Meiotic Drive vs. Meiotic Silencing. Intragenomic conflict in Neurospora

In order to prevent the uncontrolled expansion of selfish elements within their genomes, organisms have developed numerous mechanisms to monitor genome integrity. An important mechanism in fungi is meiotic silencing of unpaired DNA (MSUD). MSUD uses RNAi machinery to transcriptionally silence genomic regions that do not pair during meiosis. This mechanism has been studied thoroughly in the model fungus Neurospora crassa. In the closely related species N. sitophila, a type of meiotic drive known as spore killing has been observed. Crosses between strains containing the driving allele (Sk-1) and strains without the allele (sensitive), will result in the death of spores that do not possess Sk-1 with >90% efficiency. We have identified the Sk-1 locus, which contains ~1kb of novel sequence not found at the sensitive allele. Additoinally, analysis of the locus suggests that it was introduced from a closely related species. Based on studies in N. crassa, MSUD should silence Sk-1 in these crosses. We have observed that F1 strains from crosses between killer strains and sensitive strains from Tahiti show a reduced killing efficiency, as low as 20%, when backcrossed to the sensitive parent. To determine if this is due to the action of MSUD, we have generated a Sad-2 deletion mutant, which does not have a functional MSUD system. In the deletion strain, killing efficiency of the F1 strains is restored to nearly 100%. These results demonstrate that N. sitophila possesses an active MSUD defence system that is ineffective against Sk-1 in outcrosses, but effective in inbred lines. Furthermore, these results could not be replicated with strains from Europe, suggesting that Sk-1 is able to completely evade MSUD in these strains. This work highlights the interplay between genome defence and selfish elements and provides a powerful system in which to study MSUD and meiosis in general.