Genomic Reconstruction by Mitotic Recombination of YACs

Mitotic crossing over was first observed in the somatic tissues of genetically marked strains of fruitfly (1 ), but has proved most valuable as a mapping tool in asexual fungi (2 ,3 ). Although the yeast Saccharomyces cerevisiae has a sexual cycle, and is therefore more amenable to genetic analysis by classical meiotic approaches, mitotic crossing over within a region of homology shared between two artificial chromosomes in this host has some interesting consequences, and can be used to physically reconstruct the genomic structure of an extended region in a single molecule (4 ).