Section 12 — Plasmatic Inheritance 



Towards the various types of "chlorophyll 

 mutants" the contribution of gene mutations 

 is well recognized in general. In Arabidopsis 

 thaliana we made a detailed analysis of many of 

 these Mendelian factors considering mutation 

 rate, phenotypical manifestation in different 

 genetic backgrounds, and effects on plastid de- 

 velopment. 



Objections are constantly raised by other 

 authors as to the second genetic component of 

 plastid development, i.e. the "plastom" as being 

 the sum of the autonomous heritable elements 

 within the plastid (Renner). In the course of 

 the mutation experiments reported above, 

 however, we got some strong evidences in favour 

 of the existence of this plastom. 



Just as has been found for other plants, 

 plastom mutations in Arabidopsis are to be 

 induced by a homozygous recessive nuclear gene. 

 Plastids so mutated remain in that state indepen- 

 dent of following changes in the nuclear gene 

 contents. Moreover, plastom mutations have 

 been produced for the first time by X-raying 

 of egg cells or zygotes as well as by treatment of 

 seeds with some alkylating agents (ethyl- 

 methane-sulfonate, di-ethyl-sulfate, etc.) The 

 extranuclear constitution of the resulting so- 

 matic variegation in all three cases above is 

 supported by the following evidences: 



1. In light- and electron microscope true 

 "mixed cells" (Correns) with both normal and 

 mutated plastids have been found. The appear- 

 ance of the mutated plastids observed does not 

 seem to be principally different from that one 

 following after an equivalent gene mutation. 



2. The pattern of variegation within a leaf is 

 correlated to the margin of a cell descendence. 

 In mixed cells of our standard variety of Arabi- 

 dopsis the rate of sorting out of both plastid 

 types is rather high; this effect, however, varies 

 with different residual genotypes. Using gene 

 dependent "chlorophyll mutants" for induction 

 of plastom mutations we found the resulting 

 variegated regions to be larger, pointing to a 

 smaller competitive superiority of the initial 

 plastids in these mutants. 



3. Wherever in plant development the mutated 

 plastids reach the egg cell, a clear proof can be 

 made on their continuity by means of cytologi- 

 cal investigations as well as by demonstration 

 of non-Mendelian inheritance. 



and colour of the roots, have been effectuated, 

 obtaining a total of 12 hybrids. 



The study of reciprocal hybrids effectuated 

 by us during the 1959-1961 period showed the 

 following: 



1. The number of plants of maternal type was 

 considerably higher than those of paternal type. 

 This phenomenon was noticed in Fi and F2, 

 manifesting itself as regards the habit in general, 

 as well as the observed characters in particular 

 of form and colour of roots. For instance out 

 of the 135 plants from the four reciprocal hy- 

 brids of Fi between various sorts of the form, a 

 total of 97 plants (71.8 per cent) were of 

 maternal type, 29 plants (21.5 per cent) were of 

 intermediate type and only 9 plants (6.7 per 

 cent) were of paternal type. In F2 out of a total 

 of 639 hybrid plants, 435 plants (61.1 per cent) 

 were of maternal type, 189 plants (29.5 per cent) 

 were of paternal type and 15 plants (2.4 per 

 cent) were of intermediate type. 



2. The variation of hybrids was generally 

 very high, appearing after the segregation in a 

 large scale of colours and intermediate forms, 

 between parents. At the same time new characters 

 appeared, such as: lilac and violet colours of 

 the roots, or a longish-global form in the higher 

 part of the roots. These new characters appeared 

 in a great number of individuals (from the total 

 of 12 hybrids, 10 hybrids). 



3. The appearance of the hetorosis pheno- 

 menon is in great measure influenced by the 

 way in which the crossing is effectuated. The 

 biometrical measure of plants showed that 

 sometimes great differences exist as regards the 

 vitality between direct hybrids and the reciprocal 

 ones. In some cases the direct hybrids mani- 

 fested heterosis such as: Eiszapfen $ Red with 

 white end <§, Tara Birsei $ : Red with white end 

 ^, andSaxa$ Round white $', while the reci- 

 procal hybrids presented a biological depression. 



4. The effectuated study by us showed the 

 outstanding importance of the way of crossing 

 on the segregation of hybrids and their vitality. 



The predominant maternal type of the reci- 

 procal hybrids and the great difference as re- 

 gards their vitality could be explained by taking 

 in consideration the cytoplasmic heredity and the 

 metabolic influence of the maternal organism 

 on the hybrid seeds development. 



12.17. Reciprocal Crosses by Raphanus sativus L. 



P. Raicu and I. Popovici ( Bucharest, Rumania). 



In our researches reciprocal crosses between 

 seven sorts of radishes of various forms, size 



12.18. Hereditary Infections and Plasmatic Inheri- 

 tance in Drosophila. D. F. Poulson (New 

 Haven, U.S.A.) and B. Sakaguchi (Misima, 

 Japan). 



The demonstration that several striking in- 



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