CHROMOSOME REARRANGEMENTS THAT INVOLVE THE NUCLEOLUS ORGANIZER REGIONIN NEUROSPORA

In similar to 3% of Neurospora crassa rearrangements, part of a chromo some arm becomes attached to the nucleolus organizer region (NOR) at o ne end of chromosome 2 (linkage group V). Investigations with one inve rsion and nine translocations of this type are reported here. They app ear genetically to be nonreciprocal and terminal. When a rearrangement is heterozygous, about one-third of viable progeny are segmental aneu ploids with the translocated segment present in two copies, one in nor mal position and one associated with the NOR. Duplications from many o f the rearrangements are highly unstable, breaking down by loss of the NOR-attached segment to restore normal chromosome sequence. When most of the rearrangements are homozygous, attenuated strands can be seen extending through the unstained nucleolus at pachytene, joining the tr anslocated distal segment to the remainder of chromosome 2. Although t he rearrangements appear genetically to be nonreciprocal, molecular ev idence shows that at least several of them are physically reciprocal, with a block of rDNA repeats translocated away from the NOR. Evidence that NOR-associated breakpoints are nonterminal is also provided by in tercrosses between pairs of translocations that transfer different-len gth segments of the same donor-chromosome arm to the NOR.