Sex Bias and Maternal Contribution to Gene Expression Divergence in Drosophila Blastoderm Embryos.
PLoS Genet 2015;
11:e1005592. [PMID:
26485701 PMCID:
PMC4618353 DOI:
10.1371/journal.pgen.1005592]
[Citation(s) in RCA: 24] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2015] [Accepted: 09/18/2015] [Indexed: 11/27/2022] Open
Abstract
Early embryogenesis is a unique developmental stage where genetic control of development is handed off from mother to zygote. Yet the contribution of this transition to the evolution of gene expression is poorly understood. Here we study two aspects of gene expression specific to early embryogenesis in Drosophila: sex-biased gene expression prior to the onset of canonical X chromosomal dosage compensation, and the contribution of maternally supplied mRNAs. We sequenced mRNAs from individual unfertilized eggs and precisely staged and sexed blastoderm embryos, and compared levels between D. melanogaster, D. yakuba, D. pseudoobscura and D. virilis. First, we find that mRNA content is highly conserved for a given stage and that studies relying on pooled embryos likely systematically overstate the degree of gene expression divergence. Unlike studies done on larvae and adults where most species show a larger proportion of genes with male-biased expression, we find that transcripts in Drosophila embryos are largely female-biased in all species, likely due to incomplete dosage compensation prior to the activation of the canonical dosage compensation mechanism. The divergence of sex-biased gene expression across species is observed to be often due to lineage-specific decrease of expression; the most drastic example of which is the overall reduction of male expression from the neo-X chromosome in D. pseudoobscura, leading to a pervasive female-bias on this chromosome. We see no evidence for a faster evolution of expression on the X chromosome in embryos (no “faster-X” effect), unlike in adults, and contrary to a previous study on pooled non-sexed embryos. Finally, we find that most genes are conserved in regard to their maternal or zygotic origin of transcription, and present evidence that differences in maternal contribution to the blastoderm transcript pool may be due to species-specific divergence of transcript degradation rates.
The early embryonic stage of development is unique as this stage begins under the genetic control of the mother and transitions to genetic control of the zygote. In order to investigate how these developmental specificities have shaped gene expression evolution, we studied the contribution of maternal gene expression to the pool of transcripts in the early embryo and how gene expression differs in early female and male embryos. We find extensive female-bias in gene expression, something not observed in adults. This is largely due to the delay in the establishment of a mechanism that balances gene expression from the differing number of X chromosomes in females and males. The maternal contribution to the transcript pool in the embryo is largely conserved over species, with some exceptions. However, there may also be some species-specific differences in the degradation rate of maternal transcripts, an often-overlooked contributor to variation in transcript level.
Collapse