Gene conversion spectrum of 15 mutants giving post-meiotic segregation in the b2 locus of Ascobolus immersus

Restoration to the parental genotype of mismatches formed in recombinant DNA heteroduplex

Crosses were set up, using the b2 locus of Ascobolus immersus, to detect and measure the restoration of the parental genotype of a chromatid from hetero-duplex. From the results it can be concluded that: (1) restoration occurs, and its frequency is comparable to the frequency of conversion; (2) the relative frequency of conversion and restoration on the two homologous chromatids is that expected if the disparity in conversion to mutant and to wild-type of the marker is caused by the decision whether to excise the mutant or wild-type chain. This decision is the same on both chromatids. However, when all data are pooled, a slight, but significant, excess of conversion over restoration is found. The cause of this in discussed; (3) the absence of an excess of restoration over conversion is taken to imply that, in this system, the length of heteroduplex formed in an event is continuous; (4) these findings support the interpretation of tetrads where a crossover is separated from conversion, that they arise by independent correction giving both conversion and restoration in a heteroduplex tract continuous with the crossover.

Abuffspore colour mutant inSordaria brevicollisshowing high-frequency conversion

A mutant, YS17, at thebuffspore colour locus inSordaria brevicollis, when crossed with wild type, gives rise to aberrant asci with a frequency over 10 times that of otherbuffmutants. Over 98% of the aberrant asci have 6 wild type and 2 mutant spores. From tests with anotherbuffmutant it is concluded that loss of the mutant spore colour when YS17 shows conversion to wild type is associated with loss of the high frequency conversion, and that both characters are caused by the same mutation. A methionine-requiring mutant (met-1) has been obtained that maps 5 units to the left ofbuff, and this, together with the nicotinamide-requiring mutant (nic-1) 2 units to the right, has provided flanking markers forbuffthat can be scored with complete reliability. Crosses between YS17 and 28 otherbuffmutants have revealed close linkage to three of them which map to its right on the basis of flanking marker behaviour, all the others mapping to its left. The frequency of postmeiotic segregation at the sites ofbuffmutants near to the site of YS17 is greatly increased in the presence of YS17, and occurs in the chromatid showing conversion to wild type at YS17.

From these and other results, obtained largely by ascus analysis, the following conclusions have been drawn.

(1) The YS17 mutation is probably acting as a recognition site for an endonuclease that initiates recombination, with the result that the frequency of heteroduplex DNA within thebuffgene is much increased.

(2) The recombination initiated at YS17 is asymmetric (or at least pre-dominantly so), with the YS17 site acting as a recipient of a nucleotide chain from the other parent, not a donor to it.

(3) The frequency of crossing over associated with conversion at YS17 is variable: about 30% in crosses with most of thebuffmutants, about half this value in crosses with wild type, and almost zero in crosses with closely-linkedbuffmutants.

(4) In about one third of the crossover asci in crosses between YS17 and otherbuffmutants the crossover is not adjacent to the site of YS17 but separated from it by the site of the allele, which shows normal 4:4 segregation.

(5) It seems necessary to revive the idea of more than one recombination event in proximity, a non-crossover conversion event sometimes leading to a second event – a crossover – in the vicinity. It is tentatively suggested that both might be controlled by a single enzyme aggregate.

The interpretation of gene conversion data from ordered eight-spored asci

Currently favoured models postulate that gene conversion is due to the correction of mis-matches in heteroduplex DNA. If heteroduplex is formed reciprocally on both chromatids participating in recombination, the mis-matches due to a heterozygous site will be different on the two chromatids, and there will be four correction probabilities to be taken into account. It is shown that, given the frequencies of the five different kinds of aberrant ascus ratios, it is possible to calculate four alternative sets of values for the four correction probabilities and the total number of asci in which heteroduplex is formed. These four solutions reduce in effect to two when there are no other markers distinguishing the two chromatids. With the aid of flanking markers and the assumption that heteroduplex formation is chemically polarized, it is possible, in principle, to choose one best solution.

The method has been applied to the five one-point crosses inSordaria fimicolafrom which most data are available. The data from four different mutants crossed to wild type are compatible with a restricted model in which the correction frequencies, from mutant to wild and from wild to mutant, are the same on both chromatids. In the case of the fifth mutant the data are not consistent with this restricted model, and indicate different correction frequencies in the two chromatids.

Parameters in gene conversion: An algebraic analysis of the hybrid DNA model at thegraylocus ofSordaria fimicola

We have extended previous algebraic analyses of aberrant segregation at thegraylocus ofSordaria fimicola(Whitehouse, 1965; Emerson, 1966; Fincham, Hill & Reeve, 1980) to the more complex situation where aberrant segregations are detected in three factor crosses involving two flanking markers. This algebra has been applied to sevengrayalleles which have been extensively characterized for their pattern of gene conversion and postmeiotic segregation by Kitani & Olive (1967). It is based on seven major types of aberrant segregation which can be distinguished in the presence of flanking markers spanning the converting site, and allows us to use up to six parameters to describe hDNA formation and mismatch repair. We present solutions which predict a spectrum of aberrant segregation fitting the experimental data at theP> 0·05 level for six of the seven alleles tested. They are consistent with the following properties of hDNA at thegraylocus: (1) the single stranded DNA transferred during hDNA formation has always the same chemical polarity. (2) hDNA is mostly, if not entirely, symmetric, and its probability of formation is constant over the whole gene. (3) Disparity in aberrant segregation is mostly, if not entirely due to disparity in mismatch repair.

Gene conversion disparity in yeast: its extent, multiple origins, and effects on allele frequencies

The extent of disparity in gene conversion direction in yeast (Saccharomyces cerevisiae) is important for recombination mechanisms and for effects of conversion on allele frequencies in populations. An analysis of published and unpublished data demonstrates that yeast frequently shows significant and extensive conversion disparity, contrary to many published statements. All types of mutation--base-substitutions, frameshifts and longer deletions and additions--can show significant 6:2/2:6 and/or 5:3/3:5 disparity. There was little correlation between the occurrence of 6:2/2:6 and 5:3/3:5 disparities; when both were significant, they were more often in opposite directions than in the same direction. Surprisingly, there was little correlation between a mutation's molecular nature and its disparity properties, which generally seem unpredictable. Disparity in yeast has multiple origins. From the equations discussed, all disparity types can be explained by one or more of: correction direction disparity, chromatid invasion disparity (including cases caused by different frequencies of double-strand breaks or gaps in nonsister homologous chromatids), strand invasion disparity, and different correction frequencies for the two types of mispair for a heterozygous mutation. Levels of overall disparity and of conversion frequency mean that conversion must often change allele frequencies in sexually reproducing yeast populations.

Interchromosomal transfer of epigenetic states in Ascobolus: transfer of DNA methylation is mechanistically related to homologous recombination

The transfer of methylation between alleles represents a plausible epigenetic mutational mechanism to explain loss of imprinting in mammals and paramutation in plants. Here, we have exploited advantages unique to the fungus Ascobolus immersus to obtain direct experimental evidence that methylation transfer can occur between homologous chromosomes. A methylated allele and an unmethylated allele of the Ascobolus b2 spore color gene were brought together in individual meiotic cells. Frequent transfer of methylation to the unmethylated allele was observed. This transfer was polarized 5' to 3' along the b2 gene, as is gene conversion, and always accompanied the latter process when tested in the same cross. These and other observations strongly suggest that methylation transfer and recombination are mechanistically related.

Polarity of meiotic gene conversion in fungi: contrasting views

The frequency of meiotic gene conversion often varies linearly from one end of the gene to the other. This phenomenon has been called 'polarity'. In this review, we will primarily studies of polarity that have been done in the yeast Saccharomyces cerevisiae (ARG4 and HIS4 loci) and in Ascobolus (b2 locus) with an emphasis on possible mechanisms. The genetic and physical data obtained at these 'hotspots' of recombination strongly suggests that the formation of a polarity gradient reflects both the frequency of heteroduplex formation and the processing of this recombination intermediate by mismatch-repair-dependent processes.

Postmeiotic segregation as a source of mosaics in diploid organisms

Postmeiotic segregation (PMS) of genetic variants occurs when a DNA heteroduplex formed during meiotic recombination goes undetected by repair enzymes and is transmitted unresolved to the meiotic products. PMS provides an alternative explanation for the origin of mosaics now attributed to half-chromatid mutation. In multicellular diploid eukaryotes, PMS could result in mosaic individuals with unusual migration patterns for proteins studied by gel electrophoresis. If the gonade were mosaic, complex progenies containing as many as six phenotypic classes at a single locus could be produced.

Aberrant segregation patterns and gene mappability inAscobolus immersus

Crosses between various types of mutant giving specific patterns of aberrant segregation were performed in theb2spore colour locus ofAscobolus immersus. The map of 41 mutations showing various patterns of aberrant segregation was established. The frequency of wild-type recombinants and the map additivity, map expansion and map contraction characteristics were shown to be strongly dependent upon the pattern of aberrant segregation of the mutations used. Mutations giving no postmeiotic segregation and an excess of conversion to wild type over conversion to mutant exhibit map expansion in small intervals and a strong map contraction in large intervals. Mutations giving postmeiotic segregations also exhibit map contraction in large intervals. Mutations giving no postmeiotic segregations and an excess of conversion to mutant over conversion to wild type show map additivity and thus provide a simple way for devising gene maps. The relationship between the mapping properties and the pattern of aberrant segregations is accounted for when considering parameters of gene conversion: frequency and distribution of hybrid DNA, frequency and direction of mismatch correction.

Variation of gene conversion and intragenic recombination frequencies in the genome of Ascobolus immersus

Eighty mutants in 17 ascospore character genes were studied for their conversion patterns. The correlation between conversion pattern and mutagenic origin, previously found in genes b1 and b2 was extended to all the genes studied. Aberrant 4:4 asci were found in most genes irrespective of their conversion frequency. From gene to gene, the conversion frequency showed an almost 100 times variation. The frequency of intragenic recombination also showed sharp variation from gene to gene. The mean conversion frequency and the maximal intragenic recombination frequency were shown to be highly correlated in 5 genes for which these 2 values are known. This correlation was extended to 12 other genes in other Ascomycetes: Saccharomyces cerevisiae, Schizosaccharomyces pombe, Neurospora, and Sordaria. From this study it is concluded that, 1) the probability of hybrid DNA formation undergoes considerable changes according to the region of the genome; 2) the intragenic recombination frequency primarily reflects the frequency of hybrid DNA formation rather than the physical length of the gene; 3) for a given physical distance on the DNA, a similar fraction of the gene conversion events lead to recombination in the 5 Ascomycetes.