Mitotic intragenic recombination as a consequence of heteroduplex formation in Aspergillus nidulans

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.

Tests for recombinagens in fungi

Three types of mitotic recombination can be studied in Aspergillus nidulans and Saccharomyces cerevisiae: (1) The classical type of reciprocal mitotic crossing-over which can be detected when it occurs between non-sister chromatids at the four-strand stage followed by co-segregation of a crossing-over and a non-crossing-over chromatid in the subsequent mitotic division. Consequently, mitotic crossing-over reflects cellular responses to primary genetic damage in the G2 phase of the cell cycle. (2) Mitotic gene conversion is a unidirectional event of a localized transfer of genetic information between non-sister chromatids which in yeast can extend to segments of up to 18 cM and even beyond 22 cM in Aspergillus nidulans. Mitotic gene conversion can also occur between unreplicated chromatids and lead to the expression of the newly created genotype without any need for a subsequent mitotic cell division. It reflects a cellular response in G1. (3) Mitotic sister-strand gene conversion can be studied in a recently constructed strain with the same technical ease as classical non-sister chromatid gene conversion. It can be induced by chemicals which do not induce mutation in the Salmonella system and non-sister chromatid gene conversion. Mitotic segregation in Saccharomyces cerevisiae results almost exclusively from crossing-over and gene conversion whereas mitotic chromosomal malsegregation contributes only very little. In contrast to this, in Aspergillus nidulans, both processes contribute considerably so that mitotic segregants always have to be tested for their mechanistic origin.

Mutagenicity of pesticides evaluated by means of gene-conversion in Saccharomyces cerevisiae and in Aspergillus nidulans

Nine pesticides, Afugan, Atrazine, Benomyl, Captan, Daconil, Melthaumittel, Plantvax, Saprol, and Wepsin were tested for the induction of mitotic gene-conversion in two different eucaryotic microorganisms, Saccharomyces cerevisae and Aspergillus nidulans. In S cerevisiae the pesticides were also tested after mouse liver microsomal activation; in A nidulans all tests were performed using both resting and germinating conidia. Among the tested pesticides, only Captan revealed a consistent genetic activity, three times greater than that observed with the standard mutagen methylmethansulphonate. This genetic activity of Captan is suppressed after mammalian metabolic conversion. The use of germinating conidia in A nidulans for scoring the induction of mitotic gene-conversion did not give any further information on the activity of the tested pesticides.

MMS induction of different types of genetic damage in Aspergillus nidulans: a comparative analysis in mutagenesis

Methyl methanesulphonate (MMS) was used to test the induction of gene mutation, somatic crossing-over and mitotic non-disjunction in A. nidulans. Gene mutation was tested by inducing mutants resistant to 8-azaguanine and revertants of methG1 in a haploid strain. Somatic crossing-over was tested in heterozygous diploids, both with a selective method, i.e. inducing homozygosis to FPA resistance in a heterozygous fpa A1/+ strain, and with a non-selective method, i.e. identifying the frequencies of colour sectors. This latter method was also used to estimate the induction of non-disjunction because additional markers were present which permitted us to distinguish the two types of colour segregant. Generally, 3 different experimental procedures were used, namely the "plate test", i.e. plating of conidia in agar media containing MMS, and two types of "liquid test", i.e. brief treatment of quiescent or pre-germinated conidia in MMS solution before they were plated on agar media. Point mutations were induced with about equal efficiency with each method, whereas crossing-over was induced preferentially when germinating conidia were exposed to MMS. On the other hand, non-disjunction was induced in germinating and quiescent spores with equal efficiency, but such segregants were not recovered with the selective (fpa) method. The results are discussed for both their practical use in the mutagenic testing procedure and their theoretical implication.