The Formation of Neochromosomes during Experimental Evolution in the Yeast Saccharomyces cerevisiae

Novel, large-scale structural mutations were previously discovered during the cultivation of engineered Saccharomyces cerevisiae strains in which essential tRNA synthetase genes were replaced by their orthologs from the distantly related yeast Yarrowia lipolytica. Among those were internal segmental amplifications forming giant chromosomes as well as complex segmental rearrangements associated with massive amplifications at an unselected short locus. The formation of such novel structures, whose stability is high enough to propagate over multiple generations, involved short repeated sequences dispersed in the genome (as expected), but also novel junctions between unrelated sequences likely triggered by accidental template switching within replication forks. Using the same evolutionary protocol, we now describe yet another type of major structural mutation in the yeast genome, the formation of neochromosomes, with functional centromeres and telomeres, made of extra copies of very long chromosomal segments ligated together in novel arrangements. The novel junctions occurred between short repeated sequences dispersed in the genome. They first resulted in the formation of an instable neochromosome present in a single copy in the diploid cells, followed by its replacement by a shorter, partially palindromic neochromosome present in two copies, whose stability eventually increased the chromosome number of the diploid strains harboring it.