ABSTRACT Diploid and stable karyotypes are associated with health and fitness in animals. In contrast, whole genome duplications (WGDs) - doubling full chromosome content - are linked to genetic instability (GIN) and frequently found in human cancers 1–3 . It has been established that WGDs fuel chromosome instability through abnormal mitosis 4–8 , however, the immediate consequences of tetraploidy in the first interphase are not known. This is an essential question because single WGD events such as cytokinesis failure can promote tumorigenesis 9 . Here, we found that newly born tetraploid human cells undergo high rates of DNA damage during DNA replication in the first S-phase. Using DNA combing and single cell sequencing, we show that DNA replication dynamics is perturbed, generating under- and over-replicated regions. Mechanistically, we found that these defects result from the lack of protein mass scaling up at the G1/S transition, which impairs the fidelity of DNA replication. This work shows that within a single interphase, unscheduled tetraploid cells can acquire highly abnormal karyotypes. These findings provide an explanation for the GIN landscape that favors tumorigenesis after tetraploidization.

Genetic instability from a single S-phase after whole genome duplication