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.