LIFE CYCLES, SEXUALITY, AND SEXUAL MECHANISMS SI 



among the fungi. Recent work and re-examination of earlier studies, 

 lio\\cvcr, leave such an interpretation in some doubt; current views 

 rather favor the determination of mating type in these forms by 

 incompatibility factors (Whitehouse, 1949). 



Multiple Altenmte Sexual Factors. A somewhat more compli- 

 cated type of sexual segregation than that immediately preceding in- 

 volves the determination of several sexual strains, each typically self- 

 sterile but cross-fertile with all others. A heterothallic species of this 

 tvpe constitutes a linear series of sexual strains, each of which, with 

 the exception of the t\^'o terminal strains, reacts as 5 or as 9 depend- 

 ing upon its position in the series relative to that of its mate; each 

 of the two terminal strains reacts in a single sexual capacity, as $ 

 or as $ . This pattern of sexuality has been found in all heterothallic 

 species of the biflagellate, phycomycetous orders, Saprolegniales, 

 Leptomitales, and Peronosporales, which have been intensively in- 

 vestigated (Bishop, 1940; Bruyn, 1935, 1937; Couch, 1926; Leonian, 

 193l';Raper, 1940, 1947). 



(S I 6 and/or 9 1 9 



E87 »ll,l59—»89—>l07, 190-^88 — >I55 — >78.80. 184 



Englond I Illinois 1 



Fig. 4. In Achlya aDibisexiialis (a heterothallic species of type II) there are 

 numerous self-sterile, intergrade strains in addition to pure- $ and pure- 9 

 strains. The intergrade strains may be linearly arranged in respect to $ versus 9 

 potentialities, and each strain can react as 5 or 9 or both. (From Raper, 1947.) 



The mating pattern of a number of strains of Achlya ambisex- 

 ualis best serves to illustrate this type of sexuality (Raper, 1947). 

 Ten isolates of this species, collected from northern Illinois in 1946, 

 when mated in all possible combinations, were found to belong to 

 six strains. These strains, each self-sterile but cross-fertile in all 

 combinations, could be linearly ordered, in respect to $ and 9 

 potentialities, as shown in Fig. 4. In this series each isolate reacted as 9 

 to those on its left and as $ to those on its right. A strong $ strain, 

 E87, collected the following year in England, reacted as $ to all six 

 strains from Illinois. Any intergrade mycelium is capable of reacting 

 as $ and 9 in different portions of its thallus when mated simultane- 

 ously with strong 9 and $ plants. 



Segregation at meiosis has been observed for plants exhibiting 

 this type of sexuality in only a single species, Dictyuchus inoiiosponis, 



