SECTION 7 



CYTOGENETICS 



7.1 . Tomato Chromosomes and Their Relationship to 

 the Linkage Maps. B. Snoad (Hertford, Great 

 Britain). 



The tomato is a rather exceptional plant since 

 it has been for many years a subject for intensive 

 genetical research and also it has chromosomes 

 which are morphologically distinguishable at 

 pachytene. It is only recently, however, that 

 attempts have been made to correlate the linkage 

 maps with the chromosomes themselves. Each 

 chromosome is composed of distinct hetero- 

 chromatic and euchromatic regions and it is 

 therefore possible to compare the genetic 

 activity of these two regions. 



Such a comparison is being made by the use of 

 X-ray induced reciprocal translocations. The 

 broken chromosomes are identified at pachytene 

 and test-crosses are made with appropriate 

 genetic marker stocks. In this way linkage 

 between the visible points of chromosome 

 breakage and the selected genes can be estimated. 



The evidence so far indicates that crossing- 

 over does not occur in the heterochromatin and 

 that the active genes studied are located in the 

 euchromatin. The great majority of chromosome 

 breakage occurs in the heterochromatin so that 

 linkage between the genes and such points of 

 breakage is really a measure of the amount of 

 euchromatin separating them rather than the 

 total amount of euchromatin and heterochro- 

 matin. 



7.2. Localizing Genes by means of Induced Chromo- 

 somal Deficiencies in Tomatoes. Gurdev 

 S. Khush and Charles M. Rick (Davis, 

 U.S.A.). 



The induced deficiency method, previously 

 applied {Genetics, 46, 1389-1393) to genes on 

 chromosome 11, has been extended successfully 

 to localize other seedling marker genes. Stocks 

 homozygous for unlocated recessive genes are 

 pollinated with X-rayed pollen of a normal, non- 

 mutant parent, and all mutant progenies are 

 searched cytologically at pachytene for chromo- 

 somal alterations. More than 95 per cent of 

 such individuals are deficient for that part of the 



chromosome in which the gene in question 

 resides. In this fashion /// of chromosome 11, 

 dl of 8, and fid and clau of 4 were assigned to 

 the achromatic regions of the short arms (S) of 

 their respective chromosomes; likewise a\ to 

 the achromatic of 11L and 7i to 8L. No unequi- 

 vocal evidence has been obtained for genes in the 

 chromatic regions. More than half the deficient 

 plants are monosomic, but simple monosomies 

 have been found only for chromosome 11, the 

 remainder being translocated monosomies, in 

 which one element of a reciprocal translocation 

 has been lost. Of ten such deficiencies, the lost 

 element nearly always comprises the shorter 

 arms of the respective chromosomes, longer 

 deficiencies doubtless being less viable. Ex- 

 ceptional was a case in which centromeric 

 breaks led to translocation and loss of 8L and 

 9S. Since this plant had h phenotype and ap- 

 peared in the progeny of dl-bu-h X-irradiated 

 + + + pollen, 7i clearly lies in 8L and both bu 

 and dl in 8S. No deficiencies were transmitted to 

 any of 1429 progenies of female parents deficient 

 for arms of 1 1 and to 1 3 1 offspring of haplo- 11$. 



Research partly supported by Grant GM 

 06209 of the U.S. Public Health Service. 



7.3.Cytogenetic Correlations in Tetraploid Tomatoes. 



Peter B. Moens (Toronto, Canada). 



Five markers (dv, d, m, aw and wv) on chromo- 

 some no. 2 of the tomato, the satellited chromo- 

 some, were found to segregate 1 : 30 in the 

 progeny of duplex tetraploids. The individual 

 ratios did not differ significantly from each 

 other (x 2 =0.40, d.f. 4) and an average ratio of 

 1: 30.257 was established for the markers 

 (3.20 per cent recessives +0.05 percent, based on 

 110,000 observations). The fact that the ratios 

 do not vary with the distance from marker to 

 centromere, as is normally the case in tetraploids, 

 results from the specialized structure and be- 

 haviour of chromosome no. 2. 



Recessive zygotes in excess of 1 : 35 are the 

 result of genetic non-disjunction in quadrivalents. 

 The frequency of chromosome no. 2 quadri- 



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