Ploidy-dependent modulation of DNA replication kinetics in Xenopus

Abstract

DNA replication in multicellular organisms follows a tightly regulated spatio-temporal program. Although the mechanisms underlying this replication program remain only partially understood, studies in model systems such as Xenopus laevis have highlighted the importance of titrating low-abundance replication factors. Whole-genome duplications as a consequence of polyploidization introduce additional layers of complexity, yet comparative analyses of replication programs across closely related polyploid species are scarce. Here, we developed an interspecies in vitro replication system using X. laevis egg extracts and sperm nuclei from three Xenopus species with varying ploidy: diploid (X. tropicalis), tetraploid (X. laevis), and dodecaploid (X. eysoole). Replication in diploid X. tropicalis was faster than in tetraploid X. laevis nuclei, due to higher fork density and speed. Surprisingly, dodecaploid X. eysoole replication was also accelerated compared to tetraploid nuclei, suggesting that higher ploidy does not necessarily extend S phase. Since replication genes are highly conserved between these species, these results imply dynamic tuning of replication programs across polyploid species and shed light on the evolutionary adaptability of DNA replication in response to genome duplication.