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Double maternal effect: duplicated nucleoplasmin 2 genes, npm2a and npm2b, are shared by fish and tetrapods, and have distinct and essential roles in early embryogenesis

  • Cheung, Caroline
  • Pasquier, Jérémy
  • Bouleau, Aurélien
  • Nguyen, Thuy Thao Vi
  • Chesnel, Franck
  • Guiguen, Yann
  • Bobe, Julien
Publication Date
Jan 01, 2017
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Nucleoplasmin 2 (npm2) is an essential maternal-effect gene that mediates early embryonic events through its function as a histone chaperone that remodels chromatin. Here we report the existence of two npm2 (npm2a and npm2b) genes in zebrafish. We examined the evolution of npm2a and npm2b in a variety of vertebrates, their potential phylogenetic relationships, and their biological functions using knockout models via the CRISPR/cas9 system. We demonstrated that the two npm2 duplicates exist in a wide range of vertebrates, including sharks, ray-finned fish, amphibians, and sauropsids, while npm2a was lost in Coelacanth and mammals, as well as some specific teleost lineages. Using phylogeny and synteny analyses, we traced their origins to the early stages of vertebrate evolution. Our findings suggested that npm2a and npm2b resulted from an ancient local gene duplication, and their functions diverged although key protein domains were conserved. We then investigated their functions by examining their tissue distribution in a wide variety of species and found that they shared ovarian-specific expression, a key feature of maternal-effect genes. We also showed that both npm2a and npm2b are maternally-inherited transcripts in vertebrates. Moreover, we used zebrafish knockouts to demonstrate that npm2a and npm2b play essential, but distinct, roles in early embryogenesis. npm2a functions very early during embryogenesis, at or immediately after fertilization, while npm2b is involved in processes leading up to or during zygotic genome activation. These novel findings will broaden our knowledge on the evolutionary diversity of maternal-effect genes and underlying mechanisms that contribute to vertebrate reproductive success.

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