404 PHYSIOLOGY OF THE FUNGI 



INHERITANCE OF PHYSIOLOGICAL CHARACTERS 



The genetics of the fungi has been, in general, a neglected study. 

 Numerous papers have appeared on the sexuality of the fungi, particu- 

 larly with regard to the various sexual or compatibility groups in the 

 Basidiomycetes. The sexuality of the Mucorales has been studied to a 

 lesser extent. Genetic studies of morphological characters have been 

 decidedly fewer. Perhaps this is due to the failure to recognize definite 

 morphological differences betw^een individuals of opposite sex but of the 

 same species. An equally great handicap to such studies lies in the 

 difficulty in obtaining the perfect stage of many of the fungi which other- 

 wise might be suitable. Studies dealing with inheritance of physiological 

 characters (if sexuality is excluded) are comparatively few and recent. 



The basis of inheritance. The physical basis of inheritance is the 

 gene, located at a specific position on a certain chromosome. In mitosis 

 the chromosomes and their genes divide, and half of each goes to each 

 daughter nucleus. With the exception of parthenogenesis, all perfect 

 stages of the fungi arise as a result of the union of two nuclei. These 

 two nuclei may arise from the same haploid individual (homothallism) 

 or from separate haploid thalli (heterothallism). The union of the two 

 haploid nuclei, each with a single set of chromosomes, initiates the diploid 

 nucleus, or the syncaryotic stage, in which the chromosomes are paired. 

 The syncaryotic stage in fungi is usually short in duration, being followed 

 closely by meiosis, which involves the separation of the two chromosomes 

 (and genes) of each pair. Certain pairs of chromosomes may separate in 

 the first division, while others separate in the second. Therefore, two 

 successive nuclear divisions are necessary to complete the reduction of 

 all pairs of chromosomes (and likewise all the pairs of genes). In the 

 Ascomycetes and the Eubasidiomycetes karyogamy and meiosis occur 

 in the same cell, the ascus and the basidium, respectively. In the smuts 

 and rusts, meiosis typically takes place in a promycelium, while kary- 

 ogamy occurs in the teliospore. When a single pair of genes is considered, 

 half the haploid ascospores or basidiospores carry one gene and half carry 

 the other gene. 



Inheritance in the Ascomycetes. Some of the outstanding genetic 

 work has been done by Dodge (1927, 1928) and others with Neurospora, 

 by Ames (1934) and Doweling (1931) with Pleurage anserina, by Edgerton 

 et at. (1945), Chilton and Wheeler (1949), and their associates with 

 Glomerella, and by Lindegren (1945, 1948) and his colleagues with yeasts. 

 Most of these investigations have been concerned primarily with sexual 

 or morphological characters. The life cycle of Neurospora is shown 

 diagrammatically in Fig. 78. 



Beadle and his associates have contributed much to our knowledge 



