Zebrafish were recently used as an in vivo model system to study DNA replication timing during development. Here is detailed the protocols for using zebrafish embryos to profile replication timing. This protocol can be easily adapted to study replication timing in mutants, individual cell types, disease models, and other species.
Siefert, J. C., Clowdus, E. A., Goins, D., Koren, A., Sansam, C. L. Profiling DNA Replication Timing Using Zebrafish as an In Vivo Model System. J. Vis. Exp. (134), e57146, doi:10.3791/57146 (2018).