SACCHAROMYCODES 



23-5 



THE ACCUMULATION OF LETHAL GENES 



Saccharomycodes represents an adaptation like that in Neuro- 

 spora tetrasperma which insures the production of diploid or homo- 

 thallic strains from each ascus. This device makes it possible for 

 lethal genes to accumulate in the chromosomes further restricting 

 the flexibility of the species in its ability to outcross. Winge has 

 refuted Guilliermond's claim that the spindles in Saccharomycodes 

 do not overlap and has insisted that the spindles in Saccharomycodes 

 do overlap. It may be, however, that both situations obtain as in 

 the genus Neurospora: the spindles overlap in Neurospora tetra- 

 sperma but do not overlap in either N. crassa or N. tetrasperma. 

 Overlapping spindles are associated with first-division segregation 

 of the mating type alleles (due to their location near the centromere) 

 and favor the accumulation of the lethal genes. Overlapping spin- 

 dles insure the orientation of spores in the ascus which makes it 

 possible for N. tetrasperma to produce homothallic ascospores (fig. 

 23-3). A lethal gene carried near one of the mating type alleles or 



Fig. 23-6 Diagram of Meiosis in Saccharomycodes Ludwigii. 



near the centromere of any other chromosome would always be in- 

 troduced into a heterokaryon with another haploid nucleus carrying 

 the normal allele and thus make it possible for the lethal to live a 



