Denise Zickler

A nonmammalian homolog of the PAF7 gene(Zellweger syndrome) discovered as a gene involved in caryogamy in the fungus Podospora anserina

The car1 gene of the filamentous fungus Podospora anserina was cloned by complementation of a mutant defective for caryogamy (nuclear fusion), a process required for sexual sporulation. This gene encodes a protein that shows similarity to the mammalian PAF1 protein (Zellweger syndrome). Besides sequence similarity, the two proteins share a transmembrane domain and the same type of zinc finger motif. A combination of molecular, physiological, genetical, and ultrastructural approaches gave evidence that the P. anserina car1 protein is actually a peroxisomal protein. This study shows that peroxisomes are required at a specific stage of sexual development, at least in P. anserina, and that a functional homolog of the PAF1 gene is present in a lower eucaryote.

The Peroxisome RING‐Finger Complex is Required for Meiocyte Formation in the Fungus Podospora anserina

Peroxisomes are involved in a variety of metabolic pathways and developmental processes. In the filamentous fungus Podospora anserina, absence of different peroxins implicated in peroxisome matrix protein import leads to different developmental defects. Lack of the RING‐finger complex peroxin PEX2 blocks sexual development at the dikaryotic stage, while in absence of both receptors, PEX5 and PEX7, karyogamy and meiosis can proceed and sexual spores are formed. This suggests a complex role for PEX2 that prompted us to study the developmental involvement of the RING‐finger complex. We show that, like PEX2, the two other proteins of the complex, PEX10 and PEX12, are equally implicated in peroxisome biogenesis and that absence of each or all these proteins lead to the same developmental defect. Moreover, we demonstrate that peroxisome localization of PEX2 is not drastically affected in the absence of PEX10 and PEX12 and that the upregulation of these latter RING‐finger peroxins does not compensate for the lack of a second one, suggesting that the three proteins work together in development but independent of their function in peroxisome biogenesis.

Wood Utilization Is Dependent on Catalase Activities in the Filamentous Fungus Podospora anserina

Catalases are enzymes that play critical roles in protecting cells against the toxic effects of hydrogen peroxide. They are implicated in various physiological and pathological conditions but some of their functions remain unclear. In order to decipher the role(s) of catalases during the life cycle of Podospora anserina, we analyzed the role of the four monofunctional catalases and one bifunctional catalase-peroxidase genes present in its genome. The five genes were deleted and the phenotypes of each single and all multiple mutants were investigated. Intriguingly, although the genes are differently expressed during the life cycle, catalase activity is dispensable during both vegetative growth and sexual reproduction in laboratory conditions. Catalases are also not essential for cellulose or fatty acid assimilation. In contrast, they are strictly required for efficient utilization of more complex biomass like wood shavings by allowing growth in the presence of lignin. The secreted CATB and cytosolic CAT2 are the major catalases implicated in peroxide resistance, while CAT2 is the major player during complex biomass assimilation. Our results suggest that P. anserina produces external H2O2 to assimilate complex biomass and that catalases are necessary to protect the cells during this process. In addition, the phenotypes of strains lacking only one catalase gene suggest that a decrease of catalase activity improves the capacity of the fungus to degrade complex biomass.

The importomer peroxins are differentially required for peroxisome assembly and meiotic development in Podospora anserina: insights into a new peroxisome import pathway.

Peroxisome biogenesis relies on two known peroxisome matrix protein import pathways that are mediated by the receptors PEX5 and PEX7. These pathways converge at the importomer, a peroxisome‐membrane complex that is required for protein translocation into peroxisomes and consists of docking and RING–finger subcomplexes. In the fungus Podospora anserina, the RING–finger peroxins are crucial for meiocyte formation, while PEX5, PEX7 or the docking peroxin PEX14 are not. Here we show that PEX14 and the PEX14‐related protein PEX14/17 are differentially involved in peroxisome import during development. PEX14/17 activity does not compensate for loss of PEX14 function, and elimination of both proteins has no effect on meiocyte differentiation. In contrast, the docking peroxin PEX13, and the peroxins implicated in peroxisome membrane biogenesis PEX3 and PEX19, are required for meiocyte formation. Remarkably, the PTS2 coreceptor PEX20 is also essential for meiocyte differentiation and this function does not require PEX5 or PEX7. This finding suggests that PEX20 can mediate the import receptor activity of specific peroxisome matrix proteins. Our results suggest a new pathway for peroxisome import, which relies on PEX20 as import receptor and which seems critically required for specific developmental processes, like meiocyte differentiation in P. anserina.

Peroxisomal ABC transporters and β-oxidation during the life cycle of the filamentous fungus Podospora anserina

ATP-binding cassette transporters are ubiquitous proteins that facilitate transport of diverse substances across a membrane. However, their exact role remains poorly understood. In order to test their function in a fungus life cycle, we deleted the two Podospora anserina peroxisomal ABC transporter pABC1 and pABC2 genes as well as the three genes involved in peroxisomal (fox2) and mitochondrial (scdA and echA) beta-oxidation. Analysis of the single and double mutants shows that fatty acid beta-oxidation occurs in both organelles. Furthermore, the peroxisomal and mitochondrial fatty acid beta-oxidation pathways are both dispensable for vegetative and sexual development. They are, however, differently required for ascospore pigmentation and germination, this latter defect being restored in a DeltapABC1 and DeltapABC2 background. We report also that lack of peroxisomal ABC transporters does not prevent peroxisomal long-chain fatty acid oxidation, suggesting the existence of another pathway for their import into peroxisomes. Finally, we show that some aspects of fatty acid degradation are clearly fungus species specific.

La Theorie de la Chiasmatypie: Nouvelle interprétation des cinèses de maturation

First published in 1909 in La Cellule, F. A. Janssens’s “The chiasmatype theory. A new interpretation of the maturation divisions” proved controversial and was for several decades resisted by many geneticists and cytologists. In this month’s Perspectives, Koszul et al. revisit Janssenss findings and the surrounding controversies.Here, translated for the first time in English, GENETICS republishes the original Janssens’s article. The article is presented as closely as possible to the original publication’s format and style. GENETICS wishes to thank Romain Koszul and Denise Zickler for their labors in presenting the scientific community with this new translation.First published in 1909 in La Cellule, F. A. Janssens’s “The chiasmatype theory. A new interpretation of the maturation divisions” proved controversial and was for several decades resisted by many geneticists and cytologists. In this month’s Perspectives, Koszul et al. revisit Janssenss findings and the surrounding controversies. Here, translated for the first time in English, GENETICS republishes the original Janssens’s article. The article is presented as closely as possible to the original publication’s format and style.2 GENETICS wishes to thank Romain Koszul and Denise Zickler for their labors in presenting the scientific community with this new translation.

Asy2/Mer2: an evolutionarily conserved mediator of meiotic recombination, pairing, and global chromosome compaction

Meiosis is the cellular program by which a diploid cell gives rise to haploid gametes for sexual reproduction. Meiotic progression depends on tight physical and functional coupling of recombination steps at the DNA level with specific organizational features of meiotic-prophase chromosomes. The present study reveals that every step of this coupling is mediated by a single molecule: Asy2/Mer2. We show that Mer2, identified so far only in budding and fission yeasts, is in fact evolutionarily conserved from fungi (Mer2/Rec15/Asy2/Bad42) to plants (PRD3/PAIR1) and mammals (IHO1). In yeasts, Mer2 mediates assembly of recombination-initiation complexes and double-strand breaks (DSBs). This role is conserved in the fungus Sordaria However, functional analysis of 13 mer2 mutants and successive localization of Mer2 to axis, synaptonemal complex (SC), and chromatin revealed, in addition, three further important functions. First, after DSB formation, Mer2 is required for pairing by mediating homolog spatial juxtaposition, with implications for crossover (CO) patterning/interference. Second, Mer2 participates in the transfer/maintenance and release of recombination complexes to/from the SC central region. Third, after completion of recombination, potentially dependent on SUMOylation, Mer2 mediates global chromosome compaction and post-recombination chiasma development. Thus, beyond its role as a recombinosome-axis/SC linker molecule, Mer2 has important functions in relation to basic chromosome structure.

Peroxisome dynamics during development of the fungus Podospora anserina.

Peroxisomes are versatile and dynamic organelles that are required for the development of diverse eukaryotic organisms. We demonstrated previously that in the fungus Podospora anserina different peroxisomal functions are required at distinct stages of sexual development, including the initiation and progression of meiocyte (ascus) development and the differentiation and germination of sexual spores (ascospores). Peroxisome assembly during these processes relies on the differential activity of the protein machinery that drives the import of proteins into the organelle, indicating a complex developmental regulation of peroxisome formation and activity. Here we demonstrate that peroxisome dynamics is also highly regulated during development. We show that peroxisomes in P. anserina are highly dynamic and respond to metabolic and environmental cues by undergoing changes in size, morphology and number. In addition, peroxisomes of vegetative and sexual cell types are structurally different. During sexual development peroxisome number increases at two stages: at early ascus differentiation and during ascospore formation. These processes are accompanied by changes in peroxisome structure and distribution, which include a cell-polarized concentration of peroxisomes at the beginning of ascus development, as well as a morphological transition from predominantly spherical to elongated shapes at the end of the first meiotic division. Further, the mostly tubular peroxisomes present from second meiotic division to early ascospore formation again become rounded during ascospore differentiation. Ultimately the number of peroxisomes dramatically decreases upon ascospore maturation. Our results reveal a precise regulation of peroxisome dynamics during sexual development and suggest that peroxisome constitution and function during development is defined by the coordinated regulation of the proteins that control peroxisome assembly and dynamics.

La Theorie de la Chiasmatypie

First published in 1909 in La Cellule, F. A. Janssens’s “The chiasmatype theory. A new interpretation of the maturation divisions” proved controversial and was for several decades resisted by many geneticists and cytologists. In this month’s Perspectives, Koszul et al. revisit Janssenss findings and the surrounding controversies.Here, translated for the first time in English, GENETICS republishes the original Janssens’s article. The article is presented as closely as possible to the original publication’s format and style. GENETICS wishes to thank Romain Koszul and Denise Zickler for their labors in presenting the scientific community with this new translation.First published in 1909 in La Cellule, F. A. Janssens’s “The chiasmatype theory. A new interpretation of the maturation divisions” proved controversial and was for several decades resisted by many geneticists and cytologists. In this month’s Perspectives, Koszul et al. revisit Janssenss findings and the surrounding controversies. Here, translated for the first time in English, GENETICS republishes the original Janssens’s article. The article is presented as closely as possible to the original publication’s format and style.2 GENETICS wishes to thank Romain Koszul and Denise Zickler for their labors in presenting the scientific community with this new translation.