The timing of a genes expression can affect morphology

Modularity also allows the relative timing of two different developmental processes to shift independently of one another, a process called heterochrony. That is, the genes regulating

Beetle Springtail Shrimp Spider Centipede Onycophoran

In the insect lineage, a mutation in the Ubx homeotic gene prevents legs from forming in the abdominal segments.

Beetle

In the insect lineage, a mutation in the Ubx homeotic gene prevents legs from forming in the abdominal segments.

Beetle

Centipede

Homeotic Genes

Most arthropods have legs growing from their abdominal segments.

Centipede

Most arthropods have legs growing from their abdominal segments.

the development of one module (say, the eyes of vertebrates) may be expressed at different times in different species, relative to genes regulating development of other modules.

Heterochrony has been widely studied in salamanders, where extensive examples of this phenomenon are seen. The case of two salamander species of the genus Bolitoglossa illustrates how heterochrony can result in new morphology.

Purple dye marks the presence of BMP4.

Chick hindlimb

Duck hindlimb

Purple dye marks the presence of BMP4.

Chick hindlimb

Duck hindlimb

Bmp4 Apoptosis

In the chicken, webbing undergoes apoptosis, resulting in the separated toes of the adult.

Webbing in the adult duck's foot remains intact.

In the chicken, webbing undergoes apoptosis, resulting in the separated toes of the adult.

Webbing in the adult duck's foot remains intact.

21.6 Changes in gremlin Expression Correlate with Changes in Hindlimb Structure

The upper row of photos shows the development of the foot of a chicken; the lower row shows the development of the foot of a duck. Expression of Gremlin protein in the duck foot blocks BMP4 and prevents the embryonic webbing from undergoing apoptosis.

(a) B. rostratus (terrestrial)

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