Translocations used to generate chromosome segment duplications in Neurospora can disrupt genes and create novel open reading frames.

In Neurospora crassa, crosses between normal sequence strains and strains bearing some translocations can yield progeny bearing a duplication (Dp) of the translocated chromosome segment. Here, 30 breakpoint junction sequences of 12 Dp-generating translocations were determined. The breakpoints disrupted 13 genes (including predicted genes), and created 10 novel open reading frames. Insertion of sequences from LG III into LG I as translocation T(UK8-18) disrupts the eat-3 gene, which is the ortholog of the Podospora anserine gene ami1. Since ami1-homozygous Podospora crosses were reported to increase the frequency of repeat-induced point mutation (RIP), we performed crosses homozygous for a deficiency in eat-3 to test for a corresponding increase in RIP frequency. However, our results suggested that, unlike in Podospora, the eat-3 gene might be essential for ascus development in Neurospora. Duplication-heterozygous crosses are generally barren in Neurospora; however, by using molecular probes developed in this study, we could identify Dp segregants from two different translocation-heterozygous crosses, and using these we found that the barren phenotype of at least some duplication-heterozygous crosses was incompletely penetrant.

Neurospora crassa fmf-1 encodes the homologue of the Schizosaccharomyces pombe Ste11p regulator of sexual development.

The Neurospora crassa fmf-1 mutation exerts an unusual 'perithecium-dominant' developmental arrest; fmf-1 x fmf-1+ cross becomes arrested in perithecial development regardless of whether the mutant participates in the cross as the male or female parent. We localized fmf-1 to the LG IL genome segment between the centromere-proximal breakpoint of the chromosome segment duplication Dp(IL)39311 and the centromere. By mapping crossovers with respect to RFLP markers in this region we further localized fmf-1 to an approximately 34-kb-genome segment. Partial sequencing of this segment revealed a point mutation in the gene NCU 09387.1, a homologue of the Schizosaccharomyces pombe ste11+ regulator of sexual development. The fmf-1 mutation did not complement a NCU 09387.1 deletion mutation, and transformation with wild-type NCU 09387.1 complemented fmf-1. S. pombe Ste11 protein (Ste11p) is a transcription factor required for sexual differentiation and for the expression of genes required for mating pheromone signalling in matP and matM cells. If FMF-1 also plays a corresponding role in mating pheromone signalling in Neurospora, then protoperithecia in an fmf-1 x fmf-1+ cross would be unable to either send or receive sexual differentiation signals and thus become arrested in development.

Chromosome segment duplications in Neurospora crassa: barren crosses beget fertile science.

Studies on Neurospora chromosome segment duplications (Dps) performed since the publication of Perkins's comprehensive review in 1997 form the focus of this article. We present a brief summary of Perkins's seminal work on chromosome rearrangements, specifically, the identification of insertional and quasiterminal translocations that can segregate Dp progeny when crossed with normal sequence strains (i.e., T x N). We describe the genome defense process called meiotic silencing by unpaired DNA that renders Dp-heterozygous crosses (i.e., Dp x N) barren, which provides a basis for identifying Dps, and discuss whether other processes also might contribute to the barren phenotype of Dp x N and Dp x Dp crosses. We then turn to studies suggesting that large Dps (i.e., >300 kbp) can allow smaller gene-sized duplications to escape another genome defense process called repeat-induced point mutation (RIP), possibly by titration of the RIP machinery. Finally, we assess whether in natural populations dominant RIP suppressor Dps provide an "RIP-free" niche for evolution of new genes following the duplication of existing genes.