The mid-developmental transition and the evolution of animal body plans
Ontology highlight
ABSTRACT:
PROVIDER: PRJNA287810 | ENA |
REPOSITORIES: ENA
Dataset's files
| Action | DRS | |||
|---|---|---|---|---|
| SRR1552740.fastq.gz | Fastqsanger.gz | |||
| SRR1552750.fastq.gz | Fastqsanger.gz | |||
| SRR1552760.fastq.gz | Fastqsanger.gz | |||
| SRR1552770.fastq.gz | Fastqsanger.gz | |||
| SRR1552780.fastq.gz | Fastqsanger.gz |
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Project description:Animals are grouped into ~35 'phyla' based upon the notion of distinct body plans. Morphological and molecular analyses have revealed that a stage in the middle of development--known as the phylotypic period--is conserved among species within some phyla. Although these analyses provide evidence for their existence, phyla have also been criticized as lacking an objective definition, and consequently based on arbitrary groupings of animals. Here we compare the developmental transcriptomes of ten species, each annotated to a different phylum, with a wide range of life histories and embryonic forms. We find that in all ten species, development comprises the coupling of early and late phases of conserved gene expression. These phases are linked by a divergent 'mid-developmental transition' that uses species-specific suites of signalling pathways and transcription factors. This mid-developmental transition overlaps with the phylotypic period that has been defined previously for three of the ten phyla, suggesting that transcriptional circuits and signalling mechanisms active during this transition are crucial for defining the phyletic body plan and that the mid-developmental transition may be used to define phylotypic periods in other phyla. Placing these observations alongside the reported conservation of mid-development within phyla, we propose that a phylum may be defined as a collection of species whose gene expression at the mid-developmental transition is both highly conserved among them, yet divergent relative to other species.
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Project description:Evolutionary theory assumes that genetic variation is uniform and gradual in nature, yet morphological and gene expression studies have revealed that different life-stages exhibit distinct levels of cross-species conservation. In particular, a stage in mid-embryogenesis is highly conserved across species of the same phylum, suggesting that this stage is subject to developmental constraints, either by increased purifying selection or by a strong mutational bias. An alternative explanation, however, holds that the same ‘hourglass’ pattern of variation may result from increased positive selection at the earlier and later stages of development. To distinguish between these scenarios, we examined gene expression variation in a population of the nematode C. elegans using an experimental design that eliminated the influence of positive selection. By measuring gene expression for all genes throughout development in twenty strains, we found that variations were dramatically uneven throughout development, with a significant depletion during mid-embryogenesis. In particular, the family of homeodomain transcription factors, whose expression generally coincides with mid-embryogenesis, evolved under high constraint. Our data further shows that genes responsible for the integration of germ layers during morphogenesis are the most constrained class of genes. Together, these results provide the first evidence for developmental constraints as the mechanism underlying the hourglass model of animal evolution. Understanding the pattern and mechanism of developmental constraints provides a framework to understand how evolutionary processes have interacted with embryogenesis and led to the diversity of animal life on earth.
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Project description:To validate that EU-RNA imaging provides a direct readout of wide-spread zygotic transcription, we sought to identify the nascent transcriptome using RNA-seq. We micoinjected 5-ethynyl uridine (EU) into 1-cell Xenopus embryos, isolated total RNA from embryos at mid-blastula transition (MBT), biotinylated EU-RNA and purified it to generate cDNA libraries for sequencing. Total RNA from normal embyros were used as control. We found over 25,000 nascent genes in this EU-RNA dataset at mid-ZGA. The nascent transcriptome dataset captures nearly 90% of the transcripts present in the whole-embryo dataset and with similar or better read-depth, indicating that our EU-RNA imaging is truly representative of wide-spread zygotic transcription. Furthermore, of the known zygotic genes that are most highly induced at MBT, we detected all of them and at much higher levels than in the whole-embryo dataset. These results show that our labeling captures the nascent zygotic transcriptome. We found ~ 4-fold higher sensitivity for nascent transcripts in the EU-RNA dataset compared to the whole embryo dataset, and as excepted we did not detect thousands of maternal-only transcripts in the nascent dataset. Together, these results suggest that EU-labeling provides a visual readout of bona fide nascent zygotic transcripts.
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