Tuesday, November 25, 2008

Trichoplax adhaerens and the development of eyes

On the development of eyes

Several types of eyes exist and these include the camera-type eye, the compound eye, and the mirror eye (Figure 1). Ernst Mayr proposed that eyes evolved in all animal phyla 40 to 60 times independently.
A monophyletic program governing the development of the different eye types is proposed and the Pax6 gene is posited to be the master control gene. The Pax6 gene also plays a part in controlling the development of the nose, ears and parts of the brain.

What is needed for the developmental program of eyes?

A few core genes include:
Pax6 (eyeless [eye]) in Drosophila)
Six-type genes (E.g. Six3)
Sox-type genes (E.g. Sox2)
atonal ( E.g. Atoh7)
Retinoid receptors
Fox transcription factors (E.g. FoxN4)

Fascinating experiments have been conducted by shuffling around the genetic program architecture of genes associated with eye development in various animals.
For example in Drosophila:
Ectopic eye structures are able to be induced on the antennae, legs, and wings of fruit flies. This is done by targeted expression of the eyeless gene (Pax6 Drosophila homologue) (Figure 2). The Pax6 gene from the mouse is able to do the same job as the Drosophila version (Figure 3). And in Xenopus embryos, ectopic eye structures in can also be induced by the Drosophila eyeless (Pax6) version (Figure 4).

What about the Trichoplax adhaerens genome? Any genes for eye development?
It seems quite a chunk of the circuitry needed for eye development is present. (From table 1)
PaxB (eyeless?)
Six genes
Sox gene
Atonal gene
Retinoid X Receptor
Fox transcription factors

All that is missing seems to be crystalins (plays a part in lens formation). However, Darwin posited that "The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells and covered by translucent skin, but without any lens or other refractive body." Thus large chunks of the circuitry for eye development in Trichoplax is present but no eyes!

Now compare the developmental program to evolution.

Here is an interesting article that shows the parallels between evolution and development.

For development:
Primordial germ cells (PGC) are prevented from entering the somatic program and are demethylated (genome-wide erasure of existing epigenetic modifications). Then the gametes are imprinted (targeted DNA methylation) during gametogenesis, only to be demethylated again after fertilization. Then during development, DNA is methylated again, causing totipotential cells to become pluripotent. X-inactivation and reactivation (of the paternal gamete I think) also occurs. The whole process is governed by the genetic (and epigenetic?) program. During the unfolding of this somatic program, random variation and selection occur, ultimately leading to just a few endpoints, every time it is successful. The process is constrained (few end points) as a result of pre-existing information that is set up during the inititiation of the process. All this is controlled by information in the genome.

For evolution:
There also seems to be only a few endpoints (small subset, limited variation) out of all the possible endpoints.
In the article:
An End to Endless Forms: Epistasis, Phenotype Distribution Bias, and Nonuniform Evolution
It is argued to be as a result of genetic instructions dating earlier in evolutionary time. Preadaptations...

As already seen in the evolution of eyes, as soon as these sets of genes were formed (E.g. Pax genes), through whatever mechanism), evolution seemed to have been biased to a few end points, and these few endpoints arose 40-60 times, independently, as a result of pre-existing (preadaptations) information in the case of eyes.

What other "biased" end points can there be? Nervous systems, smell, hearing? And why would evolution be biased, as in development, to only reach a few end points over and over?


William Bradford said...

Your blogs are well researched and well organized. Did you receive a message from Telic Thoughts?

NT said...

Hi Bradford,

Thanks, I have read the e-mail and responded.