SECTION 12 



PLASMATIC INHERITANCE 



12.1. Chemical Characterization of Metagons in 

 Paramecium. I. Gibson (Edinburgh, Great 

 Britain). 



Metagons are gene-determined factors in 

 mate-killer paramecia. They control the mainte- 

 nance and multiplication of visible ia-particles 

 in the cytoplasm of these paramecia and are 

 thought to consist of RNA. Metagons have now 

 been extracted from cells and re-infected into 

 other cells. Results will be described with phenol 

 extracts and other techniques for the purification 

 and characterization of the metagons. 



The possible role of the wild type mt xm - 

 XIV and N alleles will be discussed in terms of a 

 mechanism which assumes that the mt locus 

 controls an essential reaction in the synthesis of 

 the XIV mating type substance, that this locus is 

 repressible and that it does not function in 

 cells expressing type XIII. 



This research was suppor ted by grants from the 

 U.S. Public Health Service and the National 

 Science Foundation. 



12.2. Mutations affecting Mating Type Differentiation 

 in Paramecium. Stephan R. Taub (Cambridge, 

 U.S.A.). 



The mating type of an individual Paramecium 

 aurelia is controlled by its macronucleus. Two 

 paramecia of identical genotype, however, can 

 differ in mating type. The nuclear differences 

 responsible for mating type differences become 

 established soon after the origin of the new 

 macronucleus. In certain strains (including those 

 of syngen 7), cytoplasm transmitted by the 

 previous sexual generation directs the differ- 

 entiation of the new macronucleus to control 

 permanently one of two mating type potentiali- 

 ties. The mating type-determining state of the 

 cytoplasm is, in turn, controlled by the differ- 

 entiated macronucleus and its specificity usually 

 is such as to maintain the same mating type 

 from one sexual generation to the next (Sonne- 

 born, 1954). 



In syngen 7, mutations affecting these pro- 

 cesses have occurred at two unlinked loci 

 (n and mt). Paramecia carrying the N and the 

 mt xm, xiv g enes can express either mating type 

 XIII or XIV, the type actually expressed depend- 

 ing on the state of the cytoplasm received from 

 the previous generation. Cells homozygous for 

 either the /?, the mt xm or the mtz gene express 

 type XIII regardless of the state of the inherited 

 cytoplasm. In addition, cells either heterozygous 

 or homozygous for mt xm and cells homozygous 

 for mtz carry XlV-determining cytoplasm 

 regardless of their mating type. 



12.3. Transmission and Segregation of a Cytoplasmic 

 Factor for Streptomycin Resistance in Diploid 

 Chlamydomonas reinhardi. Nicholas W. Gill- 

 ham (New Haven, U.S.A.). 



Resistance to 500 \xg per ml of streptomycin is 

 controlled by a nonchromosomal factor (sr-2) 

 which exhibits a uniparental pattern of inheri- 

 tance via the mating type plus {mt+) parent. 

 When resistant mt + cells are crossed to sensitive 

 cells (ss) of the minus mating type (mt~), the 

 resulting tetrads segregate four sr-2: zero ss 

 progeny. The remaining 10 per cent or so of the 

 tetrads are exceptional and in these tetrads the 

 sr-2 factor is transmitted to the meiotic progeny 

 by the mt~ parent. A study of these exceptions 

 has shown that the meiotic products segregate 

 pure clones of resistant or sensitive cells or 

 mixed clones containing both types of cells. 

 Therefore, segregation of resistance and sensitivi- 

 ty may occur during either the meiotic or post- 

 meiotic mitotic divisions. 



Recently, a method for obtaining diploids in 

 Chlamydomonas reinhardi has been devised by 

 Ebersold (unpublished). Cells are allowed to 

 mate as in an ordinary cross, but through the 

 use of appropriate selective markers it is possible 

 to isolate clones of cells that appear to behave 

 like diploids. In diploids made between resist- 

 ant and sensitive cells the uniparental pattern of 

 inheritance of the sr-2 factor appears to break 

 down and the factor is transmitted efficiently by 

 either parent. In the resulting diploid clones 

 segregation of resistance and sensitivity seems 

 to occur within the first few mitotic divisions. 



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