Animal development can be seen as a series of successive fate decisions where cells take internal and external (signals from other cells) information and use it to become progressively more specified. Regionalization refers to subdividing an existing embryo or tissue into smaller parts with unique fates. This can occur at a large scale, for example Bicoid and Nanos broadly regionalizing the embryo into anterior, middle, and posterior, or it can occur at a small scale, for example Shh regionalizing an autopod (hand) into thumb and pinky sides. Regionalization typically occurs in three main steps: First, a morphogen broadly patterns a tissue by forming a gradient. Next, this gradient is read out by a series of transcription and translation factors that turn the gradient into discrete domains of gene expression. Finally (or concurrently) the cells expressing these unique combinations of genes are fated and begin to exhibit different properties. This chapter first broadly examines examples of regionalization and specification and then considers the role of organizers in these processes.
- 5.1: Splitting up the A/P axis: Beginning-Hox Genes, Another Level of Regionalization
- We already took a quick look at Hox genes, both in your readings and in the Genetic Toolkit section. Now we will put them into a broader Evo-Devo context by looking at a specific example of Hox patterning and by seeing evidence that regionalization by Hox genes is conserved across animals. Some cis-regulatory elements driving Hox gene expression patterns are also conserved. However, the earlier developmental events that set up Hox expression domains are not conserved across animals.