The bare bones of fins and limbs

Perhaps the central question in developmental biology is how cells that start out as identical end up making bodies with complex shapes and a multitude of different tissues. And perhaps the central question in evo-devo is how such bodies can change into other bodies during the course of evolution. A really cool paper by Zhu et al. (2010) probes a little bit at both, and shows how relatively simple rules can produce results that are surprisingly similar to what we observe in nature.

The authors modelled the development of limb (or fin) bones in vertebrates. They used a simple model made up of the following:

  1. a virtual limb bud (let me call them “simbuds” hereafter) growing continuously
  2. a signal spreading from the tip of the bud that tells “cells” to keep growing but wanes over time (mimicking the role of the apical ectodermal ridge in real limb buds)
  3. two equations describing the activity of (1) genes that make cells differentiate into bone (“activators”), and (2) genes that prevent cells from doing so (“inhibitors”)

The shape of the simbud could be set at the start, and so could the values of all the parameters in the activator and inhibitor equations.

This is much more simple than real limb develompent. It says nothing about cell movement, and it condenses the effect of genes other than the bone activators and inhibitors into two little parameters in the equations. Yet running it with pretty much any initial settings produces something vaguely limb-like, and some sets of parameters give you simbuds that look eerily like real limbs.

Development of a simbud mimicking a chicken wing, next to drawings of the real thing.

Or fins. Or mutant limbs. Or transitional fossils.

Fully developed simbuds resembling various fossil fins: Brachypterygius was a marine reptile from the Jurassic; the other four are more or less close relatives of tetrapods, among them the famous "fishapod" Tiktaalik.

The similarity is not perfect, of course – but the model is not perfect either. Overall, it’s still pretty amazing what a variety of very realistic limb skeletons you can get out of such a simple setup – and how much you can achieve just by varying small things like how wide the limb bud is to begin with or how strongly two gene networks interact. Evolving fins into limbs should be a piece of cake for a system like that!

***

Reference

Zhu J, Zhang Y-T, Alber MS, Newman SA (2010) Bare Bones Pattern Formation: A Core Regulatory Network in Varying Geometries Reproduces Major Features of Vertebrate Limb Development and Evolution. PLoS ONE 5:e10892

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