Section 10 — Development Genetics 



10.33. The Developmental Genetics of Leaf Size in 

 Lolium. J. P. Cooper (Aberystwyth, Great 

 Britain). 



The developmental genetics of leaf size in 

 Lolium has been studied in terms of the rates of 

 leaf initiation, leaf appearance, and expansion 

 of the leaf surface at the following levels: (i) 

 differences between populations, (ii) differences 

 between high and low selection lines, and (iii) 

 the effects of major mutants. 



Population differences in leaf size in L. 

 multiflorum and L. perenne are based largely on 

 variation in cell number rather than in cell size, 

 but show marked interactions with temperature, 

 usually related to climatic origin. At 20°C 

 most populations of /.. perenne are similar in 

 leaf size, but at 5° the Mediterranean material, 

 with a winter growing season, shows considerably 

 greater rates of expansion of the leaf surface 

 than do the winter-dormant north European 

 populations. These differences are exhibited 

 mainly in the size of the individual leaves; there 

 is little variation in rate of leaf appearance. 



Selection for high and low leaf size has been 

 highly effective within populations of L. multi- 

 florum and L. perenne, with realized heritabilities 

 over four generations of 30-40 per cent. A strong 

 negative genetic correlation with rate of leaf 

 appearance has appeared, although the rate of 

 initiation remains unchanged. Response to se- 

 lection has operated mainly through changes in 

 cell number rather than in cell size. 



A number of major genes influencing leaf 

 development have been isolated, including 

 dwarf (d), branched culm (b), and shaving brush 

 (sh), which affect leaf size and rate of leaf ap- 

 pearance differentially. These single gene dif- 

 ferences are now being used to elucidate the 

 biochemical processes involved in leaf develop- 

 ment. 



10.34. Hybrid Growth-Evidence of Differential 

 Physiological Development in Heterotic and 

 Non-heterotic Maize Seedlings. Igor V. Sar- 

 kissian (Morgantown, U.S.A.). 



Earlier experiments^ 1 ) have led to an hypo- 

 thesis that stimulated photosynthetic carboxyla- 

 tion in young hybrid seedlings of barley and 

 maize is associated with heterosis in plants. 

 Additional evidence supporting this hypothesis is 

 presented from studies of photosynthetic C 14 02 

 fixation by heterotic and non-heterotic hybrid 

 maize seedlings. The patterns of distribution of 

 C 14 to the intermediary compounds were studied 

 by paper chromatography and autoradiography. 



Differential shifts in the distribution of C 14 were 

 observed in the case of the hybrids, suggesting 

 systems undergoing altered patterns of physiolo- 

 gical development. Determinations and measure- 

 ments of the respiration patterns of young seed- 

 lings also supported this conclusion. 



The significance of enhanced photosynthetic 

 CO2 fixation and differential physiological 

 development and their relationship to heterotic 

 growth will be discussed. 



1. Sarkissian and Huffaker, Proc. Nat. Acad. 

 Sci (U.S.A.) 48, 735, 1962; and in press. 



10.35. The Number of Chloroplasts in Different 

 Cells of Trisomic Sugar Beets. T. Butterfass 



( Ladenburg, Germany). 



It is shown that certain chromosomes (I, II, 

 VIII) of Beta vulgaris in the trisomic state in- 

 crease the number of chloroplasts in spongy 

 parenchyma cells by up to 60 per cent, compared 

 with eudiploids, but leave the numbers in guard 

 cells of stomata unchanged. Other chromosomes 

 (especially III, V, VI) increase the number in 

 the guard cells by up to 30 per cent, but not in 

 the spongy parenchyma cells. The augmentations 

 brought about by chromosomes II (50 per cent) 

 and VIII (60 per cent) turn out to be nearly 

 additive in double trisomies of certain combina- 

 tions, while in others they are not, thus showing 

 that some system of restriction exists which is 

 responsible for the reduced augmentation of 

 only about 45 per cent from diploid to triploid 

 level compared with 130 per cent by the summed 

 up effects of the nine chromosomes of the set. 

 The mean numbers of chloroplasts are indepen- 

 dent of cell size as well as of chloroplast size. 

 In adult epidermal and spongy parenchyma cells 

 the numbers mostly show a bimodal frequency 

 distribution. Endomitotic polyploidy, lacking in 

 guard cells, is considered one of the causes for 

 the existence of a second peak. The usefulness 

 of the plastid number as a character in differen- 

 tiation studies is emphasized. 



10.36. Influence of the D-locus on Phenylalanine 

 Metabolism in Mice. Harold Rauch and 

 Martha T. Yost (Amherst, U.S.A.). 



Using an inbred strain of mice segregating 

 alleles D (full pigmentation) and d 1 (dilute- 

 lethal), we have followed the development from 

 birth through weaning of two liver enzymes 



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