Diversification of insect wing morphology

Although all modern winged insects bear two pairs of wings, many structural, functional, and morphological differences exist between forewings and hindwings both within and between species (Fig. 5.12). For example, the wings of dragonflies (Odonata) appear rather similar, but the forewings of beetles (Coleoptera) have been modified into hardened coverings that protect the hindwings. Butterfly (Lepidoptera) forewings and hindwings are often of similar size, but have evolved different shapes and color patterns. In flies (Diptera), the hindwing has been reduced to the haltere, a small balancing organ with a different biomechanical function.

The Ubx gene regulates the differences between forewing and hindwing patterns within species. Ubx is expressed in the developing hindwing of all insects studied, and it controls hindwing-specific patterning (Fig. 5.13). Yet, the hindwings of different insects such as butterflies, beetles, and flies are all different from one another. This diversity reflects the regulation by Ubx of different sets of target genes in each animal.

Recall that in the Drosophila haltere, Ubx represses the wing-patterning genes that are required for the growth and flattening of the wing and for the development of wing veins and sensory organs (see Chapter 3). By contrast, Ubx does not repress the orthologs of these genes in the butterfly. The butterfly hindwing is large and flat, and it has similar venation and sensory organ patterning to the forewing; the developmental program that controls these features is deployed in both wing pairs. In certain butterflies, Ubx regulates target genes that control the wing shape, color pattern elements, scale pigmentation, and scale morphology— all features that have no counterpart in the fruit fly. Thus the homologous hindwings of flies and butterflies have evolved distinct morphologies in part because of independent changes in the regulatory connections between Ubx and genes in the wing hierarchy (Fig. 5.14).

Figure 5.13

Ubx expression in insect hindwings

Ubx is expressed in insect hindwings that have diverse morphologies. (a) In Drosophila, the wing is much larger than the balloon-like haltere. (b) Ubx protein (green) is expressed in the developing Drosophila haltere, but not in the wing. (c) In the butterfly Precis coenia, the hindwing is large and flat like the forewing, but has different color patterns and scale morphology. (d) Ubx protein (green) is also expressed in the butterfly hindwing. Source: Modified from Warren R, Nagy L, Selegue J, et al. Nature 1994; 372: 458-461.

Figure 5.13

Ubx expression in insect hindwings

Ubx is expressed in insect hindwings that have diverse morphologies. (a) In Drosophila, the wing is much larger than the balloon-like haltere. (b) Ubx protein (green) is expressed in the developing Drosophila haltere, but not in the wing. (c) In the butterfly Precis coenia, the hindwing is large and flat like the forewing, but has different color patterns and scale morphology. (d) Ubx protein (green) is also expressed in the butterfly hindwing. Source: Modified from Warren R, Nagy L, Selegue J, et al. Nature 1994; 372: 458-461.

Four Winged Insect Ancestor

Four-winged ancestor

Figure 5.14

Evolution of insect hindwing morphological diversity

Evolution of the Ubx regulation of hindwing development has contributed to the divergence of insect hindwing morphology. From an ancestral insect with similarly patterned forewings and hindwings (bottom), insect hindwings evolved that differ from other homologous hindwings in other species and from the forewings of the same species (top). This diversity is associated with evolutionary changes in the set of Ubx-regulated target genes in each lineage, in addition to changes in the wing developmental program itself.

Source: Modified from Carroll S. Nature 1995; 376: 479-485.

Four-winged ancestor

Sets of [/fox-regulated target genes may have diverged between species through changes in the cz's-regulatory elements that control gene expression in the wing field. For instance, the evolution of Ubx binding sites within a wing-specific cz's-regulatory element could have modified gene expression in the hindwing in a given lineage. Alternatively, the evolution of a new activating Ubx binding site could have created a novel expression pattern in the hindwing.

The evolution of developmental responses to environmental cues has also contributed to the regulation and evolution of insect wing morphology and number. For example, in ants reproductive individuals have wings while the worker and soldier castes are wingless. This difference is regulated by hormonal and environmental signals that are independent of Hox gene input. Evolution of cis-regulatory elements responsive to these cues in genes that promote wing development may underlie the evolution of wing polyphenism in ants. Due to the modular nature of the wing developmental hierarchy, such evolutionary changes in cis-regulatory elements may alter wing formation and morphology in some developmental programs without disrupting wing development entirely.

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