Mutations can result in new phenotypes

Insects are highly modular organisms; in Chapter 19 we saw the precision of the segmentation process in insect development. Insects provide examples of the evolution of morphological changes through mutations in the genes encoding transcription factors that regulate segmentation. For example, all arthropods (see Chapter 33) possess the homeotic gene Ultrabithorax (Ubx), but the insect Ubx gene has a mutation not found in the other arthropods (Figure 21.5). The Ubx protein transcribed from this mutated gene represses expression of the distal-less gene (dll), which is essential for leg formation. The Ubx protein of insects is expressed in the abdomen where it represses dll. As a result, insects have only six legs, none of which grow from the abdominal segments. In contrast, the Ubx protein of other arthropods—such as millipedes, centipedes, spiders, mites, and crustaceans—do not repress the expression of dll. Consequently, those animals all have abdominal appendages.

The evolution of the webbed feet of ducks is an example of an evolutionary change resulting from an altered spatial expression pattern of a regulatory gene. Ducks have webs that connect their toes, but chickens and most other birds do not. The developing feet of early embryos of both ducks and chickens have webs (as do those of humans; see Figure 19.11). A particular gene is expressed in the spaces between the developing bones of the toes. This gene encodes a protein called bone morphogenetic protein 4 (BMP4). This protein instructs the cells between the developing toes to undergo apoptosis— programmed cell death. The death of these cells destroys the webbing between the toes.

Embryonic duck and chicken hindlimbs both express BMP4 in the webbing between the toes, but they differ in the expression of a BMP inhibitor protein, called Gremlin (Figure 21.6). Gremlin expression occurs around the digits in both chick and duck hindlimbs. In ducks, but not in chickens, the gremlin gene is also expressed in the webbing cells. The Gremlin protein prevents the BMP4 protein from signaling for cell death in the webbing; the result is a

Figure 21.5 A Mutation Changed the Number of Legs in Insects In the insect lineage (blue) of the arthropods, a mutation in the Ubx gene resulted in |a protein that inhibits a gene that is required for legs to form. Because insects express Ubx in their abdominal segments, no legs grow from these segments. Other arthropods, such as centipedes, do grow legs from their abdominal segments.

Change in protein structure

Common ancestor

Change in protein structure

Figure 21.5 A Mutation Changed the Number of Legs in Insects In the insect lineage (blue) of the arthropods, a mutation in the Ubx gene resulted in |a protein that inhibits a gene that is required for legs to form. Because insects express Ubx in their abdominal segments, no legs grow from these segments. Other arthropods, such as centipedes, do grow legs from their abdominal segments.

Diplomonads

Beetle Springtail Shrimp Spider Centipede Onycophoran webbed foot. Experimental application of the Gremlin protein to chick hindlimbs converts them into ducklike feet (Figure 21.7).

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  • fraser
    What cell is mutated in webbed feet?
    8 years ago

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