ЦИС-эффект гомозиготного генетического фона на частоту кроссинговера у кукурузы

The cys-effect of homozygous background on crossing over in maize. From the inbred maize lines MK01 and Ku123, the isogenic lines containing mutant markers lg1, gl2, c1, sh1, wx1 were obtained. They differ from original forms by pair of mutant markers. The hybrids of isogenic lines with the original forms are heterozygous only within marked region and are almost homozygous in the rest of the genome. Homozygous background leads to increase of mean recombination rate from 5.4% to 10.1% in c1-sh1, from 15.0% to 35.6% in s1-wx1 and from 18.2% to 32.8% in lg1-gl2.

THE EFFECTS OF LETHAL SELECTION ON THE EST-6 TO PGM REGION OF CHROMOSOME 3 IN DROSOPHILA MELANOGASTER

ABSTRACT A powerful means of studying the effects of selection on chromosome segments in Drosophila melanogaster has been described by Clegg et al. (1976, 1978). This method utilizes a recessive lethal, dominant visible allele whose selection dynamics can be accurately modelled to predict the fates of nonlethal alleles at linked loci. Results of these experiments indicate that strong epistatic interactions among loci occur that affect fitnesses associated with gametic types in the basal region of chromosome 3. We have used similar methods in studying a different segment of chromosome 3, that spanned by Est-6 (3—36.8) and Pgm (3—43.4), with the aim of determining whether the results of Clegg and his colleagues could be reproduced when a different region was studied. Our experiment showed that selection did operate on a region of the chromosome marked by Pgm, but that no evidence of selection at loci marked by Est-6 was apparent. Weak evidence for epistatic interactions among loci within the marked region was also found. Three possible explanations for the discrepancies between our experiments and those of Clegg et al. are suggested. First, the geographically homogeneous origin of our populations may preclude selectively significant changes as a result of recombination. Second, the results seen by Clegg et al. may have been unique to the regions they studied, which included the basal heterochromatin of the chromosome. Finally, the three loci employed may not adequately mark the unit of selection, so that actual departures from predictions of selective neutrality may not have been apparent.

Recombination frequency and wild-type ancestry in linkage group I of Neurospora 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 12a and Lindegren 1A, and then re-isolated. Twelve crisp progeny isolated from the fifth backcross to Abbott 4A were heterogeneous for recombination frequency in the interval between cr and me-6 while thirteen crisp progeny from the fifth backcross to Lindegren 1A were homogeneous for recombination frequency in this interval. Thirteen crisp strains from the fifth backcross to Abbott 12a were homogeneous for recombination frequency between cr and nic-1 loci. However, mutant strains which had been backcrossed independently to Lindegren 1A were heterogeneous for recombination frequency in the intervals cr-aur and aur-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.