Section 10 — Developmental Genetics 



10.21. Progressive Expression of Mating-type Genes 

 in Paramaecium. R. W. Siegel (Los Angeles, 

 U.S.A.). 



Our data support the hypothesis initially 

 presented by Metz which states that the four 

 mating-type specificities in P. bursaria syngen 1 

 are brought about by the four unique combina- 

 tions of two independent pairs of complemen- 

 tary ciliary mating-type substances; substances 

 AB, aB, ab, and Ab characterize the cell sur- 

 faces of mating types I, II, III, and IV, respec- 

 tively. Conjugation depends upon the complemen- 

 tarity of either the alpha or the beta substances 

 or both. One pair of allelic genes controls the 

 alpha substances and an independently assorting 

 pair of alleles controls the beta substances. 

 Early in the life history of a clone mating does 

 not occur and neither alpha nor beta substances 

 are formed. Later, in adolescence, clones will 

 react with but two of the four standard mating 

 types and it has been shown that now some 

 clones form an alpha substance and others a 

 beta substance. With maturity, each clone forms 

 both an alpha and beta substance. These find- 

 ings mean that the two genetic loci concerned 

 with mating-type specificity come to expression 

 in a predictive temporal sequence; first neither 

 locus is expressed, then one locus alone is ex- 

 pressed and finally both loci are expressed. 



The appearance of each successive life-cycle 

 stage in P. awelia is probably a consequence of 

 the orderly progressive differentiation of nuclei 

 (Siegel, 1961). In P. bursaria nuclear differentia- 

 tions may control the expressions of (or "switch- 

 on"?) the two loci concerned with the develop- 

 ment of sexual specificity. 



10.22. Siime: a Plasmodioid Variant of Neurospora 

 crassa. Sterling Emerson (Pasadena, U.S.A.). 



The heritable slime variant of Neurospora 

 grows on an agar surface by plasmodium-like 

 flows which are devoid of cell walls at the ad- 

 vancing margins. Spherical cells with thin cell 

 walls are produced in older parts of such colo- 

 nies. Growth in liquid culture consists of spheri- 

 cal cells and atypical hyphae, both having flexible 

 cell walls. 



Heterokaryons between slime and wild type 

 have the wild-type hyphal morphology under 

 all growth conditions. In vegetative isolations 

 from such heterokaryons the slime phenotype is 

 invariably associated with nuclei of the slime 

 parent. 



Three genetic determiners, segregating in- 



dependently at meiosis, are essential to the pro- 

 duction of the slime phenotype, but are insuffi- 

 cient for its maintenance. Ascospore isolates 

 carrying these three determiners are often hyphal 

 from the start or, if germinating as slime, soon 

 become hyphal. By rigorous vegetative selection 

 from such isolates it has been possible to establi; h 

 sub-strains which retain the slime phenotype per- 

 manently. 



The unique morphological features of slime 

 make it useful in a number of kinds of investiga- 

 tions. The absence of hyphal walls permits easier 

 isolation of cellular constituents than is possible 

 in hyphal forms. Slime grows in an extremely 

 thin sheet between an agar surface and a cover 

 glass, permitting observation of living material by 

 phase microscopy under conditions in which 

 cellular structures are well spread and do not 

 overlie one another, and in which light defraction 

 from cell walls is absent. 



10.23. Transplantation of Nuclei in Neurospora 

 crassa. James F. Wilson (Salinas, U.S.A.). 



Heterokaryons of N. crassa were produced 

 artificially by cell-to-cell transfer of protoplasm 

 of genetically marked strains. The injected hyphal 

 sections were excised and cultured separately. 

 The criteria of prototrophic growth and recovery 

 of conidia of the donor genotype were used to 

 prove that functional nuclei had been transplan- 

 ted. Reciprocal injections were made with 2 

 pairs of macroconidial and 3 pairs of micro- 

 conidial strains. Growth curves and conidial 

 ratios of these artificial heterokaryons were 

 compared to the corresponding natural ones. 

 In each pair of heterokaryons produced by re- 

 ciprocal injection, the growth curve of only one 

 was equivalent to the naturally formed hetero- 

 karyon of that pair. The growth curve of the 

 other artificial heterokaryon differed either in 

 rate or in the lag period before growth occurred. 

 There was no apparent relation between conidial 

 ratios of the natural heterokaryons and the 

 artificial heterokaryons with comparable growth 

 curves. Injection of normal nuclei into a morph- 

 ological temperature-sensitive mutant pro- 

 duced an apparent heterokaryon which grew 

 with normal morphology at 34°C. The apparent 

 stability of Neurospora nuclei may make in 

 vitro studies possible. 



Published in extenso in Amer. Jour. Bot. 

 (in press). 



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