Section 10 — Developmental Genetics 



floral tissues, not only in petioles but even in 

 the archespors of carpels and stamina. In a 

 haphazard manner self-pollination therefore 

 may render possible "intra-individual crossings" 

 and may produce de novo a varying number of 

 "ever-segregating" and "ever-rearranging" in- 

 dividuals, in the same way as cv. "Harlekin" 

 is reported to have originated by crossing pink x 

 red. The investigations are being continued. 



10.30. Spontaneous Somatic Mutability in the 

 Tomato. W. Williams (Newcastle upon Tyne, 

 Great Britain). 



A case of spontaneous somatic mutability 

 in the tomato causing leaf variegation will be 

 described. The condition occurs in a large num- 

 ber of genotypes but is confined to certain stages 

 of plant development. The histological basis of 

 variegation is a failure of normal development 

 in the palisade parenchyma. In addition there 

 is failure of differentation of the sporogenous 

 tissue in the immediate pre-meiotic stage which 

 results in complete male and female sterility. 

 By a process of elimination the condition may 

 be interpreted on the basis of a highly mutable 

 controller locus acting at a specific stage in 

 development. 



10.31. Canalization of Flower Morphogenesis in 

 Wheat. O. H. Frankel and Anne Munday 

 (Canberra, Australia). 



In common wheat (Triticum vulgare) normal 

 flower morphogenesis is an invariate character. 

 It is controlled by a factor associated with Q, 

 which, in turn, controls the "vulgare syndrome". 

 In speltoid mutants, where Q is deleted or in- 

 activated, we have obtained, as previously 

 reported, a range of stable genotypes from a 

 virtual absence of flowers to perfectly normal 

 flower morphogenesis. In certain compactoids 

 this extends in the supernormal direction, with 

 extra flowers in the empty glumes. 



Flower morphogenesis in vulgare and in the 

 speltoids differs genetically and physiologically. 

 In vulgare flower morphogenesis is conditioned 

 by a single gene; in speltoids it is conditioned 

 by a complex multifactorial system. In vulgare 

 the flower formation is uniformly normal in 

 all experimental environments applied, includ- 

 ing drastic heat and radiation shocks; in the 

 speltoid series, frequencies of flower formation 

 are strongly affected by suitable combinations 

 of daylength and temperature applied during 



the critical period of flower formation. 



Thus, in vulgare flower morphogenesis is 

 buffered genetically against mutation and re- 

 combination among a multitude of polygenes, 

 and physiologically against environmental 

 shocks. There are striking analogies between 

 canalization systems uncovered in Drosophila 

 (Waddington, Rendel), mice (Fraser) and wheat. 



It can be readily understood that there can be 

 no more than one canalization factor even in a 

 hexaploid; and it is not surprising that the 

 typical polyploid series have been found in 

 genes conditioning relatively superficial func- 

 tions. Canalization systems are opportunistic; 

 their transformation in polyploids may shed 

 light on the evolutionary changes in gene func- 

 tion itself. 



10.32. Inheritance and Morphogenesis of Capsule 

 Spines in Ricinus communis. Hava Stein 

 (Rehovoth, Israel). 



At the S (spiny capsule) locus, there are 4 

 alleles: S — spiny capsule, s r — very few spines 

 present, s, sf — entirely spineless, similar to each 

 other when homozygous hence considered isoal- 

 leles. S is partially dominant: 5 s r , S s, S sf 

 plant bear spines, but their number is much 

 lower than in S S. Number of spines decreases 

 from Ssf to Ssf: S 5> Ss?> S s> Ssf In Ss 

 and S-s+ plants, spine number varies in a predicta- 

 ble manner within each plant. Number of spines 

 on pods of the first raceme is always much 

 lower than in later racemes. Within each raceme, 

 spine number increases gradually from the 

 first capsule to later ones. This indicated an 

 inverse relation between spine number and 

 growth rate. Growth inhibition induced by 

 continuous removal of leaves resulted in a highly 

 significant increase in spine number, as expected. 

 The control of spine formation on the capsule 

 by a treatment applied to other plant organs 

 may well be mediated by mobile substances. 



A spine is a multicellular cylinder crowned by 

 a single extremely elongated cell. Appearance of 

 this cell always precedes, and seems obligatory 

 to, the development of the multicellular organ. 

 Determination of spine formation is tentatively 

 assumed to consist of two steps: Excessive elon- 

 gation of random epidermal cells on the capsule 

 (perhaps controlled by the above-mentioned 

 substances) and stimulation of cell division in 

 the underlying tissues by the elongated cell 

 (through stress or similar forces). This model 

 affords an interpretation of the entire process 

 of spine morphogenesis through simple events 

 on the cellular level. 



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