Endless Forms Most Beautiful by Sean B. Carroll

Author:Sean B. Carroll
Language: eng
Format: epub, mobi
Publisher: W. W. Norton & Company
Published: 2005-06-07T16:00:00+00:00

F IG . 7.5 Evolution of wing number and form. Wing number has followed Williston’s Law, evolving from a series of gill-like appendages on all segments of extinct aquatic nymph forms to smaller numbers and reduced structures in mayflies, to the two pairs of wings in most modern flying insects. Wing number has been progressively reduced by the evolution of progressively more sites for Hox proteins in switches of genes promoting wing development. DRAWING BY LEANNE OLDS

This scenario helps explain the transition of gill form to wings, but what about the number of wings? Aerodynamic studies have concluded that two pairs of wings, located on the second and third thoracic segments, provide the best performance characteristics. How did insects reach this optimal design?

Back to Williston’s Law, Hox genes, and switches. Remember the trend that specialization is often accompanied by a reduction in the number of serially reiterated structures? This is exactly the case in the evolution of insect wing number. In the fossil record, extinct forms have been found that have fewer or smaller wings or wing-like structures on the abdomen and first thoracic segment. These forms represent intermediates between primitive aquatic forms and modern types (figure 7.5). The reduction of wing number was a matter of reducing and eliminating wing formation during development in all of the segments except for the second and third thoracic segments.

How were wings suppressed in a segment-specific pattern? The segments from which wings disappeared are zones of particular Hox proteins in all insects. Furthermore, Scott Weatherbee and Jim Langeland in my laboratory discovered that wing formation is repressed in flies in these segments by the respective Hox proteins that are expressed in them. This tells us that the modern condition of insects bearing two pairs of wings is the product of the evolution of the repression of wing formation by Hox proteins acting in the first thoracic segment and all of the abdominal segments. This repression must have evolved in stages in different groups of ancient insects, because the fossil record contains species in which wing repression was only partial. The ultimate cause of wing repression is that switches in genes involved in wing formation evolved signature sequences for Hox proteins that resulted in their being turned off in selected segments.

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