|Haas, M. Susan -|
Submitted to: Development, Genes and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 11, 2012
Publication Date: March 31, 2012
Citation: Haas, M., Beeman, R.W. 2012. Coming apart at the seams: morphological evidence for pregnathal head capsule borders in adult Tribolium castaneum. Development, Genes and Evolution. 222(2): 99-111. http://dx.doi.org/10.1007/s00427-012-0397-5. Interpretive Summary: The insect head is a complex structure that encompasses functions of sight, smell, taste, mate-detection, feeding, defense, and central nervous integration. This structure evolved from the fusion of a tandem series of body segments in a wormlike ancestor, but the details of this evolutionary fusion are obscure. We have attempted to recreate the evolutionary steps that gave rise to the modern insect head by detailed analysis of a series of mutations that cause partial reversions of head morphology to earlier evolutionary states. Our analysis suggests that the six anteriormost body segments in a wormlike ancestor fused to form the head of a modern-day insect. Understanding the evolutionary derivation and genetic regulation of vital insect structures will facilitate efforts to design improved, gene-based control strategies.
Technical Abstract: Cephalization and seamless fusion of the anterior body segments during development obscure the segmental origins of the insect head. Most of the visible seams are thought to reflect infolding for structural reinforcement rather than a merger of segmental or cuticular plate borders. Incomplete fusions, partial deletions or other modifications of segments associated with eight Tribolium mutations affecting the adult head indicate that the frontal and gular sutures are true sutures that mark borders between adjacent cuticular plates, and suggest that the anterior facial shelf is a composite of three segmental surfaces: ocular, antennal, and labro-intercalary. Additionally, midline splits of the clypeo-labrum and gula, and membranous lesions on the dorsal and lateral head capsule of mutant beetles reveal probable borders of adjacent cuticular plates where visible sutures are normally absent. The anterio-lateral lesions seen in certain mutations mark the border between ocular and antennal plates and appear to reveal the site of the often cryptic postfrontal sutures that separate these segments. While revealing or clarifying intersegmental borders between ocular, antennal and clypeo-labral plates, the various modified or unfused surfaces of the head mutations described here do not suggest the existence of a separate acronal plate at the anterior extreme of the insect head.