Throughout 500 million years of evolution, vertebrates have diversified their morphologies while somehow conserving their basic anatomical features, or the body plan. The developmental hourglass model posits an intriguing hypothesis to explain why the body plan is conserved. This model suggests that the body plan conservation can be attributed to the conserved developmental period, known as the phylotypic period, during which the body plan elements commence their development.
Consistent with this notion, previous comparative transcriptome studies have revealed evolutionary conservation during the mid-embryonic organogenesis period across vertebrates at a whole embryo level, with earlier and later stages displaying more divergence. While this mid-embryonic conservation lends support to the developmental hourglass model, whether developmental processes underlying the body plan are truly conserved remains unknown, and it remains plausible that the whole embryo-level conservation observed during the middle embryonic period might be unrelated to the evolutionary conservation of the body plan.
In this study, we examined the cell type-level conservation of the phylotypic period by conducting single-cell transcriptome comparisons across five vertebrate species: Mus musculus, Gallus gallus, Pelodiscus sinensis, Danio rerio, and Oryzias latipes. By profiling tens of thousands of single-cell transcriptomes, we identified about 25 distinct cell clusters for each species. We subsequently linked these identified cell clusters across species using orthologous gene information, leading to the identification of homologous cell clusters characterized by shared gene expression profiles. Leveraging publicly available cell atlases, we annotated these identified homologous cell types and quantified the transcriptome similarities among the examined species, serving as a proxy for assessing the evolutionary conservation of cell types.
Our results show that cell types exhibiting higher transcriptome similarities are predominantly associated with various body plan elements, including the heart, neural tube, and notochord. This finding suggests that cell types integral to the body plan exhibit a high degree of conservation across vertebrates, prompting speculation that the conservation of developmental processes at the cell type level contributes to the phenotypic constraints governing vertebrate body plans. Additionally, we will present preliminary results and discuss potential molecular mechanisms underpinning the evolution of vertebrate body plans.