Improving EvoDevo blog

Dear all,

I just added a sitemeter to my blog today. It created very powerful motivation for me to improve my blog.

If there are any comments which you would like to add, do feel free to let me know. I welcome all feedback.

Another decision I made is to post regularly on every monday so it is easier to know when to expect new posts.

Third, I hope to improve the quality of my writing-its contents and style, and the graphic presentation of my post-more pictures, color, etcetera etcetera.

That's all for now,
Regards,
The Nutty Professor

Regulatory networks and Evolution of animal body plans

Here's a little quiz. Below is an intriguing picture of the fossil remains of animals living approximately 510 million years ago. What common characteristics do you observe?

Fig. 1. Examples of Cambrian body plans from the Early Cambrian (È510 million years ago) Chengjiang Fauna of Yunnan Province, China (D to I) and the Middle Cambrian Burgess Shale Fauna of British Columbia, Canada (A to C, J). (Davidson, 2006)

(A note: Davidson and Erwin provided an excellent review discussing how changes gene regulatory networks and evolution of body plans could be linked. Refer to the journal article "Gene regulatory networks and evolution of animal body plans" published in Science in 2006 for a more indepth discussion.)

Back to the little question above, some of the common characteristics we can observe are:-

1. Bilateral

2. Appendages and their anterior-posterior organization and position
3. Body plans bear striking similarity with modern phyla

(Phylum: In biological taxonomy, phylum is one of the ranks in biological classification of eight major levels, such as species, genus, family, order, class, phylum, kingdom, domain, life.)

What can we gather from the fact that the body plans of these animals are so similar to modern phyla?

It is simply that there is little change in phylum- and superphylum-level body plans since the Early Cambrian.

(Cambrian: a major division of the geologic timescale beginning approximately 542 million years ago. The cambrian explosion is characterized by the sudden explosion of hard body fossils)

On the other hand, while there is little change in these levels since the explosion, great changes have subsequently occured within phyla and classes. In addition, the process of speciation is continuous, unlike the abrupt change in phylum-level body plans.

What explains such difference in evolution of development?

Davidson and Erwin proposed that diverse kinds of change in gene regulatory networks (GRNs) have diverse evolutionary consequences. They cleverly dissect the structure of the GRNs into smaller subcircuits such as

i) "Kernels"- evolutionarily inflexible subcircuits that perform essential upstream functions in building body parts

ii) "Plug-ins"-not dedicated to formation of body parts but are inserted into many different networks;

iii) Cis-regulatory "I/O switches"-that regulate other subcircuits

(Both plug-ins and I/O switches give variety in size and morphological patterns)

iv) "Differentiation batteries"- groups of protein-encoding genes under common regulatory control; at the periphery of GRNs; do not regulate other genes and are thus evolutionarily labile inherently

The following diagram illustrates the model proposed by Davidson and Erwin to explain the difference in evolutionary development in different biological levels (ie. phylum versus species). It shows the evolutionary consequences of changes in the different components of the GRNs.

What I like about Science

Science provided a way for exploring the realm of Nature. It is systematic, logical and clever. The vast possibilities that Science creates are also very attractive. A career as a scientist provides great training to the mind. A scientific career offers much space and alot of opportunities to build resilience and to learn, think, wonder and imagine. The learning and intellectual process in Science gives me alot of joy.

Researchers find a gene for fear

One amazing outcome of Science is that it reveals the depths of Nature's secrets. Science offers an alternative perspective, be it a deeper perspective or a different angle of looking at things. One recent example is the identification of a fear factor- a protein the brain uses to generate one of the most powerful emotions in human beings and animals by Shumyatsky and group in Howard Hughs Medical Institute in 2005. The perspective I gained from this work is that even fear and memory are genetically controlled.

A key ingredient to a good scientific work is knowing what to look for and using the right method to search for it. With the aim of understanding the neural circuitry controling fear, the place Shumyatsky and group targeted is the amygdala, a region deep in the brain known to contribute to fear and other emotions. In fact the specific site they targeted is the lateral nucleus, the portion of the amygdala that receives stimuli about fear. Thus, the lateral nucleus played a role in processing the fearful stimuli. After dissecting the individual pyramidal cells (principle cell type) in the lateral nucleus, they found two genes, known as gastrin-releasing peptide (GRP) and stathmin.

The experiments which followed are aimed at understanding the genes. Stathmin is characterized by understanding its expression in the brain and its role in controlling fear. A powerful technology used to understand and reveal the role of the gene in controlling fear is the generation of knockout mouse.

Stathmin knockout mouse have memory deficits in conditioned fear and do not have innate fear. Evidence of deficits in conditioned fear come from observation that knockout mice showed a decreased level of freezing immediately after electric shock compared to normal mice. Evidence of lack of innate fear comes from the behavior of mice in a novel environment. Normal mice when placed in a novel open field naturally avoid the open space in the center of the arena while stathmin mutant mice runs in the aversive center of the field. The behavior of mice is quantified by measuring variables such as percentage time spent freezing and time spent in the open space. Other characterization related to the mode of function of stathmin were made. Refer to the paper in Cell, Vol 123, 697.

Hopeful monsters

Last week, I posted about homeotic mutant flyheads with their antennae transformed into legs, also known as hopeful monsters.

I was wondering why they are called hopeful monsters. Monsters are supposed to be scary and powerful, like the cartoon below. Why do they need hope for?

Turns out that in reality such grossly mutated monsters don't have the phenotype that is favorable for survival or reproductive propagation. Imagine the fly with legs on its head walking upside down. It isn't going to go very far.

I'm digressing. Anyway to start understanding how this occur, let's observe the photographs closely.

Note how "clean" the transformation from antenna to leg is.

The transformation of one structure of the body did not affect the adjacent structures.

So what does this suggest?

That one structure develops independently of the other neighbouring structures.

Uh huh...but how did mutation cause the leg to end up on the head?

Have you heard of Hox genes? These are really cool genes that determine where limbs and other body segments will grow in a developing embryo.

So in the drosophila embryo, each region is specified by a cluster of Hox genes (below). So does it make sense that one part of the fly develops independently of the other parts?

Here's another blog that explains Hox genes in greater depths.

When you mutate one of the genes, mess up with one of the switches and viola! As if a magical trick, the leg ends up there.

Um, magic is not a very good explanation but I am sleepy. Stay tuned to the next blog post for explanation on switches..

Monsters in Science

I was showing pictures from a book called "The new Science of Evo Devo-Endless forms most beautiful" authored by Sean B.Carroll to my sister and she caught me smiling while looking at one of the pictures.

Here it is.

Left is a normal fly head with antennae. Right is a mutant fly in which the antennae are transformed into legs. (Photos courtesy of Dr Rudy Turner, Indiana University)

Isn't it strange to smile at photographs of flyheads?

Was I amused or was I fascinated? Or have I gone... ...mad?

Mad I wasn't but I sure am fascinated. How on earth did the leg get there? How could changing just one gene change a body so dramatically?

Stay tuned to evodevos.blogspot for the next blogpost.