A chromosome rearrangement of Neurospora that produces viable progeny containing two nucleolus organizers.

In rearrangement T(VL leads to IVL)AR33 the segment of chromosome 2 bearing the nucleolus organizer is translocated to the end of chromosome 4. When AR33 is crossed by Normal sequence (N), one third of the viable progeny contain a stable nontandem duplication with two organizers per nucleus. The organizer-deficient complementary products are inviable. Chromosomes and nucleoli have been examined during meiosis and postmeiotic nuclear divisions in the ascus, comparing heterozygous AT33 X N crosses with N X N and with crosses heterozygous for other interchanges. When AR33 is heterozygous, asci are of three types having the nucleolus organizer dupliciated in 0, 1 or 2 of the meiotic products. Frequencies of the ascus types are as expected from the known positions of rearrangement break points. Nucleoli formed by two organizers frequently fuse. Deficiency nuclei that contain no nucleolus organizer may form one or more small nucleolus-like bodies.