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 × N). We describe the genome defense process called meiotic silencing by unpaired DNA that renders Dp‐heterozygous crosses (i.e., Dp × N) barren, which (...) provides a basis for identifying Dps, and discuss whether other processes also might contribute to the barren phenotype of Dp × N and Dp × 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. (shrink)

RésuméĽétude du développement?on;un organisme fait appel à diverses approches choisies en fonction des objectifs fixés par ľexpérimentateur. Ainsi, la mise en évidence?on;une corrélation entre deux paramétres, par exemple biochimique et morphologique, permet de les considérer comme associés, sans nécessairement dégager entre eux un lien direct de causalité. Ce dernier peut être explicité par une autre approche, comme ľillustre une étude portant sur la relation entre une enzyme ‐glycohydrolase) et la différentiation sporale chez Neurospora crassa.SummaryThe study of the development (...) of an organism requires diverse approaches selected on the basis of experimental objectives. For instance, the finding of a correlation between two parameters, i.e. biochemical and morphological, allows them to be considered as associated without necessarily pointing to a direct causal relationship. This latter may be brought out by another approach, as illustrated by this study bearing on the relation between an enzyme ‐gly‐cohydrolase) and differentiation of spores in Neurospora crassa.ZusammenfassungDas Studium der Entwicklung eines Organismus erfordert verschiedenartige Annäherungen, je nach den vom Experimendierenden gestellten Ziele. Daher erlaubt der Nachweis einer Korrela‐tion zwischen zwei Parametern, zum Beispiel einem biochemischen und einem morphologischen, sie als verbunden zu betrachten ohne notwendigerweise eine Kausalbeziehung zwischen Ihnen zu beweisen. Eine solche kann von einer anderen Seite geklärt werden, wie es eine Studie der Beziehung zwischen einem Enzym ‐glycohydrolase) und der Sporendifferenzierung bei Neurospora crassa zeigt. (shrink)

Sporulation in the mold Neurospora crussa can proceed along three very different pathways, leading to the production of three types of spores. Two asexual sporulation pathways that lead to the formation of macroconidia and microconidia involve budding from hyphae by two different mechanisms. A much more complex sexual reproductive pathway involves the formation of a fruiting body called a perithecium, in which meiosis takes place and ascospores are formed in sac‐like cells called asci. Numerous mutations exist that affect these (...) developmental pathways, and genes have been isolated that are expressed preferentially during sporulation. The Neurospora sporulation pathways offer a simple system with which to study mechanisms and regulation of development that are usually obscured by complex cell‐cell interactions involved in animal and plant development. (shrink)

In the heterothallic species Neurospora crassa, strains of opposite mating type, A and a, must interact to give the series of events resulting in fruiting body formation, meiosis, and the generation of dormant ascospores. The mating type of a strain is specified by the DNA sequence it carries in the mating type region; strains that are otherwise isogenic can mate and produce ascospores. The DNA of the A and a regions have completely dissimilar sequences. Probing DNA from strains (...) of each mating type with labelled sequences from the A and the a regions has shown that, unlike in Saccharomyces cerevisiae, only a single copy of a mating type sequence is present in a haploid genome. The failure to switch is explainable by the physical absence of DNA sequences characteristic of the opposite mating type. While the mating type sequences must be of the opposite kind for mating to occur in the sexual cycle, two strains of opposite mating type cannot form a stable heterokaryon during vegetative growth; instead, they fuse abortively to give a heterokaryon incompatibility reaction, which results in death of the cells along the fusion line. The DNA sequences responsible for this reaction are coextensive with those sequences in the A and a regions which are necessary to initiate fruiting body formation. The genus Neurospora also includes homothallic species – ones in which a single haploid nucleus carries all the information necessary to form fruiting bodies, undergo meiosis, and produce new haploid spores. One such species, N. terricola, contains one copy each of the A and the a sequences within each haploid genome. Another homothallic species, N. africana, contains a single copy of the A sequence in the haploid genome, but does not contain any sequences corresponding to the a sequence. We present a model for the role of the mating type‐specific sequences in heterothallic and homothallic species of Neurospora and we speculate on the origin of the different modes of reproduction in the genus Neurospora. (shrink)

Here we elucidate a paradox: how a single chemoattractant‐receptor system in two individuals is used for communication despite the seeming inevitability of self‐excitation. In the filamentous fungus Neurospora crassa, genetically identical cells that produce the same chemoattractant fuse via the homing of individual cell protrusions toward each other. This is achieved via a recently described “ping‐pong” pulsatile communication. Using a generic activator‐inhibitor model of excitable behavior, we demonstrate that the pulse exchange can be fully understood in terms of (...) two excitable systems locked into a stable oscillatory pattern of mutual excitation. The most puzzling properties of this communication are the sudden onset of oscillations with final amplitude, and the absence of seemingly inevitable self‐excitation. We show that these properties result directly from both the excitability threshold and refractory period characteristic of excitable systems. Our model suggests possible molecular mechanisms for the ping‐pong communication. (shrink)

Since the practice turn, the role technologies play in the production of scientific knowledge has become a prominent topic in science studies. Much existing scholarship, however, either limits technology to merely mechanical instrumentation or uses the term for a wide variety of items. This article argues that technologies in scientific practice can be understood as a result of past scientific knowledge becoming sedimented in materials, like model organisms, synthetic reagents or mechanical instruments, through the routine use of these materials in (...) subsequent research practice. The proposed theoretical interpretation of technology is examined through a case where a model organism—Drosophila melanogaster—acted as a technology for investigating a contested biological effect of a mechanical instrument: Hermann J. Muller’s experiments on X-ray mutagenicity in the 1920s. The article reconstructs how Muller employed two synthetic Drosophila stocks as tests for measuring X-rays’ capacity to cause genetic aberration. It argues that past scientific knowledge sedimented in the Drosophila stocks influenced Muller’s perception of X-ray-induced mutation. It further describes how Muller’s concept of X-ray mutagenicity sedimented through the adoption of X-ray machines as a ready-made resource for producing mutants by other geneticists, for instance George Beadle and Edward Tatum in their experiments on Neurospora crassa, despite ongoing disputes surrounding Muller’s conclusions. Technological sedimentation is proposed as a potential explanation why sedimentation and disputation may often coexist in the history of science. (shrink)

During meiosis, homologous chromosomes must pair in order to permit recombination and correct chromosome segregation to occur. Two recent papers1,2 show that meiotic pairing is also important for correct gene expression during meiosis. They describe data for the filamentous fungus Neurospora crassa that show that a lack of pairing generated by ectopic integration of genes can result in silencing of genes expressed during meiosis. This can result in aberrant meioses whose defects are specific to the function of the (...) unpaired gene. Furthermore, mutations affecting the silencing mechanism have been selected in a gene encoding a putative RNA‐dependent RNA polymerase. This finding indicates the involvement of a meiotic specific post‐transcriptional gene silencing mechanism (PTGS) similar to that observed in vegetative cells in N. crassa and other organisms. Finally, this gene product is essential for normal meiosis, suggesting that RNA‐dependent processes are fundamental to the sexual cycle. BioEssays 25:99–103, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Graphical AbstractThe fungus Neurospora comprises a novel model for testing hypotheses involving the role of sex and reproduction in eukaryotic genome evolution. Its variation in reproductive mode, lack of sex-specific genotypes, availability of phylogenetic species, and young sex-regulating chromosomes make research in this genus complementary to animal and plant models.

This book explains the role of simple biological model systems in the growth of molecular biology. Essentially the whole history of molecular biology is presented here, tracing the work in bacteriophages in E. coli, the role of other prokaryotic systems, and also the protozoan and algal models—Paramecium and Chlamydomonas, primarily—and the move into eukaryotes with the fungal systems Neurospora, Aspergillus and yeast. Each model was selected for its appropriateness for asking a given class of questions, and each spawned its (...) own community of investigators. Some individuals made the transition to a new model over time, and remnant communities of investigators continue to pursue questions in all these models, as the cutting edge of molecular biological research flowed onward from model to model, and onward into higher organisms and, ultimately, mouse and man. (shrink)

Genetic analysis is revealing molecular components of circadian rhythms. The gene products of the period gene in Drosophila and the frequency gene in Neurospora oscillate with a circadian rhythm. A recent paper(1) has shown that the PERIOD protein can undergo both intermolecular and intramolecular interactions in vitro. The effects of temperature and two period mutations on these molecular interactions were compared to the effects of the mutations and temperature on the in vivo period length of circadian rhythms. The results (...) suggest that the molecular interactions may compete to maintain a rhythm with a constant period over a wide temperature range. (shrink)

The existence and properties of the chloroplast genome were established by a combination of genetic methods which identified chloroplast mutations and placed them into a linear sequence or map; and by chemical methods, CsCl density gradient ultracentrifugation and base analysis, which identified non‐nuclear DNA extracted from isolated chloroplasts. These studies, carried out in the 1950s and 1960s, primarily with Chlamydomonas, as well as parallel studies of mitochondrial DNA with yeast and Neurospora, laid the framework for distinguishing organelle and nuclear (...) genomes. On this basis, the coding and regulatory functions of three genomes – nuclear, chloroplast, and mitochondrial – are being addressed in modern plant molecular biology. (shrink)

Mutations that disrupt biological rhythms have existed in microbial and metazoan eukaryotes for some time. They have recently begun to be studied with increasing intensity, both in terms of phenotypic effects of the relevant genetic variants, and with regard to molecular isolation and analysis of the genes defined by two of the ‘clock mutations’. These genetic loci, called period (per) in Drosophila and frequency (frq) in Neurospora, influence not only the basic characteristics of circadian rhythmicity, but also temperature compensation (...) of such daily cycle durations, ‘re‐setting’ of the rhythms' phases, and (for the per mutants) behavioral oscillations associated with much shorter than circadian periodicities. Molecular cloning of Drosophila's per gene, accompanied crucially by the locus's identification in germ‐line transformants, has led to information on its expression ‐ temporally, spatially, and in regard to heterogeneity of mRNA types. Nucleotide‐sequencing analyses of genomic DNA (and/or cDNA) from normal and mutated per alleles have (1) led to the suggestion that this clock gene encodes a family of proteoglycans (which was further indicated by application of antibody reagents obtained by manipulation of one of the gene's exons); and (2) shown that the three types of per mutations ‐ which shorten or lengthen rhythm periodicities or appear to eliminate them ‐ are associated with interesting amino‐acid substitutions or a stop codon, respectively. Analogous molecular findings are awaited from Neurospora, whose frq gene has very recently been cloned and definitively identified, in part via transformation experiments. (shrink)