Construction and analysis of a two-chromosome double reciprocal translocation that functions as a ‘balancer’ inNeurospora crassa

T(IIL; VL;IIR; VR) BLNC-1 is a compound chromosome rearrangement inNeurospora crassa that combines two reciprocal translocations:T(IIL; VL) AR30 which interchanges the left end of linkage group II with the left end of linkage group V, andT(IIR;VR) ALS154 which interchanges the right end of linkage group II with the right end of linkage group V.BLNC-1 acts as a crossover suppressor for most of both linkage groups II and V since single crossovers between the rearrangement breakpoints result in progeny with lethal unbalanced duplications and deficiencies. The integrity ofBLNC-1 following meiosis was tested in crosses of markedBLNC-1 by marked Normal sequence, with markers located at critical points on linkage groups II and V. Although recombination between distal markers in the four arms was reduced markedly, double crossovers in the long intervening regions occurred with a frequency of 21%. Of these double crossovers, most were coincidental crossovers, one in each of the long intervening regions, resulting in the resolution of the complex into its component rearrangements (16%), while a minority of the double crossovers (5%) were crossovers involving only one of the two component linkage groups, and resulted in the insertion of a segment between the breakpoints. - TheBLNC-1 balancer can be used for: (1) mapping new loci to linkage groups II and V, especially for identifying markers mapping near the tips of the linkage groups; (2) for isolating genetically intact chromosomes from natural populations or for quantitative genetic studies; and (3) for studying recombinational hot-spots which can be detected as escapes from crossover suppression. -Based on experience withBLNC-1, future two-chromosome balancers should be designed with two breakpoints near, but not at, the opposite ends of the chromosome to be balanced, and the other two breakpoints close to, but spanning, the centromere of a second chromosome. Such a construction when combined with appropriately placed selective markers should prevent breakdown of the complex, and should resemble an inversion in eliminating crossover products. Contribution no. 85-218-J from the Department of Plant Pathology, Kansas Agricultural Experiment Station, Kansas State University, Manhattan.