Does a trans-species polymorphism affect meiotic silencing by unpaired DNA in Neurospora?

Meiotic silencing by unpaired DNA (MSUD) was discovered in crosses made in the Neurospora crassa standard OR genetic background. We have found that crosses in the novel N. crassa B/S1 and the N. tetrasperma 85 backgrounds show a relatively less efficient silencing of unpaired genes and increase in inappropriate silencing of paired genes. Crosses in the OR, B/S1, and 85 backgrounds heterozygous for an ::r^ec transgene produced, respectively, > 97%, 24%, and 7% MSUD-induced round ascospores, whereas the corresponding ::r^ec-homozygous crosses produced < 1%, 6%, and 1.4%. Additionally, Dp-heterozygous crosses in 85 were non-barren whereas those in OR were barren. Analysis of the f1 progeny from the OR x B/S1 crosses suggested that as few as three unlinked loci might determine these differences between the parental strains.

Results of Novak and Srb (Can. J. Genet. Cytol. 15: 685-693, 1973) suggest that MSUD efficiency in the N. tetrasperma T-220 strain might be similar to that in N. crassa OR. If such is the case, the difference between OR and B/S1 in N. crassa would be paralleled by that between T-220 and 85 in N. tetrasperma, and suggest a trans-species polymorphism (TSP) in MSUD efficiency. TSPs in Neurospora are associated with self/non-self recognition systems. Allorecognition protects against the spread of mycoviruses, and MSUD might protect against the spread of transposable elements, and it is not inconceivable that alternative alleles of one or more allorecognition system have differential effects on MSUD efficiency.

Acknowledgements: DPK is an Indian National Science Academy Senior Scientist, and DAG is a recipient of the Council of Scientific and Industrial Research-University Grants Commission Senior Research Fellowship.