Spontaneous IR duplications generated at mitosis in Aspergillus nidulans: further evidence of a preferential site of transposed attachment.

A radiation-induced translocation, T(IIR----IIIL), has been shown to be nonreciprocal and to have most of IIR, including its terminus, attached uninverted to the terminus of IIIL.--Progeny with the IIR segment in duplicate, obtained from crosses of T(IIR----IIIL) to strains with a standard genome, were unstable at mitosis; like earlier duplication strains, they suffered deletions from either duplicate segment. Frequent mitotic crossing over occurred between the duplicate IIR segments so that, following deletions, more than two classes of stable, balanced products arose from each heterozygous duplication strain.-- Spontaneous, mitotically arising duplications of the IR segment, bearing the rate-limiting adE20 allele, can be selected on adenine-free medium on which they emerge as vigorous sectors from the stunted adE20 colony. It was shown previously that most such duplications, when selected from a strain with standard genome, had the terminal IR segment attached to the end of IIR. Selection has now been made from an adE20 strain carrying T(IIR----IIIL), and seven of the 13 independent IR duplications were linked to the III-IIR translocation complex. In three strains analyzed further, the duplicate IR segments, which included the IR terminus, were attached uninverted to the terminus of IIR; the segments of IR were of approximately equal genetic length.--This supports earlier suggestions that there is a preferential site for the initiation of IR duplications and a preferential site, the IIR terminus, for their attachment.