Double maternal effect: duplicated nucleoplasmin 2 genes,npm2aandnpm2b, are shared by fish and tetrapods, and have distinct and essential roles in early embryogenesis

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 twonpm2(npm2aandnpm2b) genes in zebrafish. We examined the evolution ofnpm2aandnpm2bin a variety of vertebrates, their potential phylogenetic relationships, and their biological functions using knockout models via the CRISPR/cas9 system. We demonstrated that the twonpm2duplicates exist in a wide range of vertebrates, including sharks, ray-finned fish, amphibians, and sauropsids, whilenpm2awas 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 thatnpm2aandnpm2bresulted 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 bothnpm2aandnpm2bare maternally-inherited transcripts in vertebrates. Moreover, we used zebrafish knockouts to demonstrate thatnpm2aandnpm2bplay essential, but distinct, roles in early embryogenesis.npm2afunctions very early during embryogenesis, at or immediately after fertilization, whilenpm2bis 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.Author SummaryThe protein and transcript of thenpm2gene have been previously demonstrated as maternal contributions to embryos of several vertebrates. Recently, twonpm2genes, denoted here asnpm2aandnpm2b, were discovered in zebrafish. This study was conducted to explore the evolutionary origin and changes that occurred that culminated in their current functions. We found that an ancient local duplication of the ancestralnpm2gene created the current two forms, and while most vertebrates retained both genes, notably, mammals and certain species of fish lostnpm2aand, albeit rarely, bothnpm2aandnpm2b. Our functional analyses showed thatnpm2aandnpm2bhave diverse but essential functions during embryogenesis, asnpm2amutants failed to undergo development at the earliest stage whilenpm2bmutants developed, although abnormally, until the zygotic genome activation stage after which their development was arrested followed subsequently by death. Our study is the first to clearly demonstrate the evolution, diversification, and functional analyses of thenpm2genes, which are essential maternal factors that are required for proper embryonic development and survival.