Allan Carrillo-Baltodano: Different developmental tracks, to form a belly and a back: comparative developmental biology shines light in the evolution of animal body plans

Photo description: larvae of four different species of annelids covered in this study. 

From left to right: Spirobranchus lamarcki, Owenia fusiformis, Platynereis dumerilii and Capitella teleta. The larvae have been stained with fluorescent markers to see the muscle (magenta) and cilia (yellow). 

Credit: Allan M. Carrillo-Baltodano. Creative Commons Attribution 4.0 International License.

An international group of scientists from Queen Mary University of London,
Academia Sinica and the University of St Andrews just published a new study in
Nature Communications showing evidence of developmental system drift in annelids
(earthworms, ragworms). Developmental system drift or DSD is a usual overlooked
evolutionary outcome where extant species use different genes to form and pattern
an ancestral structure or tissue (in this case the back of the body).

Having a back and a belly is a characteristic present in the so called bilaterian
animals, that include annelids and us, humans. In the majority of bilaterian animals,
the patterning of this body axis is accomplished by the activation of a cell signalling
pathway called Bone Morphogenetic Protein (BMP) pathway. However, in annelids
studied so far, BMP does not play a role, instead relying on a separate pathway
called Nodal. Moreover, annelids can undergo two types of cleavage during early
development – equal, where the zygote will divide into two equally sized daughter, or
unequal, where one of the daughters is bigger. Notably, all annelids studied so far
had unequal cleavage, which is thought to have derived from the ancestral equal
cleavage multiple times throughout evolution.

Using pharmacological and cell ablation experiments in combination with
bioinformatic analyses in four different species of annelids (two equal and two
unequal), the researchers led by Allan Carrillo-Baltodano, postdoctoral research
associate and Chema Martín-Durán, Professor at Queen Mary’s School of Biological
and Behavioural Sciences, found that those annelids with equal cleavage have
maintained the usage of the conserved BMP to pattern the body axis, while those
with unequal have not only changed to use Nodal (or BMP just to pattern the head),
but also have changed what genes are part of the network that pattern the belly vs
the back.

Crucial to this work was the validation of candidate genes obtained from the
transcriptomic analyses. During the LIDo summer research placement in 2021,
Artenis Pashaj from Kingston University expended two months learning from Allan
how to analyse part of the transcriptomic data, and performing gene expression
analyses in the bench to look at the spatial expression of the candidates during the
development of some of the annelids.

This work provides the first evidence-based results in annelids that genetic
underpinnings deployed during development has changed from an ancestral network
(BMP) to a derived network (Nodal), although the outcome remains the same
(having a back). Future research is exploring how these changes affect specific cell
types in species with equal and unequal cleavage, to determine the impact of DSD in
the formation of tissues and organs.

Link to the paper: https://www.nature.com/articles/s41467-026-71950-7 

Allan’s LinkedIn profile: https://www.linkedin.com/in/allan-carrillo-baltodano-705976384/   

Allan’s Bluesky profile: https://bsky.app/profile/allancarbal.bsky.social 

Martin-Duran lab website: https://www.martinduranlab.com/