74 SEX IN MICROORGANISMS 



polyphyletic origin of the various groups make less difficult the 

 rationalization of the existing sexuality of the fungi. There would 

 remain the same difficulties mentioned above, only partially obscured 

 by the introduction of additional uncertainties. 



The nearest approach to a biologically feasible system which 

 could account for the existing sexual complexity would reject both 

 of the simple propositions stated above, but would constitute a partial 

 synthesis of the two. The following suggestion would appear success- 

 fully to avoid the various objections to simpler derivation. 



Homothallic forms, having gametic copulation, could well have 

 given rise to a primitive homothallic group within which occurred 

 the major evolutionary changes in life cycles and sexual mechanisms 

 — the progressions from predominantly haploid to predominantly 

 dicaryotic cycles and from gametic through gamete-gametangial and 

 gametangial to somatic copulation. Each distinct type of heterothal- 

 lism could have been independently evolved, at the appropriate level, 

 from this primitive homothallic stem. Once the several types of heter- 

 othallism had evolved, existing homothallic species could well have 

 been derived from them by relatively simple means. For example, 

 several kinds of chromosomal aberrations causing a slight dislocation 

 of the locus of a sexual or incompatibility factor could result, after 

 transfer of the factor to the homologous chromosome through cross- 

 ing-over, in genetically stable self-fertile individuals. Such self-fertile 

 individuals, being assured the immediate benefits of sexual reproduc- 

 tion and being genetically isolated, might well prosper and constitute 

 a significant factor in speciation in the fungi. 



This scheme, unduly indirect at first glance, would account for 

 numerous awkward facts implicit in any simpler hypothesis. The 

 more important of these are: (1) the occurrence of homothallic and 

 heterothallic species within groups having unique features which must 

 have been evolved relatively recently, the genera Achlya and Copri- 

 nus for example; (2) the occurrence of the same precise types of 

 heterothallism throughout large groups embracing widely divergent, 

 morphological characters, the Hymenomycetes for example, with 

 tetrapolar and bipolar species, the latter possibly a transitional stage 

 between the former and homothallism, and the aquatic, biflagellate 

 Phycomycetes with their peculiar multiple-sexual-strain type of 

 heterothallism; (3) the lack of strictly heterothallic species that can 

 be reasonably interpreted as intermediate between two distinct types 



