46 SEX IN MICROORGANISMS 



stitute a sizable proportion of all known species, approximately 20 

 per cent according to Bessey (1950). Because of the failure to ob- 

 serve rarely occurring sexual stages, the actual number of exclusively 

 asexual species must be somewhat less than reported, but it must still 

 be very large. The entire group known as the Fungi Imperfecti be- 

 longs here as well as numerous species which are clearly assignable 

 by morphological characteristics to various groups throughout the 

 perfect fungi, such as PenicilHwii iiotatniu, the producer of the drug 

 penicillin. 



Certain of the benefits of sexuality are provided in many sexually 

 sterile species by the association of nuclei of different origins in heter- 

 ocaryotic mycelia, in which different genetic characters are ex- 

 pressed in much the same way as in dicaryotic mycelia or in diploid 

 organisms (Pontecorvo, 1946). Of particular interest in this connec- 

 tion is the recent demonstration by Pontecorvo of recombination in 

 low frequency of genetic factors in heterocaryotic fungal systems 

 similar to that shown by Lederberg and Tatum (1946) in bacteria. 

 The exact mechanism whereby such recombinations are achieved in 

 fungi has not been fully elucidated. It has been shown, however, that 

 there are formed occasional diploid nuclei, heterozygous for the 

 characters carried by the heterocaryotic components, and that these, 

 through a pseudo-meiotic rearrangement lacking reduction, produce 

 diploids which are homozygous for one or more characters and which 

 represent new genetic combinations (Pontecorvo and Roper, 1952; 

 Roper, 1952). 



HAPLom Cycle 



The predominant type of life cycle found in the Phycomycetes 

 and the more primitive Ascomycetes is completely haploid with 

 the exception of a single, diploid, nuclear generation, the fusion or 

 zygote nucleus. This type of life cycle is the simplest possible one 

 that allows for sexual fusion and the recombination of genetic fac- 

 tors and in all probability represents the primitive type from which 

 the more complicated cycles have evolved. The general correlation 

 between this type of cycle and the relative morphological simplicity 

 of the forms exhibiting it, not only in the fungi but also in the algae, 

 would tend to support this view. 



