The recent proliferation of studies on moveable genetic elements has suggested that genome rearrangement can contribute to the underlying mechanisms involved in cell differentiation1. Evidence to support this notion is rapidly accumulating in both prokaryotic and eukaryotic organisms 2-5. So far, however, relatively few eukaryotes have lent themselves to genetic analysis with resolution sufficient for the detection of genome rearrangement during cell differentiation. The filamentous fungi as exemplified here by Schizophyllum commune, provide an unusual experimental opportunity for detecting moveable genetic elements in that the organism grows as a binucleate heterokaryon. Each cell contains two compatible haploid nuclei which can be experimentally dissociated into uninucleate cells, and these cells can then be used in genetic analysis to detect genetic changes. We now present evidence that nuclei in cells from differentiated mound bodies show a specific genetic change whereas nuclei from surrounding non-mound vegetative cells show no detectable genetic changes. The binucleate cells of differentiated mound bodies were found to contain haploid nuclei one of which remained unchanged with regard to its genetic marker genes whereas the other nucleus, and always the same one, contained specific marker genes from the other nucleus. A unilateral transfer of gene copies between nuclei thus seems to be associated with mound cell differentiation. The genetic change is not the result of mutation and also is distinguished from somatic recombination6, but is similar to single factor transfer7. The transferred copies are stably integrated into the recipient genome and subsequently behave as mendelian genes.
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