TABLE 6

Analysis of interference based on random spores

Random sporesIndividual spores from Set II tetrads
Met+ spores
 that are
 NAT-hph
 noncrossoversMet+ spores
 that are NAT-HPH
 crossovers
 (P < 0.0001)Nonselected
 NAT-hph
 noncrossover
 sporesNonselected
 NAT-HPH
 crossover spores
 (P = 0.0005)Total
 NAT-hph and
 NAT-HPH spores
A. Crossing over in the lys5-NAT interval among individual sporesa
Crossovers 82 37 518 25 543
Noncrossovers39742920632122275
χ2 P (interference)0.29 (none)<0.0001 (positive)0.48 (none)0.0012 (positive)
Random sporesIndividual spores from Set II tetrads
Met+ spores
 that are
 NAT-hph
 noncrossoversMet+ spores
 that are NAT-HPH
 crossovers
 (P = 0.0015)Nonselected
 NAT-hph
 noncrossover
 sporesNonselected
 NAT-HPH
 crossover spores
 (P = 0.029)Total
 NAT-hph and
 NAT-HPH spores
B. Crossing over in the HPH-cyh2 interval among individual sporesa
Crossovers 57 27 266 13 279
Noncrossovers42243923462222568
χ2 P (interference)0.18 (none)0.007 (positive)0.67 (none) 0.042 (positive)
  • a A comparison of MET13 prototrophic random spores derived from a met13-1/met13-5 diploid (first two columns) with equivalent segregants obtained by treating the 1451 tetrads in set II as individual spores (last two columns). Only NAT-containing spores were counted so that crossover and noncrossover events were counted only once, to permit statistical comparisons with the random Met+ spore data.

  • The χ2 P-values at the heads of columns 2 and 4 compare the crossovers in those columns to the noncrossovers in columns 1 and 3, respectively. The χ2 P-values in the last rows of parts A and B compare the data in those columns with the total number of NAT-hph and NAT-HPH segregants in set II, treated as single spores. Random spores were obtained from a yeast diploid strain obtained by a cross between haploid strains isogenic to those used for tetrad analysis (Table 1, set II). The diploid used to generate Met+ random spores was derived from the haploids MATα lys5 NAT met13-1 cyh2 KAN and MATa ade5,7 leu1 HPH trp5 met13-5 crl3. The randomness of the spores was verified by monitoring phenotypes at both the ADE5,7 and the LEU1 loci (480 Ade and 465 Ade+; 434 Leu and 511 Leu+). Among Met+ spores, there were 466 crossovers (371 Nat Hph and 95 Nat+ Hph+) and 479 noncrossovers (370 Nat+ Hph and 109 Nat Hph+) for the flanking markers. The inequalities in both the crossover and noncrossover classes suggest that 78% of Met+ spores are the result of conversion of met13-1. This bias is expected from the sign and magnitude of the slope of the conversion gradient at the MET13 locus, where met13-5 is at the low-conversion end (J. McCusker and J. E. Haber, unpublished data).