Citation: Erkenbrack, E. M., Ako-Asare, K., Miller, E., Tekelenburg, S., Thompson, J. R., & Romano, L. (2016). Ancestral state reconstruction by comparative analysis of a GRN kernel operating in echinoderms. Development Genes and Evolution, 226(1), 37-45.
Animals consist of a wide variety of cells that serve different functions depending on their location in the body. Cells with similar functions, or cell types, in different animal species are related both by an evolutionary line of descentÐsimilar to the relatedness of species themselvesÐand by a developmental line of descent in the embryo. Networks of interacting genes, or gene regulatory networks, control gene expression in the cell, thereby specifying cell type identity. Understanding how new cell types arise by changing gene regulatory networks is critical both to comprehending fundamental aspects of human biology and to manipulating cell types in the laboratory. We approached this question by studying endometrial stromal fibroblast (ESF) cells from the uterus of humans and opossums, two distantly related mammals. We showed that the distantly related cell type in opossum expresses a similar set of regulatory genes as the human cell, but in response to pregnancy-related signals, the opossum cells induce a stress response. In the human cells, these signals induce differentiation into decidual cells, a specialized cell type present in humans and closely related mammals. These results suggest that a gene regulatory network that modulated an ancestral, pregnancy-related stress response was hijacked and repurposed to function in differentiation and specification of the decidual cell type.
This work provides direct evidence of evolutionary rewiring of gene-regulatory circuitry accompanying divergence of two subclasses of echinoderm, the cidaroid and euechinoid sea urchins. These forms descend from a known common Paleo- zoic ancestor, and their embryos develop differently, offering an opportunity to probe the basic evolutionary process by which clade divergence occurs at the gene-regulatory net- work (GRN) level. We carried out a systematic analysis of the use of particular genes participating in embryonic skeleto- genic cell specification, building on an established euechinoid developmental GRN. This study revealed that the well-known and elegantly configured regulatory circuitry that underlies skeletogenic specification in modern sea urchins is largely a novel evolutionary invention. The results dramatically dis- play extensive regulatory changes in a specific developmental GRN, underlying an incidence of cladistic divergence at the subclass level.
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