Old Questions and Young Approaches to Animal Evolution by José M. Martín-Durán & Bruno C. Vellutini

Author:José M. Martín-Durán & Bruno C. Vellutini
Language: eng
Format: epub
ISBN: 9783030182021
Publisher: Springer International Publishing

7.4 Where Do We Go from Here?

The more detailed and fine-scale experiments using novel molecular approaches in germ layer specification have shed a great deal of light on this crucial embryological process that has fascinated embryologists for centuries. The examples we have looked at provide some clues as to why there is so much controversy surrounding the germ layer theory since its inception. It is clear that the molecular mechanisms of germ layer specification are remarkably similar across a diverse array of animal taxa that have characteristically different body plans given the conserved nature of GRN kernels, therefore, providing support for the homology of germ layers. However, the evolution of these processes involves the integration of multiple cell signaling events and fine-scale regulatory interactions, making germ layer specification a highly complex developmental event, and making it sometimes difficult to identify the similar evolutionary origins shared by these different germ layers. Therefore, further experimentation carried out in phylogenetically informative taxa using more powerful and novel techniques will help to gain further evolutionary insights into the molecular control of germ layer specification.

The future looks promising, given the innovative, novel techniques that have been developed for experimental analysis of animal embryology. Novel sequencing techniques have made it possible to generate high-resolution transcriptome data at the single cell level. Recent studies on Xenopus and zebrafish have generated whole embryo transcriptomes at different, critical developmental time points at single-cell resolution (Farrell et al. 2018; Wagner et al. 2018; Briggs et al. 2018). These studies have made it possible to characterize the developmental trajectories of individual cells which acquire characteristic fates by transitioning through different transcriptional states depending on the signals they receive during embryonic development. For example, during zebrafish development, some cells express genes that are characteristic of multiple developmental fates, and they have the ability to trans-specify from one fate to another (Farrell et al. 2018). Taking this approach one step further, novel techniques are being developed to better characterize the regulatory events underlying this transitioning through different transcriptional states during early embryogenesis. A recent study looking at changes in the chromatin regulatory landscapes during Drosophila embryogenesis has used a novel technique called single-cell combinatorial indexing assay for transposase accessible chromatin with sequencing (sci-ATAC-seq) to characterize the chromatin accessibility in over 20,000 single nuclei (Cusanovich et al. 2018). The findings of this study have shown that there is spatial heterogeneity in the accessibility of the regulatory genome before gastrulation, in line with future cell fates of these different cells. Furthermore, the individual cell types can be identified by their chromatin accessibility, and these cells maintain a signature that can be used to identify their germ layer origin. Interestingly, they have also identified common regulatory elements between cells of the endoderm and non-myogenic mesoderm, a finding that further supports the hypothesis of a bifunctional endomesoderm giving rise to endoderm and mesoderm (Cusanovich et al. 2018). With the help of these novel techniques, we are beginning to understand the complexity of the process of germ layer

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