Evolution, 4th Edition

384 CHAPTER 15

from a velvet worm in the phylum Onychophora, the sister group of the arthro-

pods. The difference between these Ubx effects was traced to a new poly-alanine

domain that evolved in insects and inhibits transcription of a gene (Distal-less) that

is necessary for leg development (FIGURE 15.18) [17]. Thus, when insects evolved,

Ubx became a leg repressor.

A more radical change in the function of a transcription factor has been

described for fushi tarazu (ftz). In Drosophila, this gene is important in the seg-

mentation of the embryo [37]. In the grasshopper Schistocerca and the flour beetle

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D. melanogaster Other Drosophila D. biarmipes

Dll Dll Dll

Ancestral

yellow yellow yellow

Co-option Diversication

Distal-less expression

Wing pattern

yellow expression

Enhancer

FIGURE 15.17 Evolution of a transcrip-

tion factor underlies a novel character. The

images at top show the wing pigmentation

patterns and the expression patterns of

Distal-less and yellow genes in D. mela-

nogaster (left), certain other Drosophila

species (center), and D. biarmipes (right).

The diagrams at bottom provide a model of

the evolution of regulatory changes. Left: In

D. melanogaster, the enhancer upstream of

yellow (y) does not bind the regulatory pro-

tein produced by Distal-less (Dll). Dll has no

effect on expression of y. Center: In some

Drosophila species, the enhancer evolved

the ability to bind Dll, as highlighted in blue.

This caused enhanced expression of y in

regions of the wing where Dll is expressed.

Right: The next evolutionary step, found in

D. biarmipes, involved increased expression

of Dll in one region of the wing (darker red

patch in middle row; blue in the diagram).

This caused increased expression of y and

darker pigmentation in that patch. (From [4].)

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Fly (Diptera)

WFQNRRMKLKKEIQAIKELNEQEKQAQAQKAAAAAAAAAAVQGGHLDQ*

WFQNRRMKLKKEIQAIKELNEQEKQAQAQKAAAAAAAVAAQVDPN*

WFQNRRMKLKKEIQAIKELNEQDKRITPSKLHSNC-SSPTGILVTMKKMK-SFNLITE*

WFQNRRMKLKKEMQTIKDLNEQEKK---QRDTLSTV*

Beetle (Coleoptera)

Brine shrimp (Crustacea)

Velvet worm (Onychophora)

FIGURE 15.18 In insects, the Ubx gene

suppresses development of legs on the

abdomen, but it lacks this activity in crusta-

ceans and other non-insect arthropods, as

well as in the related phylum Onychophora

(velvet worms). Part of the Ubx protein

sequence is shown for these species; the

letters in the sequences represent amino

acids (e.g., A = alanine) encoded by the

corresponding DNA sequences. The region

in brown has high sequence homology

among all the taxa. The long sector in blue

is a novel poly-alanine sequence that is

responsible for repression of legs on the

abdomen of the insects. (After [17].)

15_EVOL4E_CH15.indd 384 3/22/17 1:30 PM