Goin’ a-hunting

It’s a little-known fact that before/in between wanting to be a biologist, I almost got sucked into astronomy. The cosmos still fascinates me, from the menagerie of space rocks and gas balls that fill our own solar system to the mysteries at the edge of the known universe. To the evolutionist in me, the possibility of life on other worlds is an especially tantalising idea. And now we are finding other worlds at a breakneck pace. I don’t think we will ever know what life is like on any of them, though detecting its existence may once become possible.

Did I mention planet hunting is awesome?

I am talking about the citizen science project Planet Hunters, of course. This is only one of the amazing projects you can participate in at the Zooniverse (which gets its name from Galaxy Zoo, the project that started it all). The main mission of Planet Hunters is, of course, to find planets orbiting other stars. You, the user have to look at a month’s worth of brightness measurements from a star, and search for the tell-tale dips that betray an extra-solar eclipse. Like this:

Most of the more spectacular ones have already been found by this point – either by your fellow hunters, or by the team operating the Kepler space telescope, which provides all the data. However, there are so many other gems to discover among those messy light curves that it almost doesn’t matter if your planet-hunting thunder is perpetually stolen.

Sometimes, you find pure beauty. One of the most common types of Interesting Stuff that the Kepler data offer is eclipsing binaries. These are pairs of stars orbiting each other in a way that we see their orbits edge on. Like the planets, these binaries eclipse their companion stars. Since stars are bigger and brighter than planets, the eclipses are much bigger compared to the noise in the data, so an EB has neat, clean dips in its light curve, occurring with clockwork regularity.

Some of them are so close together and orbit so fast that at Kepler’s resolution, a month of their light looks more like lace than a pattern of ups and downs.

And then there are all the others; dwarfs and giants, variable stars regular and haphazard, huge flares, weird things like cataclysmic variables. Even if you are in it for the planets, you can’t help but learn a lot about the stars. After a while, they become like family. You look at a light curve and you can immediately guess whether it’s a dwarf or a giant, whether it’s cool or hot, whether it’s a binary or a loner, or even if its’s one of the rarer breeds of stars you might come across. It’s a bit like birdwatching. If you’ve ever got disproportionately excited from recognising a rare bird (or flower, or insect, or sports car), you know what I mean. (If you haven’t, what are you waiting for? ;))

I’m grateful to the people who make these adventures possible. It’s great that I can play at astronomy, see all that neat stuff, contribute to a field I have absolutely no expertise in, and learn from the knowledgeable folks that hang around the forums. The Zooniverse deserves every one of its hundreds of thousands of users and millions of clicks, is all I’m saying 🙂

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!

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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