Evolutionary diversification of <i>MCM3</i> genes in <i>Xenopus laevis</i> and <i>Danio rerio</i>

<div><p>Embryonic cell cycles of amphibians are rapid and lack zygotic transcription and checkpoint control. At the mid-blastula transition, zygotic transcription is initiated and cell divisions become asynchronous. Several cell cycle-related amphibian genes retain 2 distinct forms, maternal and zygotic, but little is known about the functional differences between these 2 forms of proteins. The minichromosome maintenance (MCM) 2–7 complex, consisting of 6 MCM proteins, plays a central role in the regulation of eukaryotic DNA replication. Almost all eukaryotes retain just a single <i>MCM</i> gene for each subunit. Here we report that <i>Xenopus</i> and zebrafish have 2 copies of <i>MCM3</i> genes, one of which shows a maternal and the other a zygotic expression pattern. Phylogenetic analysis shows that the <i>Xenopus</i> and zebrafish zygotic <i>MCM3</i> genes are more similar to their mammalian <i>MCM3</i> ortholog, suggesting that maternal <i>MCM3</i> was lost during evolution in most vertebrate lineages. Maternal MCM3 proteins in these 2 species are functionally different from zygotic MCM3 proteins because zygotic, but not maternal, MCM3 possesses an active nuclear localization signal in its C-terminal region, such as mammalian MCM3 orthologs do. mRNA injection experiments in zebrafish embryos show that overexpression of maternal <i>MCM3</i> impairs proliferation and causes developmental defects, whereas zygotic <i>MCM3</i> has a much weaker effect. This difference is brought about by the difference in their C-terminal regions, which contain putative nuclear localization signals; swapping the C-terminal region between maternal and zygotic genes diminishes the developmental defects. This study suggests that evolutionary diversification has occurred in <i>MCM3</i> genes, leading to distinct functions, possibly as an adaption to the rapid DNA replication required for early development of <i>Xenopus</i> and zebrafish.</p>