Question

Snakes have lost their limbs over the course of evolution. Recent data show a reduction in Shhexpression in their limb fields due to loss of a portion of the limb enhancer, noting that other enhancers remain intact.

First, indicate whether this is an example of heterotopy, heterochrony, heterometry, or heterotypy, defining your chosen term. Second, describe how you might use the mouse embryo as a model system to test the hypothesis that this reduction in Shhexpression in the limb field is responsible for the loss of limbs in snakes. Be sure to provide a brief description of the relevant technique(s), and to include at least one control

Answer 1

1. This is an example of heterometry, defined as loss of a particular morphological feature in terms of timing of modification or simply change in the level of gene expression. During the evolution, snakes lost their limbs due to the reduction of loss of Shh gene expression in the limb fields.

2. Transgenic mouse can be used as a model organism to study this hypothesis. To generate this mouse, microinjection of DNA containing this gene is introduced and its expression is studied.