EvoluTion And dEvEloPmEnT 385
Tribolium, which is more closely related to flies, this gene was found to have a
homeotic function instead: it determines organ identity rather than segmentation.
Inserting ftz from these insects into Drosophila caused a transformation of anten-
nae into legs, a homeotic effect that was traced to an amino acid motif (YPWM)^2 in
the Ftz protein of the grasshopper and beetle, which is lacking in Drosophila (FIG-
URE 15.19). However, the Ftz protein of the beetle and fly has an amino acid motif
(LXXLL, where “X” stands for any amino acid) that is required for segmentation
function because it interacts with a cofactor protein when it binds to cis-regulatory
elements. The ftz gene of Drosophila has lost homeotic function by loss of one short
motif and gained segmentation function by acquiring a different motif.
Thus, a transcription factor may retain the amino acid motif that binds to a
cis-regulatory element, and be highly conservative in evolution, but evolve other
functions by changing amino acid motifs that interact with other proteins and
cofactors. The result is that gene regulatory networks, sets of interacting genes that
determine phenotypic traits, evolve by forming new interacting combinations of
transcription factor modules and cis-regulatory modules, at various levels in regu-
latory cascades.(^2) Each of the 20 amino acids is represented by a letter.
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_15.19.ai Date 01-05-2017
(A)
(B)
(D)
(C)
Dm-ftz
Dm-ftzYPWM
TC-ftz
YPWM
–YPWM
+LXXLL
Ftz
Exd
Ftz-F1
FIGURE 15.19 Evolutionary changes in function of the Ftz (Fushi
tarazu) transcription factor (represented by red circle). (A) In
ancestral insects, the amino acid sequence YPWM enabled Ftz
to interact with another protein (Exd, blue circle) and to help de-
termine segment identity (a homeotic function). This condition is
retained in the grasshopper. In the ancestor of beetles and flies,
Ftz evolved an additional amino acid motif (LXXLL) that interacts
with the cofactor Ftz-F1 (green circle). This condition is seen in
beetles, where Ftz and its cofactors affect both segment identity
and the formation of segments. In the ancestor of Drosophila,
the YPWM motif was deleted so that Ftz interacts only with Ftz-
F1. This changed the protein’s function: it determines segment
formation but not identity. The results of these evolutionary
changes can be demonstrated by experimentally changing
the ftz gene sequence in Drosophila and seeing the effects of
these changes on the fly’s antennae. (B) The normal Drosophila
antenna (labeled Dm-ftz) has three short segments (A1, A2, and
A3) and a featherlike structure (arista: Ar). (C) When the YPWM
motif that was lost from Drosophila is added back to the protein
(Dm-ftxYPWM), the antenna’s terminal segment and the arista are
partly transformed into a leglike structure. (D) When the fly gene
is replaced with a beetle gene (TC-ftz), the homeotic transforma-
tion to a leg with a five-segmented tarsus (as in many beetles)
is complete. The numerals designate specific segments of the
tarsus. (Diagram after [39]; antenna images from [37].)
15_EVOL4E_CH15.indd 385 3/22/17 1:30 PM