Fhy

1l 12 13

1l 12 13

Figure 5.10

Dll and Ubx are coexpressed in the pleuropod appendages of beetles and grasshoppers

Lepidoptera (butterflies)

Coleoptera (beetles)

Orthoptera (grasshoppers)

Figure 5.10

Dll and Ubx are coexpressed in the pleuropod appendages of beetles and grasshoppers

The evolution of Hox repression of abdominal limb development and Dll expression in insects evolved in at least two steps. First, abd-A gained the ability to repress limb formation in most abdominal segments (A2-A7) near the base of the insect clade. In the ancestral lineage of butterflies and flies, Ubx also evolved the ability to repress limb formation and Dll expression in the first abdominal segment (A1). Dipterans do not form an A1 appendage, and Dll (green) is not expressed in that segment (top right). In beetles and grasshoppers, a small pleuropod appendage forms in the A1 segment (bottom right). Both Dll (green) and Ubx (red) are expressed in these pleuropods (overlap in yellow), indicating that Ubx does not repress pleuropod formation or Dll expression in these animals.

Source: Modified from Palopoli MF, Patel NH. Curr Biol 1998; 8: 587-590.

and smaller winglets on the abdomen. The fossil record suggests that over time abdominal wing structures became reduced in size and ultimately disappeared, as did the wing-like structures on the first thoracic segment (Fig. 5.11).

The restriction of insect wings to the second and third thoracic segments suggests that the Hox genes sculpted the evolution of wing number and pattern. Indeed, genetic analysis has established that specific Hox genes repress the initiation of wing development in particular segments. In Drosophila, wing development is repressed in the first thoracic segment by Scr and in the abdominal segments by Ubx, abd-A, and Abd-B. The evolution of Hox repression of wing development over the course of insect evolution could explain the reduction and loss of wings on these segments (Fig. 5.11). Initially, Scr, Ubx, abd-A, and Abd-B may have modified the expression of wing-patterning genes to control the size of abdominal and T1 winglets. Over time, Hox regulation completely suppressed the formation of wings to sculpt the modern insect body plan. The evolution of Hox protein binding sites in the cz's-regulatory regions of genes required for wing formation could account for the repression of wing development during evolution.

Figure 5.11

Evolution of insect wing number

The segmental domains of Hox gene expression did not change during insect evolution and were probably similar in ancestral insects (shown as abbreviations in each body segment: S, Scr; A, Antp; U, Ubx; AA, abd-A; B, Abd-B). The evolutionary progression of insect wing number reflects the modification or repression of wing development by Scr, Ubx, abd-A, and Abd-B. Primitive insects were wingless (a), and the first insects with wing-like structures were larval forms (b, c). Wing development became repressed in the first thoracic segment (red/Scr) and in abdominal segments (yellow/Ubx, green/abd-A, blue/Abd-B) of fossil (d) and modern (e, f) adult insects. In dipterans (f), Ubx also regulates the reduced size and modified shape of the haltere. Source: Modified from Carroll SB, Weatherbee SD, Langeland JA. Nature 1995; 375: 58-61.

Figure 5.11

Evolution of insect wing number

The segmental domains of Hox gene expression did not change during insect evolution and were probably similar in ancestral insects (shown as abbreviations in each body segment: S, Scr; A, Antp; U, Ubx; AA, abd-A; B, Abd-B). The evolutionary progression of insect wing number reflects the modification or repression of wing development by Scr, Ubx, abd-A, and Abd-B. Primitive insects were wingless (a), and the first insects with wing-like structures were larval forms (b, c). Wing development became repressed in the first thoracic segment (red/Scr) and in abdominal segments (yellow/Ubx, green/abd-A, blue/Abd-B) of fossil (d) and modern (e, f) adult insects. In dipterans (f), Ubx also regulates the reduced size and modified shape of the haltere. Source: Modified from Carroll SB, Weatherbee SD, Langeland JA. Nature 1995; 375: 58-61.

Figure 5.12

Diversity of insect wing morphologies

Dragonflies have very similar forewings and hindwings, whereas other insects display a diversity of wing morphologies. For example, the forewings of beetles have been modified into specialized protective coverings (elytra). The color patterns of butterfly wings can differ dramatically between species. (Specimens are not shown to scale.)

Source: Specimens courtesy of Department of Entomology, University of Wisconsin-Madison. Photographs courtesy of Leanne Olds.

Figure 5.12

Diversity of insect wing morphologies

Dragonflies have very similar forewings and hindwings, whereas other insects display a diversity of wing morphologies. For example, the forewings of beetles have been modified into specialized protective coverings (elytra). The color patterns of butterfly wings can differ dramatically between species. (Specimens are not shown to scale.)

Source: Specimens courtesy of Department of Entomology, University of Wisconsin-Madison. Photographs courtesy of Leanne Olds.

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