title: Molecular asymmetry in the 8-cell stage Xenopus tropicalis embryo described by single blastomere transcript sequencing.
abstract: Correct development of the vertebrate body plan requires the early definition of two asymmetric, perpendicular axes. The first axis is established during oocyte maturation, and the second is established by symmetry breaking shortly after fertilization. The physical processes generating the second asymmetric, or dorsal-ventral, axis are well understood, but the specific molecular determinants, presumed to be maternal gene products, are poorly characterized. Whilst enrichment of maternal mRNAs at the animal and vegetal poles in both the oocyte and the early embryo has been studied, little is known about the distribution of maternal mRNAs along either the dorsal-ventral or left-right axes during the early cleavage stages. Here we report an unbiased analysis of the distribution of maternal mRNA on all axes of the Xenopus tropicalis 8-cell stage embryo, based on sequencing of single blastomeres whose positions within the embryo are known. Analysis of pooled data from complete sets of blastomeres from four embryos has identified 908 mRNAs enriched in either the animal or vegetal blastomeres, of which 793 are not previously reported as enriched. In contrast, we find no evidence for asymmetric distribution along either the dorsal-ventral or left-right axes. We confirm that animal pole enrichment is on average distinctly lower than vegetal pole enrichment, and that considerable variation is found between reported enrichment levels in different studies. We use publicly available data to show that there is a significant association between genes with human disease annotation and enrichment at the animal pole. Mutations in the human ortholog of the most animally enriched novel gene, Slc35d1, are causative for Schneckenbecken dysplasia, and we show that a similar phenotype is produced by depletion of the orthologous protein in Xenopus embryos.
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, Mill Hill, London NW7 1AA, UK.
authors.forename: Elena
authors.initials: E
authors.lastname: De Domenico
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, Mill Hill, London NW7 1AA, UK.
authors.forename: Nick D L
authors.initials: ND
authors.lastname: Owens
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, Mill Hill, London NW7 1AA, UK.
authors.forename: Ian M
authors.initials: IM
authors.lastname: Grant
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, Mill Hill, London NW7 1AA, UK.
authors.forename: Rosa
authors.initials: R
authors.lastname: Gomes-Faria
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, Mill Hill, London NW7 1AA, UK. Electronic address: mike.gilchrist@crick.ac.uk.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Gilchrist
mesh_terms: D000818:Animals; D001757:Blastomeres / Q000378:metabolism*; D019521:Body Patterning / Q000235:genetics; D005260:Female; D055785:Gene Knockdown Techniques; D006801:Humans; D008297:Male; D023421:Models, Animal; D009004:Monosaccharide Transport Proteins / Q000037:antagonists & inhibitors / Q000235:genetics; D009154:Mutation; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D014981:Xenopus / Q000196:embryology* / Q000235:genetics* / Q000378:metabolism; D029867:Xenopus Proteins / Q000037:antagonists & inhibitors / Q000235:genetics
chemical_list: D009004:Monosaccharide Transport Proteins; D012333:RNA, Messenger; C425807:SLC35D1 protein, human; D029867:Xenopus Proteins
keywords: Cortical rotation; Dorsal determinants; Molecular asymmetry; Polar enrichment of mRNAs; Spatial distribution of maternal mRNAs; Xenopus tropicalis