DNA sequence duplications trigger gene inactivation in Neurospora crassa

Transforming sequences are faithfully replicated in vegetative cells of Neurospora but are typically subject at high frequency to sequence alterations and methylation in the period between fertilization and nuclear fusion. Previous work showed a correlation between the occurrence of these radical changes, referred to by the acronym RIP, and the presence of sequence duplications resulting from the introduced DNA. Various possible causes for the RIP process were investigated. Introduction of a single copy of a DNA fragment containing the Neurospora am gene into a strain having a deletion of this DNA did not lead to RIP, whereas introduction of two or more copies of the same fragment did. A conventional cross of strains having single copies of am at unlinked chromosomal locations was used to build a strain duplicated for am. Both copies of the duplicated gene were subject to the RIP process in crosses of this strain. We conclude that sequence duplications, per se, trigger RIP.