The neurospora transposon Tad is sensitive to repeat-induced point mutation (RIP)

Abstract

RIP (repeat-induced point mutation) efficiently mutates repeated sequences in the sexual phase of the Neurospora crassa life cycle. Nevertheless, an active LINE-like retrotransposon, Tad, was found in a N. crassa strain from Adiopodoume. The possibility was tested that Tad might be resistant to RIP, or that the Adiopodoume strain might be incompetent for RIP. Tad elements derived from the Adiopodoume strain were found to be susceptible to RIP. In addition, strains lacking active Tad elements, including common laboratory strains and strains representing seven species of Neurospora, were found to have sequences closely related to Tad but with numerous mutations of the type resulting from RIP (G:C to A:T). Even the Adiopodoume strain showed Tad-like elements with mutations characteristic of RIP. Results of crossing of an Adiopodoume transformant with progeny of Adiopodoume suggest that the Adiopodoume strain is proficient at RIP. We conclude that Tad is an old transposable element that has been inactivated by RIP in most strains. Finding relics of RIP in both heterothallic and homothallic species of Neurospora implicates RIP across the genus.