Tad, a LINE-like transposable element of Neurospora, can transpose between nuclei in heterokaryons

The Tad transposon of Neurospora crassa appears to be a LINE-like element with very restricted distribution within the genus Neurospora. When forced heterokaryons were constructed between strains which did and did not contain Tad, the nuclei of the naive nuclear type rapidly acquired Tad elements. The elements acquired by naive nuclei are active, since they can pass Tad to other naive nuclei in subsequent heterokaryons. When heterokaryons are passaged by serial transfer, the load of acquired Tad elements appears to increase, indicating that transposition is continuing in these heterokaryons, even after all of the naive nuclei have acquired Tad. In normal heterokaryons of Neurospora, nuclei do not fuse. An experiment to test for the possibility that Tad promotes nuclear fusion gave negative results. Thus Tad appears to have a cytoplasmic intermediate in its transposition. When heterokaryon incompatible strains were cocultured, there was no indication that Tad elements could be transferred to the naive strain, suggesting that Tad is not a virus. These data are consistent with the transposition of Tad via RNA and cDNA intermediates, as has been postulated to occur with LINE-like elements.