MIP: An Epigenetic Gene Silencing Process in Ascobolus immersus

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The filamentous fungus Ascobolus immersus is a haploid and heterothallic Ascomycete belonging to the class Discomycetes (“cup fungus”). Ascobolus was introduced as material for genetic studies by Georges Rizet (Lissouba et al. 1962). It has been used for years as a model organism for investigating meiotic recombination. In this species, each ascus is composed of eight haploid spores (four pairs), and each pair of spores is issued through a mitotic division from one of the four meiotic products arising from an individual meiosis. Wild-type spores display a dark-brown pigmentation. Spore color mutants exhibiting white ascospores can be screened, using a binocular microscope. In crosses between wild-type and mutant strains, mendelian segregations yield four wild-type (dark-brown) and four mutant (white) spores. Asci showing departures from mendelian segregations are easily scored. They reflect the occurrence of gene conversion events resulting from the process of meiotic recombination. One spore color gene, named b2, was extensively used for studying meiotic recombination. The genetic study of segregation and recombination patterns in crosses heterozygous for one or several allelic mutations in b2 provided valuable information about the formation of recombination intermediates (reviewed in Rossignol et al. 1988; Nicolas and Rossignol 1989).