“Karyotype Engineering by Chromosome Fusion Leads to Reproductive Isolation in Yeast”, 2018 (; backlinks; similar):
Extant species have wildly different numbers of chromosomes, even among taxa with relatively similar genome sizes (for example, insects)1,2. This is likely to reflect accidents of genome history, such as telomere-telomere fusions and genome duplication events3–5. Humans have 23 pairs of chromosomes, whereas other apes have 24. One human chromosome is a fusion product of the ancestral state6. This raises the question: how well can species tolerate a change in chromosome numbers without substantial changes to genome content?
Many tools are used in chromosome engineering in Saccharomyces cerevisiae7–10, but CRISPR-Cas9-mediated genome editing facilitates the most aggressive engineering strategies.
Here we successfully fused yeast chromosomes using CRISPR-Cas9, generating a near-isogenic series of strains with progressively fewer chromosomes ranging from 16 to two. A strain carrying only two chromosomes of about 6 megabases each exhibited modest transcriptomic changes and grew without major defects.
When we crossed a 16-chromosome strain with strains with fewer chromosomes, we noted two trends. As the number of chromosomes dropped below 16, spore viability decreased markedly, reaching less than 10% for 12 chromosomes. As the number of chromosomes decreased further, yeast sporulation was arrested: a cross between a 16-chromosome strain and an 8-chromosome strain showed greatly reduced full tetrad formation and less than 1% sporulation, from which no viable spores could be recovered. However, homotypic crosses between pairs of strains with 8, 4 or 2 chromosomes produced excellent sporulation and spore viability. These results indicate that 8 chromosome-chromosome fusion events suffice to isolate strains reproductively.
Overall, budding yeast tolerates a reduction in chromosome number unexpectedly well, providing a striking example of the robustness of genomes to change.