METABOLISM AND REPRODUCTION 375 



The most recent claimed difference, a color reaction with strong alkali 



(292), was found invalid almost immediately (46). Enzyme differences 

 between opposite mating types in Neurospora crassa (103) and growth 

 differences between (+) and (— ) strains of Ustilago spp. (281) are 

 exposed, until confirmed, to a similar skepticism. 



The long debate over the existence of perithecium-inducing hor- 

 mones in Neurospora sitophila is reviewed by Raper (236); the work 

 of Aronescu (3, 4) in particular seems conclusively negative. This con- 

 clusion must stand unless challenged by new evidence. However, in 

 Glomerella cingulata, like Neurospora a member of the Sphaeriales, 

 there is preliminary evidence for a diffusible metabolite which induces 

 the formation of perithecia (92, 200). Since the various steps of sexual 

 reproduction in G. cingulata are gene-controlled (308), it is perhaps 

 not too fancifuf to envision a one gene-one hormone relation. 



In Ascobolus stercorarius the induction of ascogonia and other early 

 steps in sexual reproduction are controlled by diffusible materials, 

 presumably hormones (31). The branching of the trichogyne in Neu- 

 rospora sitophila (9) and the suppression of conidiation in Glomerella 

 cingulata by filtrates from perithecial cultures (309) are also very 

 probably hormone effects. There is no evidence as yet for hormonal 

 mechanisms in basidiomycetes. 



The literature on the chemical stimulus for the aggregation of myx- 

 amoebae in the Acrasiae has been thoroughly reviewed by K. B. Raper 

 (238) and by Sussman (279). The substance responsible has been 

 termed acrasin (34) and has now been separated from the cells; it was 

 found to consist of two components and to be destroyed by some sys- 

 tem, probably an enzyme, of the cell (260, 280). It is especially interest- 

 ing that acrasin not only attracts scattered myxamoebae but also 

 stimulates them in turn to produce acrasin; the effect is that of a chain 

 reaction which extends the range of the chemotactic system much be- 

 yond any possible range based only on diffusion from a single point. 



1 1 . METABOLISM AND REPRODUCTION 



Throughout this chapter we have stressed the concept that the major 

 stimulus to reproduction is exhaustion of the medium. However, 

 many different metabolic changes occur at the same time in a culture, 

 and the possibility must be kept in mind that one or more of these 

 changes is the primary stimulus to spore formation. In particular, 

 the mycelium begins to autolyze (Chapter 1). In yeast colonies on 

 agar, ascospores appear in that part of the colony which is undergoing 

 autolysis (191). Now, it is possible that some factor in normal cells is 



