A genetic toolkit consisting of highly conserved regulatory genes governs pattern formation in multicellular organisms. Genetic toolkit genes act independently among modules of a developing embryo. The expression of these developmental genes can differ among species in various ways, often resulting in major morphological differences. They can differ in amount of expression (heterometry), in the timing of expression (heterochrony), or in the location of expression (heterotopy).
learning outcomes
You should be able to:
Use a specific example to defend the claim “Genes controlling development are highly conserved.”
Provide plausible evolutionary explanations for cases where different populations or closely related species differ in genetic switches.
Interpret experimental results to assess the role of BMP4 in beak development.
How does the story of the eye-
Substituting the fruit fly genetic switch for eye formation in the mouse embryo for the mouse genetic switch still resulted in eye formation in the mouse, and vice versa. This indicates that the genetic switches for development are conserved in evolution.
In a series of experiments on chick embryos, researchers applied different concentrations of BMP4 to the embryos’ beak growth region. They measured the size of the beak cartilage at a later stage of development. Based on the data in the table below, what could you conclude about the role of BMP4 in beak growth?
| Amount of BMP4 | Cartilage diameter (mm) |
|---|---|
| None (control) | 0.5 |
| 0.1 unit | 0.7 |
| 0.3 unit | 1.0 |
| 1.0 unit | 1.8 |
BMP4 is involved with cartilage development.
Plasmodium vivax is a protist that causes a form of malaria. When P. vivax enters the blood, it attaches to a glycoprotein on the red blood cells. Some human populations in Africa are immune to P. vivax because they lack this particular glycoprotein on their red blood cells. In a variety of tissues the transcription of this glycoprotein is under the influence of enhancers, and the enhancer that is normally expressed in red blood cell precursors is mutated in individuals immune to this form of malaria. What does this case illustrate in evolutionary developmental terms?
The observations are consistent with the hypothesis that there has been selection in some human populations for mutations of the enhancer that controls expression of the glycoprotein in red blood cells. This genetic change would be expected to have a selective advantage in human populations that are exposed to malaria at high levels, because the mutation confers greater resistance to malaria in humans who carry it.
You have seen how the genetic toolkit guides morphogenesis in individual organisms, and how differences in genetic switches contribute to differences among species. Now we discuss the roles that some of these same tools play in the evolution of new forms and new species.
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