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1.
Summary New 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.  相似文献   

2.
Summary A fine structure map of gene b5 has been established in Ascobolus immersus and the unstable mutant site b5-301 (phenotype: ascospore coloration) has been found to map within this gene. This map was constructed using seven b5 mutants induced by ICR170 and is based on the additivity of recombinant frequencies and confirmed by three point tests. The unstable site 301 is located between the induced sites. In particular, mutant 249 is located to the left of site 301, whereas sites 601 and 754 are located to the right.Previous studies showed that the inducing gene of mutant b5-301 reversions are either closely linked to the b5 locus or within it in certain strains. The study of asci resulting from reciprocal recombination between unstable mutant site and several induced mutant sites showed that neither crossovers located on the left nor the right of site 301, separate the unstable site from the inducing gene. Thus, the inducing gene was found to map within gene b5 as did the inducible site.These results constitute a genetic argument showing the presence of an insertion element. In this case, the insertion structure contains at least the integration site (inducible site) and the inducing gene which allows the excision.  相似文献   

3.
Summary The conversion spectrum of fifteen mutants giving post-meiotic segregations and located in the b2 locus of Ascobolus immersus was studied in 77 mutant x wild-type crosses. These mutants all yield aberrant 4:4 asci, mutants located in the right portion of the locus yielding more aberrant 4:4 than left mutants. The basic frequency of conversion is higher in the left portion. The frequency of hybrid DNA, its symmetrical or asymmetrical distribution and the frequency of correction of the mismatch in hybrid DNA were estimated. The left region shows a higher frequency of hybrid DNA formation than the right region. The fraction of hybrid DNA with a symmetrical distribution tends to increase from left to right in the locus. The frequencies of mismatch correction show considerable variation from one mutant to another and have no relationship to their location. The implications of these observations on the molecular models of genetic recombination are discussed.  相似文献   

4.
Summary Sixty two ascospore colour mutants have been induced in Ascobolus immersus: 25 by an acridine (ICR170), 18 by N-methyl-N-nitro-N-nitrosoguanidine (NG) and 19 by ethyl methanesulfonate (EMS). All these mutants have been crossed to the wild type strain and their conversion spectrum has been determined. It appears that the conversion spectrum is closely related to the origin of the mutants studied with respect to the mutagen by which they were induced. All NG mutants gave numerous asci with postmeiotic segregation and an excess of conversion to wild type over conversion to the mutant type. ICR mutants gave no postmeiotic segregation and an excess of conversion to the mutant type. The majority of EMS mutants behave like NG mutants, but some showed only meiotic segregation with either an excess of conversion to the mutant type or an excess of conversion to wild type. These data are in good agreement with the hypothesis that the nature of the mutation has a strong influence on the conversion spectrum.  相似文献   

5.
Summary Pasadena strains of Ascobolus immersus were used to study the controls of gene conversion. Formulae were derived for estimating conversion parameters according to different hybrid DNA models of recombination. Genetic factors influencing conversion were determined by using genetically different isolates and using alleles before and after they had undergone conversion. Conversion properties at particular sites were greatly affected by genetic factors very near to those sites: the nature of the mutations and factors elsewhere in the genome were much less important. These genetic factors and temperature both affected the frequency of hybrid DNA formation at particular sites, the efficiency of mispair correction and the relative frequencies of correction to wild-type or mutant. The effects of temperature on conversion parameters were usually complex.  相似文献   

6.
As a result of a search for genetic markers to be used in interallelic recombination studies, 23 temperature-sensitive mutants of Ascobolus immersus were isolated following UV-irradiation. Three of these failed to grow at 37 C but grew normally or, in one case with a colonial phenotype, at 25 C. Twenty mutants are like the wild type at 25 C but express a colonial phenotype at 37 C. Crosses of the mutants to wild type indicate that all are due to single-gene mutations. Crosses among certain of the mutants indicate that several different loci are affected.  相似文献   

7.
Summary Gene conversion can be used to study: the topography and pairing relationships of the four chromatids of a bivalent at the time of crossing over and hybrid DNA formation, the lengths of intimately paired segments and the frequency of intimate pairing at particular sites. Conversion ratios of different types, corresponding-site interference, co-conversion, and the range and distribution of conversion frequencies are discussed in relation to DNA and chromatid pairing, and synaptinemal complex organisation. Conversion data from Ascobolus immersus and other fungi are compared with electron microscope data from various organisms and with models of the synaptinemal complex.  相似文献   

8.
A composite cross was made between 12 strains of the fungus Ascobolus immersus, six with wild-type red ascospores (w1+) and six with white ascospore mutation w1-78. A high postmeiotic segregation (PMS) frequency line was set up from colonies from ascospores from dehisced octads showing PMS, 5+ : 3w and 3+ : 5w. A low PMS line was started from ascospores from 4+ : 4w or 6+ : 2w octads, and a 'no selection' line was set up from ascospores from random octads. Colonies were crossed to tester strains to determine PMS frequencies and the selected lines were continued from ascospores of crosses of the red ascospore strain with the most extreme (e.g. high for the high line) PMS frequency with the white-ascospore strain of most extreme PMS frequency and of opposite mating type. Significant responses to selection were obtained for increased (+100%) and decreased (-58%) PMS, giving a 4.8-times difference in generation 4, with little change in the frequencies of conversion classes showing meiotic segregation (6+ : 2w and 2+ : 6w). The continuous, symmetrical, roughly normal distributions for PMS frequencies obtained when generation 5 strains were crossed to unselected tester strains are those expected if PMS frequencies are controlled by a number of polygenes, not major genes. Crosses of selected fifth-generation red-ascospore strains with extreme PMS values to base-substitution mutant w1-78, to frame-shift mutant w1-3C1 and to white-ascospore mutants w-BHj and w-9 at two loci unlinked to w1 showed that the effects of selection were not allele specific, locus specific or mutation-type specific.  相似文献   

9.
Direct evidence for horizontal transfer of a mitochodnrial plasmid from the discomyceteAscobolus immersus to the pyrenomycetePodospora anserina is presented. Southern blot hybridisation analysis, polymerase chain reaction (PCR) amplification, and DNA sequencing demonstrate transmission of a linear plasmid upon hyphal contact. DNA extraction from isolated organelles indicates a mitochondrial localisation for the plasmid inP. anserina. This is the first report of horizontal gene transfer among unrelated fungi. These results have important evolutionary implications for plasmid propagation in fungi.  相似文献   

10.
Summary Only 1.4% of the double mutant recombinants expected on the basis of wild-type recombination frequencies were observed in the combined data from two-factor crosses between a gene 37 amber mutant, amB280, and eighteen different temperature sensitive mutants which were also defective in gene 37. Similar, though less extreme, deficiencies of double mutant recombinants were observed by Doermann and Parma (1968) for mutants in several other genes. In our amB280xts crosses, frequencies of wild-type recombinants were in reasonably good agreement with those expected from the map positions of the mutants determined in crosses not involving amB280. Wild-type and double mutant recombinants were found at comparable frequencies when each of three other gene 37 amber mutants was crossed to a gene 37 temperature sensitive mutant.Experiments were performed to test whether the deficiency of double mutant recombinants in the amB280xts crosses could be explained by assuming that they occurred primarily in heterozygous particles, where their expression was masked. However, no evidence in support of this explanation was found. Other possible explanations, that the deficiency of double mutants was due to their inviability or the inability of double mutant chromosomes to replicate, were also inconsistent with our observations. The hypothesis considered to most plausibly explain our evidence is that the process by which double mutant recombinant chromosomes are formed is inhibited in the vicinity of a poorly suppressed am mutation.  相似文献   

11.
Summary Linear, extrachromosomal DNA's of the filamentous fungus Ascobolus immersus are localized within the mitochondria. These linear plasmids have no homology to the high molecular weight mtDNA (hmw mtDNA). For analysis of plasmid replication an in organello DNA synthesis system was developed, in which radionucleotides were incorporated into intact mitochondria. Plasmid DNA is labelled preferentially in this system. From replication analysis of a specific plasmid there is evidence of a virus-like protein-primed replication. Sequence analysis of this plasmid reveals that a viral DNA polymerase is encoded. Thus, these genetic elements presumably are viral remnants rather than true plasmids.  相似文献   

12.
7 mus (mutagen-sensitive) mutants of Neurospora crassa, which are more sensitive to the toxic effects of MMS (methyl methanesulfonate) than wild-type, were investigated for cross-sensitivities to other mutagens and inhibitors. These mutants have recently been mapped in 5 new genes, mus-7 to mus-11, and mutant alleles from each gene were checked for their effects on mutation frequencies. It was found that mutants in 3 of these 5 genes showed radiation-induced mutation frequencies similar to wild-type. These included 2 alleles of the gene mus-10, which were cross-sensitive only to UV and were the only mutants that produced some viable ascospores in homozygous crosses. The mutant of the second gene, mus-8, was especially sensitive to UV and mitomycin C and produced slightly reduced frequencies of spontaneous mutation. In contrast, the mutant of the third gene, mus-7, was not UV-sensitive but showed some cross-sensitivity to X-rays; mus-7 was highly sensitive to MMS and also to histidine, which inhibits various repair-defective mutants at concentrations well below those that reduce wild-type growth. None of these mus resemble mutants previously found in Neurospora, nor do they conform clearly to mutant types identified in E. coli or yeast. On the other hand mutants in 2 further genes, mus-11, and especially 2 alleles of mus-9, are very similar to uvs-3 of Neurospora and generally resemble mutants that are considered to be defective in "error-prone" repair. They were UV- as well as X-ray-sensitive, and showed strong spontaneous mutator effects but almost no increase in recessive lethal frequencies in heterokaryons after UV-treatments.  相似文献   

13.
    
We have used a biological phenomenon that occurs inNeurospora crassa, termed Repeat-Induced Point mutation (RIP), to create partially functional mutant alleles of thealbino-3 (al-3) gene encoding geranylgeranyl pyrophosphate synthase, an enzyme involved in the biosynthesis of carotenoids and diverse prenylated compounds. A total of 70 RIP-inducedal- 3 mutants were identified by their pale albino phenotype, resulting from inactivation of carotenoid biosynthesis. Nucleotide sequence analysis of theal-3 gene in five of the RIP-induced mutants revealed that in each case RIP had introduced no more than six point mutations. The low frequency of RIP mutants (0.42%) and the isolation of only leaky mutants with very few mutations suggest that ascospores containing a heavily mutatedal-3 gene do not survive. These results are evidence that the RIP phenomenon, used to inactivate and silence duplicated genes inN. crassa, may be exploited in its mild version as a method of sequence-specific in vivo mutagenesis to obtain functional mutant alleles ofNeurospora genes. This mild form of mutagenesis may be particularly advantageous in selecting for leaky mutations in essentialNeurospora genes.C.B. and M.C. contributed equally to this work  相似文献   

14.
Summary The mating pattern between 38 strains collected at various places in Europe and Southern India was determined. There are at least three compatibility groups: A (23 strains) and B (9 strains) comprising the European isolates, and C (6 strains), the Indian isolates. Within each compatibility group sexual reproduction is, as expected, controlled by a bipolar mechanism of homogenic incompatibility. No fertile offspring are obtained in any intergroup crossing showing that there is genetic separation by heterogenic incompatibility. However, the European group B seems to be closer related to the Indian (C) group in that sterile fruit bodies are produced between + and – mating types. An indication for a further subdivision is the occurrence of a barrage-like phenomenon between representatives of all three groups. The data thus indicate how the start of speciation may be occurring in Ascobolus immersus by means of both spatial and genetic isolation.  相似文献   

15.
《Gene》1996,170(1):155-156
The structural gene encoding S-adenosyl-l-methionine synthetase (SAM-S) in the fungus Ascobolus immersus has been cloned and sequenced. It contains a 1179-bp ORF, interrupted by three introns, encoding a 393-amino-acid protein (42 978 Da) that is 90% homologous to the SAM-S of the filamentous fungus Neurospora crassa, indicating that these fungi are closely related species  相似文献   

16.
A Mutant Affecting Meiosis in Neurospora   总被引:3,自引:0,他引:3       下载免费PDF全文
David A. Smith 《Genetics》1975,80(1):125-133
Many mutants affecting meiosis increase the occurrence of aneuploid meiotic products. In Neurospora, mutants of this type cause ascospore abortion which is reflected by an increase in the proportion of ascospores failing to develop black pigment. The usefulness of the criterion white-ascospore-production as a signal for the presence of a mutant affecting meiosis is demonstrated by the recovery of several such mutants. One of these is mei-1 (meiotic-1), a recessive mutant on linkage group IV. Crosses homozygous for mei-1 produce 90% white ascospores (vs. 5% in wild-type crosses). Viable ascospores, invariably black, are always disomic for one or more linkage groups; the chromatids assorted into viable ascospores do not engage in crossing over in meiosis. The distribution of viable ascospores in individual asci suggests that all meioses are defective in the first meiotic division, and that most meioses are defective in both divisions.  相似文献   

17.
Summary Six mutant strains (301, 102, 203, 104, 305, and 307) affected in their nitrate assimilation capability and their corresponding parental wild-type strains (6145c and 21gr) from Chlamydomonas reinhardii have been studied on different nitrogen sources with respect to NAD(P)H-nitrate reductase and its associated activities (NAD(P)H-cytochrome c reductase and reduced benzyl viologen-nitrate reductase) and to nitrite reductase activity. The mutant strains lack NAD(P)H-nitrate reductase activity in all the nitrogen sources. Mutants 301, 102, 104, and 307 have only NAD(P)H-cytochrome c reductase activity whereas mutant 305 solely has reduced benzyl viologen-nitrate reductase activity. Both activities are repressible by ammonia but, in contrast to the nitrate reductase complex of wild-type strains, require neither nitrate nor nitrite for their induction. Moreover, the enzyme from mutant 305 is always obtained in active form whereas nitrate reductase from wild-types needs to be reactivated previously with ferricyanide to be fully detected. Wild-type strains and mutants 301, 102, 104, and 307, when properly induced, exhibit an NAD(P)H-cytochrome c reductase distinguishable electrophoretically from contitutive diaphorases as a rapidly migrating band. Nitrite reductase from wild-type and mutant strains is also repressible by ammonia and does not require nitrate or nitrite for its synthesis. These facts are explained in terms of a regulation of nitrate reductase synthesis by the enzyme itself.  相似文献   

18.
A convenient and rapid method for screening and identifying rod mutants of Bacillus subtilis is described. At the restrictive temperature (45 °C), all rod mutants of B. subtilis screened lost their ability to sporulate. The morphology and colour of mutant colonies grown on sporulation agar plates differed from those of rod+ cells, which were able to sporulate even at elevated temperature. These characteristics provide an alternative approach for the identification of rod mutants in B. subtilis culture by streaking the cells onto a minimal glucose agar plate and incubating at the restrictive temperature. After 30 h of incubation at this temperature, rod mutants are easily identified. This method will facilitate the screening and isolation of rod mutants of B. subtilis.  相似文献   

19.
We have isolated and characterised the nuclear gene that codes for the 30.4-kDa subunit of the peripheral arm of complex I from Neurospora crassa. The single-copy gene was localised on chromosome VI of the fungal genome by restriction fragment length polymorphism mapping. An extra copy of the gene was introduced into a strain of N. crassa by transformation. This strain was crossed with another strain in order to inactivate, by repeat-induced point mutations, both copies of the duplication carried by the parental transformant. Ascospore progeny from the cross were analysed and a mutant strain lacking the 30.4-kDa protein, nuo30.4, was isolated and further characterised. The mutant appears to assemble the membrane arm of complex I, while formation of the peripheral arm is prevented. Nevertheless, the mutant grows reasonably well – indicating that this well conserved protein is not essential for vegetative growth – and is able to mate with other strains both as male or female. Strains with multiple mutations are readily obtained from heterozygous crosses between different complex I mutants of N. crassa. On the other hand, homozygous crosses between several mutants, including nuo30.4, fail to produce ascospores. These results suggest that complex I plays an essential role during the sexual phase of the life cycle of the fungus. Received: 24 February 1997 / Accepted: 23 September 1997  相似文献   

20.
A scheme has been devised for efficient isolation of recessive meiotic mutants of Neurospora crassa. These mutants were detected by their reduced fertility or by the abortion of ascospores. Their isolation involved the selection and screening of the strains arising from ascospores disomic (n + 1) for linkage group I (LG I), which bears the mating-type locus. These strains are self-fertile heterokaryons that contain two types of haploid nuclei of opposite mating types (A + a). Selfings of these strains are homozygous for genes on all linkage groups except LGI and therefore allow the expression of recessive mutants with an altered sexual cycle. Using this selection procedure, three classes of mutants were detected. In one class, mutants had an early block in perithecial development (class I), and in another mutants had altered perithecia, but apparently unaltered fertility (class III). No recessive mutants were observed and all mutants tested (eight of class I and two of class III) were expressed only when used as the maternal parent. A third mutant class displayed normal production of perithecia, but defective formation of asci (class IIA), or black ascospores (class IIB). Four of 13 class IIA mutants were analyzed, and two of them [asc(DL131) and asc (DL400)] were definitely recessive analysis of 10 of 13 class IIB mutants disclosed six recessive, mutually complementing mutants: ase(DL95), asc(DL243), asc(DL711), asc(DL879), asc(DL917m) and asc(DL961). Mutants asc(DL95), asc(DL243) and the previously studied mei-1 mutant (Smith 1975) complemented one another in crosses, but did not recombine. These may be alleles of the same gene, or they may comprise a gene cluster.  相似文献   

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