Interallelic recombination at the his-5 locus in Neurospora crassa

Use of fluorescent protein to analyse recombination at three loci in Neurospora crassa

We have inserted a histone H1-GFP fusion gene adjacent to three loci on different chromosomes of Neurospora crassa and made mating pairs in which a wild type version of GFP is crossed to one with a mutation in the 5' end of GFP. The loci are his-3, am and his-5, chosen because recombination mechanisms appear to differ between his-3 and am, and because crossing over adjacent to his-5, like his-3, is regulated by rec-2. At his-3, the frequencies of crossing over between GFP and the centromere and of conversion of 5'GFP to GFP(+) are comparable to those obtained by classical recombination assays, as is the effect of rec-2 on these frequencies, suggesting that our system does not alter the process of recombination. At each locus we have obtained sufficient data, on both gene conversion and crossing over, to be able to assess the effect of deletion of any gene involved in recombination. In addition, crosses between a GFP(+) strain and one with normal sequence at all three loci have been used to measure the interval to the centromere and to show that GFP experiences gene conversion with this system. Since any gene expressed in meiosis is silenced in Neurospora if hemizygous, any of our GFP(+) strains can be used as a quick screen to determine if a gene deleted by the Neurospora Genome Project is involved in crossing over or gene conversion.

Genetic analysis of recombination at the g locus in Sordaria fimicola

Interallelie crosses of mutants at the grey (g) spore colour locus in Sordaria fimicola, heterozygous for flanking markers, give rise to a large number of aberrant ascus genotypes, 45 of which can arise through relatively simple events and have been chosen for study. These genotypes comprise 50–75% of the aberrant asci, depending on the mutants crossed.

Comparison of the results from 10 pairwise crosses involving 7 alleles reveals that linked postmeiotic segregation and co-conversion decrease rapidly in frequency with increasing separation of the mutant sites.

The data from reciprocally recombinant asci, from asci with normal segregation at one of the two mutant sites, and from flanking marker behaviour in one- and two-point crosses, agree with the Holliday-Sobell formulation, with the following additional features:

(1) The nuclease, which nicks homologous polynucleotides and then degrades one of the two nicked chains when a mutant enters the hybrid DNA structure, can show preferential degradation of the mutant (or the wild-type) chain. In addition, a second nuclease is involved in the excision-repair process that introduces an additional preferential (marker specific) bias in the degradation of the mutant (or the wild-type) chain. This could explain why asci with odd-ratio conversion (5:3 and 3:5 ratios) sometimes show a different bias, as first reported by Emerson for Ascobolus, from those with even-ratio conversion (6:2 and 2:6 ratios), since the latter but not the former require, in addition, the action of a mismatch correction enzyme to account for them.

(2) The migratory hybrid DNA structure which enters the gene at one end may be of a different size from that which enters from the other end.

(3) Mismatch correction at the end of the hybrid DNA structure leads to a non-recombinant outside marker genotype and modifies the 1:1 ratio of parental:recombinant flanking markers that is otherwise found.

Recombination within the am gene of Neurospora crassa

Maps have been made showing the order of am mutant sites using (a) frequencies of am+ recombinants from crosses between am mutants and (b) the modes of distribution of the am+ recombinants among the two parental and two recombinant classes with respect to the flanking markers inos and sp.

It is possible to arrange the sites in an order such that, in almost all the crosses yielding useful numbers of am+ recombinants, the sp allele originally in coupling with the distal am mutant site occurs in the majority of the recombinants. No such consistent pattern was found with respect to the inos marker. The order obtained by reference to sp agreed with the best order deducible from recombination frequencies.

The data are consistent with the hypothesis that am+ recombinants arise by a process of gene conversion, that there is a gradient of conversion frequencies from the right (inos or distal) end of the gene to the left (sp or proximal) end, and that conversion tends to be associated (though less than 50% of the time) with crossing-over, especially on the distal side.

There is no obvious relationship between the map position of a given am mutant and the properties of the protein product of the mutant gene.

The orientation of gene maps by recombination of flanking markers for the am locus of Neurospora crassa

Fincham (1967), Smyth (1973 b) and Rambosek and Kinsey (1983) have each generated fine-structure maps of the am gene of Neurospora crassa. Each map had a consistent linear order of alleles but the assignment of an orientation with respect to other linkage group-V loci differed. Fincham found the end marked by the am6 allele to be at the distal end of the locus, Smyth found am6 to be at the proximal end while the data of Rambosek and Kinsey did not suggest an orientation. Smyth s orientation has been adopted as the standard, but not unreservedly. We have aligned the genetic and physical maps of the am gene, showing that am6 is at the distal end, supporting Fincham's orientation. However, we suggest that an assumption used to orient fine structure genetic maps is flawed and that the conflicting orientation between these three studies follows from the different choice of flanking markers.

Recombination in the AM gene of Neurospora crassa--a new model for conversion polarity and an explanation for a marker effect

It is argued that polarised intragenic recombination is not necessarily due to hybrid DNA extending into the gene for variable distances from one side; it can as well be explained by hybrid DNA usually covering the whole gene, the two complementary DNA strands of the gene having unequal chances of undergoing the interchromatid transfer involved in hybrid DNA formation, and excision from base-pair mismatches proceeding predominantly in one chemical direction (perhaps 5' to 3'). This alternative model for polarity is thought to be in better accord with the behaviour of flanking markers in the am data, which appears to show that conversion-associated crossing-over can occur with almost equal likelihood on either side of the gene (a feature previously held to support negative interference--Fincham, 1974). It also provides an explanation for an unusual marker effect involving two am mutational sites separated by only three base pairs. The nature of the sites (defined by amino acid replacement analysis) is such that one forms a purine-purine mismatch on the same chromatid as the other forms a pyrimidine-pyrimidine mismatch, and vice versa. They show an approximately two-fold difference in recombination frequencies in crosses to other am mutants mapping to the left, and a difference of similar magnitude, but opposite sign, in crosses to mutants mapping to the right.

Recombination and chromosome structure in eukaryotes

Allelic recombination frequency and pattern in fungi show strong gene-to-gene variation. At least in two fungal species,NeurosporaandSchizophyllum, recombination frequencies in certain chromosomal segments are genetically determined. The possibility that chromosomal proteins and/or synaptinemal complexes may play an important role in regulation of recombination frequency and pattern is discussed.

Non-reciprocal recombination at the pan-2 locus in Neurospora crassa

From a cross involving alleles at the pan-2 locus 11 asci were selectively recovered each of which showed non-reciprocal recombination for the pan-2 locus. Analysis of these asci provide data which may be explained on the basis of a model similar to the hybrid DNA-polaron model of recombination proposed by Whitehouse and Hastings (1965).

Multiple alterations in metabolite uptake in a mutant of Neurospora crassa

A temperature-conditional lethal mutant of Neurospora crassa, un-t (55701), was resistant to neutral amino acid analogues by virtue of a decreased ability to transport these analogues and their natural congeners across the cell membrane. The uptake of acidic, but not basic, amino acids was also impaired, as was the uptake of potassium ions. After preincubation above the tolerated temperature, the ability to take up a still wider variety of metabolites was greatly reduced. Protoplasts of the mutant were more osmotically fragile than those of wild type. The possibility that the mutant has a generalized membrane defect is discussed.

Genetic control of recombination and the incompatibility system inSchizophyllum commune

Crossing-over between the α and the β loci constituting theAincompatibility factor gives rise to two new specificities which are compatible with both parental As and with each other. The frequency of monokaryotic mycelia carrying re-combinant A factors is shown to be under genotypic control in a multiple crosses programme (6 × 6), selection for high and low recombination frequencies (two generations), and crosses between the selection lines. The recombination values based on samples of 100 monokaryons, range from 0 to 21%; however, the more accurately estimated values of the ‘High’ and ‘Low’ selections are 14% and 4%, each being based on approximately 2,000 mycelia.

The data are compatible with a gene-system consisting of the postulated locusrecwhich has a major effect on recombination and which is linked to the A factor, and several minor effects by other loci. Alternative interpretations are presented and discussed. The apparent dominance of low frequencies of recombination on high frequencies can be related to the breeding behaviour ofS. commune.Thus close linkage between α and β allows a high number of A specificities to be maintained in a population as well as high out-breeding potential, while the inbreeding potential (i.e. dikaryotic combinations between monokaryons originating from a single fruit-body) is kept low and near its minimum.

The significance of the two-locus structure of the incompatibility factors is examined theoretically in an Appendix at the end of the Discussion section.