Abstract

DNA replication is a ubiquitous and conserved process. However, mechanistic exceptions exist that challenge our understanding of this system. This is exemplified by the replication band (RB) in spirotrich ciliates, which is a motile hub that traverses the macronucleus while replicating DNA . Here, we used a combination of computational and experimental approaches to examine the function and evolution of the RB. We show that the RB is paraphyletic due to its divergence in the spirotrich, Phacodinium metchnikoffi. Rather than a band, P. metchnikoffi contains a system we termed the replication envelope, wherein replication initiates at the nuclear envelope and advances inwards in a centripetal wave. Furthermore, we identified genes involved in cellular transport that are associated with the RB and may be involved in its translocation. These findings reveal the complex evolution of motile replication systems in ciliates and raise new possibilities for how nuclear organization can be regulated.