44 



BIOLOGIC BASIS OF SEX 



tion Avhich affect type compatibilities. The 

 work of Esser and Straub (1958) on 21 ir- 

 radiation mutant strains is cited. Of the 21 

 strains, 18 were sterile when grown alone. 

 These mutants could be divided into quali- 

 tatively different groups in terms of the 

 stages at which sexual isolation was 

 achieved. Three strains developed only vege- 

 tative mycelia; 3, ascognia but no proto- 

 perithecia; 6, few to many protoperithecia 

 but no perithecia; 6, sterile perithecia; and 

 3, fertile perithecia with nondischarging asci. 

 Four mutant types were observed more than 

 once. The results suggested single factor 

 changes each affecting a single essential fac- 

 tor product significant to further sexual de- 

 velopment. Restoration occurred by pairing 

 with wild type alleles in heterokaryons de- 

 rived from hyphal fusions between the two 

 defective strains. Fertility is sometimes 

 noted in pairing of self-fertile strains ap- 

 parently mediated through the cytoplasm. 

 Diffusible agents, hormones and surface 

 agents may all play a part in fertility. In 

 some features the results in fungi suggest the 

 wide range in fertility phenotypes of gene 

 origin so frecjuently isolated in Drosophila 

 experiments. Together with much other ma- 

 terial they furnish further evidence for a 

 multigenic basis for sex-controlled charac- 

 teristics. 



Physiologic differentiation of individuals 

 of a species into diverse mating types was 

 notably extended by Sonneborn's (1937) 

 discovery that in Paramecium aurelia two 

 classes of individuals exist. Members of dif- 

 ferent classes unite for conjugation; mem- 

 bers of the same classes do not. Information 

 on these types has accumulated rapidly dur- 

 ing the last few years (Jennings, 1939; Son- 

 neborn, 1947, 1949, 1957). As a rule P. au- 

 relia has two and only two interbreeding 

 mating types in a variety. In general mating 

 types from different varieties do not react 

 to each other. Death or low viability follows 

 in a few cleavages for some of the rarely 

 interbreeding mating types. In P. bursa n'a 

 Jennings and his group discovered six vari- 

 eties each of them comprising a set of mat- 

 ing types. A system of at least four mating 

 types is known for varieties I, III and VI; 

 eight are found in variety II; IV has two 

 and V is represented by but one. Simihir 



mating type systems are found in other 

 genera, i.e., Euplotes (Kimball, 1939) . 



Mating systems have been demonstrated 

 for some species of bacteria. A strain in 

 order to show conjugation followed by 

 transfer of separate genetic entities requires 

 at least two different types of individuals. 

 Ravin (1960) has recently reviewed this 

 subject for bacteria. The F+ and the F" 

 cell types when intermixed will conjugate 

 and show detectable rates of interchange for 

 genetic materials in tests of subsequent 

 progenies. The F~ and F~ when mixed do 

 not show recombination. The F+ and F + 

 when mixed may show low rates of inter- 

 change which have been suggested as due to 

 the presence of physiologic F~ variants 

 sometimes found in genetically F+ isola- 

 tions. The postulated F+ and F~ factors 

 suggest relations similar to those of some of 

 the genes in higher animals or plants. The F+ 

 factor has a property that may set it apart 

 from these genes. In the presence of F + 

 cells, F~ cells adopt the F+ mating type. 

 The reaction is suggestive of that taking 

 place within the male sex in Bonellia as 

 observed by Baltzer. 



In the absence of visible morphologic dif- 

 ferences which enforce exchanges and re- 

 combinations of genetic materials, the phys- 

 iologic or submicromorphologic conditions 

 found in either haploid or diploid unicellular 

 organisms accomplish the same objectives 

 by establishing mating types. AVhether these 

 differences are really comparable with sex 

 and sex determination is still an open ques- 

 tion. There may be some ground for think- 

 ing that there are precursors which assist in 

 the develoiiment of such systems. 



Vin. Environmental Modifications 

 of Sex 



A. AMPHIBIA 



Amphibian sex chromosomes, Amby- 

 stoma, Siredon, Pleurodeles, Triton, Tri- 

 turus,Rana, and Xenopus as found in nature 

 are ZZ for the males and WZ for the fe- 

 males. Sex reversal of the normal sex pheno- 

 types has been particularly successful in 

 this class of animals and under a variety of 

 conditions (for further information see 

 Chapter l)y Burns). The haploid chromo- 

 sonu" nuiuhers foi' the males of these various 



