phenomenon in "colchicine reactive" genotypes. 

 To explain the appearance of true breeding 

 mutants a mutational effect was postulated in 

 conjunction with somatic chromosome reduction 

 followed by restoration to the original chromo- 

 some number. This hypothesis was tested by 

 treating tetraploid seedlings of "colchicine 

 reactive" genotypes. Out of 80 treated seedlings 

 there were 9 diploids (one a chimera of diploid 

 and tetraploid tissue) among the 17 survivors, 

 while no diploids occurred among the 118 

 control tetraploid plants. Histological study 

 indicated that diploid cells occurred at the base 

 of the apical dome 2 to 3 days after colchicine 

 application and that these may proliferate to 

 take over the growing point at the 5th to 6th day. 

 As an additional test, 441 seedlings containing 

 structural chromosome markers (reciprocal 

 translocations) in the heterozygous condition 

 were treated. Of 258 survivors 6 were true- 

 breeding mutants homozygous for the structural 

 markers and 4 appeared to be chimeras of mu- 

 tant and original tissue and still heterozygous. 

 No homozygosity occurred in the 108 surviving 

 untreated controls. These tests indicate that after 

 colchicine treatment of sorghum seedlings of a 

 reactive genotype under certain environmental 

 conditions somatic chromosome reduction oc- 

 curs. Cells so formed may take over the growing 

 point to form a plant containing half the original 

 chromosomes or homozygous if restored to the 

 original number. 



7.12. Cytogenetic Studies of Perennialism in Deriva- 

 tives of Interspecific Hybrids of Zea*. Donald 

 L. Shaver (Upton, U.S.A.). 



Perennialism is recovered in 4x derivatives 

 of hybrids between 4x maize and 4x perennial 

 teosinte by sib-mating among the most perennial 

 segregates of each generation. Similarly, only 3 

 generations of selection at the 75 per cent maize 

 level have resulted in a progressive recovery of 

 the perennial expression. A high degree of maize- 

 likeness therefore appears compatible with the 

 perennial expression at the 4x level. 



Diploid derivatives of maize and perennial 

 teosinte are produced by crossing the 30 chromo- 

 some triploid hybrid of 4x perennial teosinte 

 x2x maize back to 2x maize, and then inter- 

 crossing among the resulting array of euploid 

 and aneuploid plants. The average chromosome 

 number in the first post-triploid generation is 

 24.6, and in the second post-triploid generation 

 is 20.7. The third post-triploid generation is 

 almost entirely euploid. The study of pollen 

 abortion and seedling lethality in euploids of the 

 post-triploid generations indicates the presence 

 of genetic lesions which are functionally viable 

 in 4x teosinte but which act as diploid-lethals. 

 In spite of the high degree of preferential segre- 

 gation of markers in the triploid indicating 

 that post-triploid material should carry a large 

 proportion of perennial teosinte genes, derived 

 diploids are very maize-like. The apparent 

 tendency to lose teosinte genes is probably 

 connected with the elimination of diploid-lethal 

 factors. However, even among euploids and 

 aneuploids of the first post-triploid generation 

 which have not yet suffered elimination of 

 teosinte genes, it has not been possible to demon- 

 strate true perennialism. One exceptional 21 

 chromosome plant has been found which 

 appears truly perennial. It has been cloned up 

 through the fourth generation of culms. Data 

 on its cytology and genetic progeny ratios will 

 be presented. 



* Research carried out at Brookhaven National 

 Laboratory under the auspices of the U.S. 

 Atomic Energy Commission. 



7.13. The Use of Hypoploids in Identifying [Natu- 

 rally Occurring Duplications in Maize. J. R. 



Laughnan (Urbana, U.S.A.). 



We have commenced using various hypoploids 

 in an arm-by-arm search for naturally occurring 

 duplications in the maize genome. Assuming that 



120 



