Recombination frequency and wild-type ancestry in linkage group I ofNeurospora crassa

A selection of single and double mutant strains was backcrossed repetitively at least four times to each of the wild-types Abbott 4A, Abbott 12aand Lindegren 1A, and then re-isolated. Twelve crisp progeny isolated from the fifth backcross to Abbott 4Awere heterogeneous for recombination frequency in the interval betweencrandme-6while thirteen crisp progeny from the fifth backcross to Lindegren 1Awere homogeneous for recombination frequency in this interval. Thirteen crisp strains from the fifth backcross to Abbott 12awere homogeneous for recombination frequency betweencrandnic-1loci. However, mutant strains which had been backcrossed independently to Lindegren 1Awere heterogeneous for recombination frequency in the intervalscr-aurandaur-nic-1. In general, recombination frequencies in crosses between strains of the same wild-type ancestry including Abbott 4 and Abbott 12 were significantly higher than those in crosses of Lindegren × Abbott 4 or Lindegren × Abbott 12 ancestry but exceptions were found. Recombination frequencies between the same markers usually, but not always, increased on inbreeding and the changes in frequency were non-uniform in the marked region in some crosses. The limitations of backcrossing as a means of transferring a mutant marker into a wild-type genome were discussed. It was concluded that inducing marker mutations anew in a given wild strain, preferably Lindegren 1A, might be more successful in obtaining constancy of map distance.