Chapter 23 

 THE BALANCED LETHAL - SACCHAROMYCODES 



Lindegren (1932) developed a technique for calculating link- 

 age relations in Neurospora based on the cytological fact that cen- 

 tromeres always undergo reduction at the first meiotic division. 

 In Neurospora crassa first- and second-division segregation are 

 easily distinguishable by the arrangement of the spores in the ascus 

 (Dodge, 1929); the spindles do not overlap and the first-division 

 segregation produces an ascus in which four spores at one end dif- 

 fer from the four at the other end (fig. 23-1). Any gene-pair which 

 always or nearly always undergoes first-division reduction is near 

 the centromere. A single cross -over between the locus of the gene 

 and the centromere results in second-division segregation (fig. 23- 

 2). Each second-division ascus produces two cross -over and two 

 non-cross-over chromatids and the percentage of crossing over (or 

 the map distance from the centromere) is theoretically equal to one 

 half the percentage of second-division segregation. This relation- 

 ship may be affected by the non- random nature of exchanges be- 

 tween chromatids (Lindegren & Lindegren, 1942). 



In Neurospora tetrasperma (Dodge, 1927) the location of the 

 gene controlling mating type near the centromere and the orienta- 

 tion of the spindles controls the production of homothallic asco- 

 spores. In this species, the self-sterility alleles segregate at the 

 first-division and the spindles of the second division overlap as in- 

 dicated in fig. 23-3. This produces an ascus containing four homo- 

 thallic spores, since each contains nuclei of complementary mating 

 types. Occasional mononucleate spores are produced which give 

 rise to haploid clones; thus giving the species sufficient plasticity 

 to make a high degree of variability available for adaptations to new 

 conditions. 



Winge and Laustsen showed that Saccharomycodes ludwigii is 

 a heterozygous balanced lethal. S. ludwigii normally has no haplo- 

 phase for the spores appear in the ascus in pairs and copulate be- 

 fore beginning growth (fig. 23-4). In S. ludwigii two genes, one of 

 which has a lethal allele (N/n) and the other controlling length of 

 cell (L/1), are both segregated at the first division of the zygote 

 nucleus and the second spindles overlap. The zygote is always het- 

 erozygous for both pairs of alleles and the asci (which contain two 

 spores at each pole) produce zygotes by the fusion of these which 

 are similarly heterozygous. Only two kinds of asci are produced 

 (fig. 23-5). 



23-1 



