Transposons are ubiquitous genetic elements discovered so far in all investigated prokaryotes and eukaryotes. In remarkable contrast to all other genes, transposable elements are able to move to new locations within their host genomes. Transposition of transposons into coding sequences and their initiation of chromosome rearrangements have tremendous impact on gene expression and genome evolution. While transposons have long been known in bacteria, plants, and animals, only in recent years has there been a significant increase in the number of transposable elements discovered in filamentous fungi. Like those of other eukaryotes, each fungal transposable element is either of class or of class II. While class I elements transpose by a RNA intermediate and employ reverse transcriptases, class II elements transpose directly at the DNA level. We present structural and functional features for such transposons that have been identified so far in filamentous fungi. Emphasis is given to specific advantages or unique features when fungal systems are used to study transposable elements, e.g., the evolutionary impact of transposons in coenocytic organisms and possible experimental approaches toward horizontal gene transfer. Finally, we focus on the potential of transposons for tagging and identifying fungal genes.

/pdf/transposons-in-filamentous-fungi-facts-and-perspectives-2v9gxjs7j2.pdf

Transposons in filamentous fungi--facts and perspectives.

When a gene complex called Spore killer is heterozygous, ascospores representing two of the four products of each meiosis are killed. Only those that receive the killer complex survive. This article reviews what is known of the mode of action of the Neurospora Spore killers, their chromosomal basis, and their occurrence in nature. Similar genes or gene com- plexes have been found in other fungi. An example from Podospora anserina is analyzed. The best-studied example of meiotic drive in fungi is Spore killer in Neuro- spora, which has much in common with the Segregation Distorter (SD) complex in Drosophila and the t complex in mice. The effects are manifested postmeioti- cally in all three systems, but the results of drive are seen most directly in Neuro- spora. With the SD complex and with the t haplotypes, it is necessary to produce a heterozygous male and then to obtain and analyze diploid progeny in order to assess the effects of that heterozygosity in the father. Electron microscopy of heterozygous SD crosses can be used to confirm that about half of the sperm deteriorate, but this procedure is far too demanding for routine use. In Neuro- spora, by contrast, Spore killer is manifested in ascospores that can be seen under low magnification with a dissecting microscope. These ascospores are the immediate products of meiosis; yet they differ from sperm in being themselves the progeny. In Neurospora crassa, each fertilization event in a cross results in a fruiting body within which haploid nuclei proliferate in dikaryotic fashion, producing many pairs of nuclei of opposite mating type that fuse and go into meiosis, each in a separate ascus. As with male meiosis in plants and animals, all four meiotic products normally survive. Each mature ascus normally contains eight viable black ascospores, which originated from the same meiosis (the four immediate meiotic products undergo an additional mitosis before ascospore walls enclose the eight daughter nuclei). The result is essentially four sets of identical twins. A feature of ascospore formation that is important for Spore killer is the timing of cell-wall formation. Meiosis and the postmeiotic mitosis occur in a common cytoplasm. No cell walls are present in the developing ascus until the ascospore walls are formed. (For a diagram of the Neurospora life cycle, see Perkins and Barry 1977; for an illustrated account of events in the ascus, see Raju 1980.) Spore killer was discovered because of its dramatic effect on the appearance of ascospores. Viable ascospores are black, but inviable ascospores remain small and unpigmented. When a Spore-killer strain is crossed with the standard wild

Meiotic Drive in Neurospora and Other Fungi

A plasmid-like DNA molecule was isolated from the fungus Ascobolus immersus in strains which exhibited genetic instability. It was found to be inherited maternally. The molecule had a density of 1.708 g/cc and a molecular weight of 4.1×106 daltons. Digestion simultaneously by two restriction enzymes demonstrated the linearity of the molecule. Electron microscopy of alkaline-denatured and self-renatured molecules revealed that the ends are inverted repeated sequences, 1200 base pairs in length.

Isolation and characterization of a linear DNA molecule in the fungus Ascobolus immersus

In seven out of eleven wild strains of the Ascomycete Ascobolus immersus plasmid DNA was found. There was great variability with respect to size and number of the plasmids in the strains concerned. For a further analysis two plasmids originating from one wild strain were submitted to restriction analysis and electron microscopy. Both turned out to be linear having different molecular weights (pAIl = 7.9 kb, pAI2 = 5.6 kb). Denaturation of pAI2 and subsequent renaturation revealed the presence of inverted repeats (0.7 kb) at both ends. After treatment with proteinase K and 5′ and 3′ specific exonucleases it became evident that the 5′ ends of pAI2 are linked with proteins. In this respect it is similar in structure to other linear genetic elements such as the linear plasmids found in Zea mays and the genomes of adenoviruses.

Proteins are attached to the ends of a linear plasmid in the filamentous fungus Ascobolus immersus

In the first edition of The Mycota Volume II, published in 1995, one paper by Poggeler (Fasbender and Kuck 1995) and a second one by Kempken (1995) discussed reverse transcriptase activity and plasmids, respectively. Transposons were barely covered at that time, mostly due to the fact that little was known about transposons in mycelial fungi. When we were both asked by the editor to provide updated reviews for our subjects, we got together and quickly decided that a joint article would be the best way to provide a comprehensive overview dealing with diverse genetic elements such as introns, plasmids or transposons.

Mobile Genetic Elements in Mycelial Fungi

Genetic instability in Ascobolus immersus: modalities of back-mutations, intragenic mapping of unstable sites, and unstable insertion. Preliminary biochemical data.

SummaryNew unstable mutants of Ascobolus immersus involving the color or size of ascospores were sought among spontaneous mutants. Among the 34 unstable mutants isolated, 31 had white spores, 2 had pink spores and 1 had a large sized spores. The unstable mutants involve 11 loci whose mutation leads to white spores and 2 loci whose mutations give pink spores, among the 19 loci known to be implicated in this character; 1 locus is defined by only one large spore mutant. All these genes are localized on at least 7 different chromosomes. Unstable mutants of the same locus may correspond to several different sites, but the number of these sites is very limited.The frequency of unstable mutations was estimated: among 36 mutants belonging to 8 different genes, 20 were stable and 16 were unstable. This high frequency of unstable mutants is undoubtedly underestimated. The moment of reversion of 23 of these new mutants was also sought: 15 of them revert as does mutant B, previously studied, in the very young mycelium, at high temperature and with a reversion frequency of 0.004 to 0.34, according to the mutant; 5 of them revert as mutant 301, also previously studied, during the development of the fruit-body and with a frequency of 0.009 to 0.035; two of these mutants revert very early in the ascospore as soon as the first mitoses or in the very young mycelium at 22° C, with a very high reversion frequency that may reach 1.0; finally, the last mutant studied reverts in the fruit-body with a frequency reaching 0.40, but with modalities different from mutant 301. The mutants of the same locus may revert with different modalities. The same modality may correspond to different sites of the same gene.In unstable double-mutant strains involving two different genes, the reversion of one is independent of the reversion of the other, whether or not the reversion modailities of each mutant are identical.Results indicate the existence of inducers common to several unstable mutants which present the same modalities of reversion.These data support the previously formulated hypothesis of transposable elements.

Unstable ascospore color mutants of Ascobolus immersus: III. Mapping, temporal occurrences and modalities of back-mutations of 34 New unstable mutants

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Genetic instability in Ascobolus immersus: modalities of back-mutations, intragenic mapping of unstable sites, and unstable insertion. Preliminary biochemical data.

Unstable ascospore color mutants of Ascobolus immersus: III. Mapping, temporal occurrences and modalities of back-mutations of 34 New unstable mutants