BMP Signalling: The Ancient Body Axis Blueprint That Keeps Reinventing Itself

A review published in the journal Development Genes and Evolution by David Mörsdorf, Paul Knabl and Grigory Genikhovich examines this question by surveying how bone morphogenetic protein (BMP) signalling builds the second, dorso-ventral body axis across the animal kingdom. The central message is striking: the system is at once highly conserved and extremely evolvable, so much so that even its core components can be deployed differently or lost outright in different species.

Bilateria, the group encompassing the vast majority of animal phyla, are defined by bilateral symmetry. They possess a main body axis connecting their anterior and posterior ends, a second axis running between their dorsal and ventral surfaces, and a left side that is roughly a mirror image of the right. BMP signalling has widely conserved functions in the formation and patterning of that second, dorso-ventral axis, although to different extents in different bilaterian species. Early findings in the fruit fly Drosophila and the frog Xenopus highlighted similarities between these evolutionarily very distant species, suggesting a single ancient mechanism might underlie the pattern.

More recent analyses featuring other model organisms have revealed considerable diversity in the mechanisms underlying dorsoventral patterning. As phylogenetic sampling becomes broader, the review argues, the axis patterning system proves so evolvable that even its core components can be deployed differently or lost in different model organisms. The authors highlight the diversity of ways by which BMP signalling controls bilaterality in different animals, some of which do not even belong to Bilateria, underscoring how deep the roots of this patterning system may run.

The review notes that vertebrates possess Smad8 and Smad9 alongside Smad1 and Smad5, but the authors refer to all Smad1/5/8/9 homologs as Smad1/5 for consistency. Similarly, Drosophila gene and protein names are mentioned on first encounter, after which the authors keep using their vertebrate orthologue names even when discussing fly genes, a terminological choice that keeps the cross-species comparison readable.

Historically, the field owes much to the French naturalist Étienne Geoffroy Saint-Hilaire, who lived from 1772 to 1844. He noticed that the relative positions of the central nervous system, heart and gut in a lobster were the same as in a mammal, but the ventral-most lobster structure corresponded to the dorsal-most mammalian structure. This observation gave rise to the so-called inverted lobster hypothesis, an early hint that dorsal and ventral identities might be flipped between major animal lineages while sharing the same underlying patterning logic.

The review also engages with the question of how far back bilaterality goes. The authors conclude that with the current review they cannot answer whether the last common ancestor of Cnidaria and Bilateria was bilaterally or radially symmetric, or whether the emergence of the Chordin-mediated BMP shuttling was the evolutionary event that made bilaterally symmetric body plans possible.

Phylogenetic details add further texture to the story. Analyses showed that the xenacoelomorphs Xenoturbella bocki and Symsagittifera roscoffensis possess two ADMP genes, orthologous to the ADMP1 and ADMP2 of ambulacrarians. The authors note this can be seen either as additional evidence in favour of the phylogenetic position of Xenacoelomorpha as sister to Ambulacraria, or as indicating multiple independent losses of ADMP2 across Bilateria.

Perhaps the most conceptually important point in the review concerns convergence. The authors write that often superficial differences originate from a detectable ancestral pattern, whilst superficial similarities may arise convergently based on the intrinsic properties of the BMP-Chordin system to break symmetry and activate downstream transcription factors in a concentration-dependent manner.

Looking forward, the authors argue that future research combining functional analyses and modelling is bound to give some understanding as to where the limits to the extent of the evolvability of BMP-dependent axial patterning may lie. No datasets were generated or analysed during the current study, the review states, marking it as a synthesis of existing literature rather than a source of new experimental data.

Subject of Research: BMP signalling in secondary axis patterning of Cnidaria and Bilateria

Article Title: Highly conserved and extremely evolvable: BMP signalling in secondary axis patterning of Cnidaria and Bilateria

Article References: Mörsdorf, D., Knabl, P., & Genikhovich, G. (2024). Highly conserved and extremely evolvable: BMP signalling in secondary axis patterning of Cnidaria and Bilateria. Development Genes and Evolution, 234(1), 1-19. https://doi.org/10.1007/s00427-024-00714-4

Image Credits: AI Generated

DOI: 10.1007/s00427-024-00714-4

Keywords: BMP signalling, dorso-ventral axis, bilateria, cnidaria, chordin, evolutionary developmental biology, body symmetry, smad1/5, axis patterning, evolvability, Highly, conserved