Genome quality control: RIP (repeat-induced point mutation) comes to Podospora.

RIP (repeat-induced point mutation) is a silencing process discovered in Neurospora crassa and so far clearly established only in this species as a currently occurring process. RIP acts premeiotically on duplicated sequences, resulting in C-G to T-A mutations, with a striking preference for CpA/TpG dinucleotides. In Podospora anserina, an RIP-like event was observed after several rounds of sexual reproduction in a strain with a 40 kb tandem duplication resulting from homologous integration of a cosmid in the mating-type region. The 9 kb sequenced show 106 C-G to T-A transitions, with 80% of the replaced cytosines located in CpA dinucleotides. This led to the alteration of at least six genes, two of which were unidentified. This RIP-like event extended to single-copy genes between the two members of the repeat. The overall data show that the silencing process is strikingly similar to a light form of RIP, unaccompanied by C-methylation. Interestingly, the N. crassa zeta-eta sequence, which acts as a potent de novo C-methylation RIP signal in this species, is weakly methylated when introduced into P. anserina. These results demonstrate that RIP, at least in light forms, can occur beyond N. crassa.

ami1, an orthologue of the Aspergillus nidulans apsA gene, is involved in nuclear migration events throughout the life cycle of Podospora anserina.

The Podospora anserina ami1-1 mutant was identified as a male-sterile strain. Microconidia (which act as male gametes) form, but are anucleate. Paraphysae from the perithecium beaks are also anucleate when ami1-1 is used as the female partner in a cross. Furthermore, in crosses heterozygous for ami1-1, some crozier cells are uninucleate rather than binucleate. In addition to these nuclear migration defects, which occur at the transition between syncytial and cellular states, ami1-1 causes abnormal distribution of the nuclei in both mycelial filaments and asci. Finally, an ami1-1 strain bearing information for both mating types is unable to self-fertilize. The ami1 gene is an orthologue of the Aspergillus nidulans apsA gene, which controls nuclear positioning in filaments and during conidiogenesis (at the syncytial/cellular transition). The ApsA and AMI1 proteins display 42% identity and share structural features. The apsA gene complements some ami1-1 defects: it increases the percentage of nucleate microconidia and restores self-fertility in an ami1-1 mat+ (mat-) strain. The latter effect is puzzling, since in apsA null mutants sexual reproduction is quite normal. The functional differences between the two genes are discussed with respect to their possible history in these two fungi, which are very distant in terms of evolution.

A proposed new contiguous gene syndrome on 8q consists of Branchio-Oto-Renal (BOR) syndrome, Duane syndrome, a dominant form of hydrocephalus and trapeze aplasia; implications for the mapping of the BOR gene.

The analysis of a de novo 8q12.2-q21.2 deletion led to the identification of a proposed previously undescribed contiguous gene syndrome consisting of Branchio-Oto-Renal (BOR) syndrome, Duane syndrome, hydrocephalus and trapeze aplasia. This is the first reported localization of the genes responsible for Duane syndrome and this dominant form of hydrocephalus. In contrast, we report a new localization for the gene responsible for BOR syndrome which is more telomeric to an initial placement. Linkage analysis of affected families consistently mapped the gene responsible for BOR and Branchio-Oto (BO) syndromes to within the deletion. Using new algorithms, a YAC contig was constructed and used to localize the breakpoint of another chromosomal rearrangement associated with BO syndrome to a 500 kb interval within the deletion. The 8q12.2-q21.2 deletion suggests that reduced dosage of the relevant genes is sufficient to cause Duane syndrome, BOR syndrome and this dominant form of hydrocephalus.