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Drosophila

Vertebrates

Homologous genes are shown in the same color.

21.3 Genes Show Similar Expression Patterns

Homologous genes for certain transcription factors are expressed in similar patterns along the anterior-posterior axes of both insects and vertebrates.

21.4 Altering Homeobox Genes Changes Morphology (a) Deletion of the Ubx gene in Drosophila converts the third thoracic segment,which does not normally bear wings, into a duplication of the second thoracic (forewing-bearing) segment. (b) Deletion of the Hoxc-8gene in mice transforms a lumbar (abdominal) vertebra into a copy of a thoracic (ribbed) vertebra.

21.4 Altering Homeobox Genes Changes Morphology (a) Deletion of the Ubx gene in Drosophila converts the third thoracic segment,which does not normally bear wings, into a duplication of the second thoracic (forewing-bearing) segment. (b) Deletion of the Hoxc-8gene in mice transforms a lumbar (abdominal) vertebra into a copy of a thoracic (ribbed) vertebra.

Ubx Mouse Rib
Extra rib

served in thousands of species that display an incredibly vast array of seemingly unrelated morphologies.

However, the vast differences in morphological form that result from such similar genetic instructions means that the instructions cannot be the whole story. The anterior-posterior axes of both human and fruit fly embryos are specified by the same genes, but humans don't develop wings. Humans use Pax6 to specify eye-forming regions, but the human visual system forms from different precursor structures than the fly system does. So there must be important differences as well as important similarities. Both similarities and differences are expected under descent with modification, but how can differences in developmental genetics explain structural differences?

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