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Amphioxus Hox complex

Figure 4.9

ParaHox and Hox Complexes

An ancestral ProtoHox complex of three or four genes (center) duplicated in early metazoan history to create two sister complexes, the ParaHox complex and the Hox complex. The number of genes in the Hox complex increased through gene duplication and divergence before and during the radiation of bilaterian phyla. Both the ParaHox complex (top) and the Hox complex (bottom) are present as tightly linked arrays in Amphioxus.

Source: Redrawn from Brooke NM, Garcia-Fernandez J, Holland PWH. Nature 1998; 39: 920-922.

ancestral arthropod Hox cluster beetle Hox cluster fly

Hox clusters

Ubx abdA Abd-B

gain of insect Q/A motif, loss of crustacean Ser/Thr motif

Ubx abdA Abd-B

ancestral arthropod Hox cluster beetle Hox cluster fly

Hox clusters

gain of insect Q/A motif, loss of crustacean Ser/Thr motif

Figure 4.10

Evolution of new developmental functions for insect Hox proteins

During insect evolution, the ancestral Hox genes zen and ftz evolved new developmental roles in dorsoventral axis formation and segmentation, respectively, and lost ancestral roles in regional patterning of the anteroposterior axis. In the dipteran lineage, the zen gene duplicated to give rise to bicoid. The Ubx protein gained a new motif associated with the repression of insect abdominal limbs. Source: Modified from Mann RS, Carroll SB. Molecular mechanisms of selector gene function and evolution. Curr Opin Genet Dev 2002; 12: 592-600.

Figure 4.10

Evolution of new developmental functions for insect Hox proteins

During insect evolution, the ancestral Hox genes zen and ftz evolved new developmental roles in dorsoventral axis formation and segmentation, respectively, and lost ancestral roles in regional patterning of the anteroposterior axis. In the dipteran lineage, the zen gene duplicated to give rise to bicoid. The Ubx protein gained a new motif associated with the repression of insect abdominal limbs. Source: Modified from Mann RS, Carroll SB. Molecular mechanisms of selector gene function and evolution. Curr Opin Genet Dev 2002; 12: 592-600.

insects in Drosophila indicate that ftz has slowly lost its Hox function (retained somewhat in beetles and grasshoppers) and gained a pair-rule function. The expression of ftz in the central nervous system of several insects and a cirripede crustacean is more reminiscent of Hox genes, with a clear anterior boundary of expression in the thorax. The probable ancestral Hox expression pattern is reflected in the deployment of the ftz ortholog in a myriapod (centipede) and a chelicerates (mite), where this gene is expressed in a Hox-like pattern in the embryonic ectoderm.

Hox proteins can also acquire new functional motifs while retaining their Hox function. In the insect lineage the Ubx protein has gained a functional motif that is exploited as part of its role in regulating regional identity along the A/P axis. Insect Ubx proteins contain a carboxy-terminal glutamine and alanine-rich transcription repression domain that is not present in crustacean or onychophoran Ubx proteins, and which is implicated in the evolution of morphological segmental diversity in insects. We will go into more detail on the role of Hox protein evolution in arthropod body plan diversification in Chapter 5.

The evolution of the Hox complex illustrates several important mechanisms underlying the evolution and functional diversification of gene families. First, an early duplication of the small, ancestral Hox complex created the sister ParaHox complex. Second, tandem duplication events expanded the number of paralogous Hox genes within the complex. Third, the homeodomains of many Hox genes diverged and became constrained before the bilaterian radiation. Fourth, large-scale or genomic duplications at the base of the vertebrate lineage (and again in teleosts) increased the number of vertebrate Hox complexes. Fifth, rapid changes in homeodomain sequence accompanied the evolution of new developmental functions for some insect Hox genes. Sixth, evolution of new motifs has modified Hox protein functions.

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