THE NEUROSPORA TRANSPOSON TAD IS SENSITIVE TO REPEAT-INDUCED POINT MUTATION (RIP)

RIP (repeat-induced point mutation) efficiently mutates repeated seque nces in the sexual phase of the Neurospora crassa life cycle. Neverthe less, an active LINE-like retrotransposon, Tad, was found in a N. cras sa strain from Adiopodoume. The possibility was tested that Tad might be resistant to RIP, or that the Adiopodoume strain might be incompete nt for RIP. Tad elements derived from the Adiopodoume strain were foun d to be susceptible to RIP. In addition, strains lacking active Tad el ements, including common laboratory strains and strains representing s even species of Neurospora, were found to have sequences closely relat ed 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 RTP. Results of crossing of an Adiopodoum e transformant with progeny of Adiopodoume suggest that the Adiopodoum e strain is proficient at RIP. We conclude that Tad is an old transpos able element that has been inactivated by RIP in most strains. Finding relics of RIP in both heterothallic and homothallic species of Neuros pora implicates RIP across the genus.