SEXUAL REPRODUCTIONS 12-18 



will grow; on a medium supplying one nutrilite, one of the parental 

 combination and the L, L' recombinants will grow; on a medium sup- 

 plying both nutrilites, all four types will grow. 



4) Segregation of the A/a and B/b alleles is studied only among the 

 ''biochemical" recombinants, and these must necessarily originate 

 from cross mating." 



CROSS-FERTIUZATION IN SACCHAROMYCES 

 AND OTHER ORGANISMS 



There is a variety of mechanisms in different plants and ani- 

 mals for insuring cross -fertilization, but all of these differ some- 

 what from that found in yeasts; the symbols a and a were given to 

 the yeast alleles to indicate their uniqueness. 



Self -Sterility Alleles 



Most hermaphroditic, flowering plants are self-sterile due to 

 a genetic mechanism which prevents pollen shed by the flower from 

 growing down the styles of the parent plant. A mechanism that may 

 be fundamentally similar prevents the sperms of an individual her- 

 maphroditic sea squirt, Ciona, from fertilizing eggs produced by 

 the ovaries of the same individual. 



Sexual Dimorphism 



In higher animals and some plants sexual dimorphism insures 

 cross-fertilization. The genetic mechanism simply operates to 

 reduce the probability of functional intersexes or hermaphrodites 

 occurring. 



Plus -Minus Factors 



This mechanism in Rhizopus is not a sexual mechanism be- 

 cause no unmistakable sex organs are involved and therefore it 

 cannot be called a self -sterility mechanism. It is more precise 

 to consider this a special case in which a single pair of alleles 

 controls copulation. 



Neurospora 



Fig. 12-3 describes the life cycle of Neurospora and shows that 

 the alleles controlling cross -fertilization are a specific type of 

 self -sterility allele. Both plus and minus thalli contain both male 

 and female sex organs but self-fertilization does not occur. The 

 Neurospora mechanism differs from the self -sterility mechanism 

 found in flowering plants in that the zygote is invariably hetero- 

 zygous for the same pair of plus -minus alleles, and only one pair 

 exists. In flowering plants a series of multiple alleles exists and 

 a great variety of heterozygotes abounds. 



