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Evolution of maternal and zygotic mRNA complements in the early Drosophila embryo

ABSTRACT: The earliest stage of animal development is controlled by maternally deposited mRNA transcripts and proteins. Once the zygote is able to transcribe its own genome, maternal transcripts are degraded, in a tightly regulated process known as the maternal to zygotic transition (MZT). While this process has been well-studied within model species, we have little knowledge of how the pools of maternal and zygotic transcripts evolve. To characterize the evolutionary dynamics and functional constraints on the timing of early embryonic expression, we created a transcriptomic dataset for 14 Drosophila species spanning over 50 million years of evolution, at developmental stages before and after the MZT, and compared our results with a previously published Aedes aegypti developmental time course. We found deep conservation over 250 million years of a core set of genes transcribed only by the zygote. This select group is highly enriched in transcription factors that play critical roles in early development. However, we also identify a surprisingly high level of turnover of transcripts represented at both stages over the phylogeny. While mRNA levels of genes with maternally deposited transcripts are more highly conserved than zygotic genes, those maternal transcripts that are completely degraded at the MZT vary dramatically between species. We also show that hundreds of genes have different isoform usage between the maternal and zygotic genomes. Our work suggests that maternal transcript deposition and early zygotic transcription are surprisingly dynamic over evolutionary time, despite the widespread conservation of early developmental processes.

ORGANISM(S): Drosophila ananassae Drosophila erecta Drosophila miranda Drosophila mauritiana Drosophila yakuba Drosophila pseudoobscura Drosophila melanogaster Drosophila sechellia Drosophila mojavensis Drosophila persimilis Drosophila santomea Drosophila simulans Drosophila willistoni Drosophila virilis

PROVIDER: GSE112858 | GEO | 2018/12/04

REPOSITORIES: GEO

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Project description:Maternal gene products supplied to the egg during oogenesis drive the earliest events of development in all metazoans. After the initial stages of embryogenesis, maternal transcripts are degraded as zygotic transcription is activated; this is known as the maternal to zygotic transition (MZT). Altering the abundances of maternally deposited factors in the laboratory can have a dramatic effect on development, adult phenotypes and ultimately fitness. Zygotic transcription activation is a tightly regulated process, where the zygotic genome takes over control of development from the maternal genome, and is required for the viability of the organism. Recently, it has been shown that the expression of maternal and zygotic transcripts have evolved in the Drosophila genus over the course of 50 million years of evolution. However, the extent of natural variation of maternal and zygotic transcripts within a species has yet to be determined. We asked how the maternal and zygotic pools of mRNA vary within and between populations of D. melanogaster. In order to maximize sampling of genetic diversity, African lines of D. melanogaster originating from Zambia as well as DGRP lines originating from North America were chosen for transcriptomic analysis. Single embryo RNA-seq was performed before and after zygotic genome activation to determine which transcripts are maternally deposited and which are zygotically expressed within and between these populations. Differential gene expression analysis has been used to quantify quantitative changes in RNA levels within populations as well as fixed expression differences between populations at both stages. Generally, we find that maternal transcripts are more highly conserved, and zygotic transcripts evolve at a higher rate. We find that there is more within-population variation in transcript abundance than between populations and that expression variation is highest post- MZT between African lines. Determining the natural variation of gene expression surrounding the MZT in natural populations of D. melanogaster gives insight into the extent of how a tightly regulated process may vary within a species, the extent of developmental constraint at both stages and on both the maternal and zygotic genomes, and reveals expression changes allowing this species to adapt as it spread across the world.

Project description:During early stages of embryonic development the genome is transcriptionally inactive and cells are under the control of maternally provided mRNA and proteins. At a key point in development, known as the maternal to zygotic transition (MZT), the genome becomes activated and the maternally provided mRNAs begin to degrade. In the early zebrafish embryo, when under maternal control, cells divide rapidly and synchronously with cell cycles that lack gap phases. At cell cycle ten the introduction of gap phases lengthens the cell cycle and synchronised division is lost. During the MZT, zygotic activation of microRNAs leads to the targeted degradation of a number of maternally provided mRNAs, thus linking genome activation to maternal mRNA degredation.While the MZT has been studied in several different organisms the molecular mechanisms that coordinate genome activity and mRNA degradation remain largely unknown. For example, while the bulk of zygotic transcription occurs at cell cycle ten we do not understand why there is a minor wave of transcription before this time point. Similarly maternal mRNAs degrade at different rates, with only a percentage undergoing microRNA-mediated degredation. The aparant different rates of maternal mRNA degredation may be obscured by zygotic transcription.In order to gain an understanding of the MZT I intend to establish a precise understanding of transcription in the early embryo by using solexa sequencing to perform transcript counting at five different developmental stages that span the MZT. Specifically I intend to use crosses from two different zebrafish strains SAT (Sequenced AB and Tbingen, Zv9) and WIK. This project will allow one to understand the overall transcription profiles of genes in the early embryo, but importantly, the SNPs between the two different strains will determine if transcripts are maternal or zygotic (paternal). As a proof of principle we will first use one lane of sequencing to identify transcripts from a cross of an SAT and a WIK fish. This will allow us to observe SNPs between the two different strains. This will be run over one solexa lane. We will then sequence from five different time points on four different crosses SAT male and WIK female, WIK male and SAT female, WIK male and WIK female and SAT male with SAT female. We will prepare the libraries, which will be sequenced paired-end 54 bp over a total of seven lanes of solexa.. This data is part of a pre-publication release. For information on the proper use of pre-publication data shared by the Wellcome Trust Sanger Institute (including details of any publication moratoria), please see http://www.sanger.ac.uk/datasharing/

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Evolution of maternal and zygotic mRNA complements in the early Drosophila embryo

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