The insect upper lip (labrum) is a nonsegmental appendage‐like structure

Abstract

The insect upper lip--the labrum--is a lobe-like structure anterior to the mouth opening. Whether the labrum represents a fused pair of segmental appendages or evolved independently is heavily debated. Here, we identify additional similarities of the regulatory gene network active in labrum and trunk appendages. However, we do not find a labral parasegment boundary and we show that labral Tc-Dll expression is independent of Tc-wg and Tc-hh signals. In contrast, Tc-Dll expression in all trunk appendages does require these signals. Finally, we identify crucial differences between the location of the labrum and trunk appendages: the labrum develops in median rather than lateral tissues and is part of an anterior nonsegmental tissue marked by and dependent on Tc-six3 activity. To reconcile these seeming contradictory results, we propose that the genetic network evolved in either labrum or trunk appendages and became redeployed at a novel location to form the other structure.

Head patterning and Hox gene expression in an onychophoran and its implications for the arthropod head problem

“…From the expression studies so far it is clear that six3 marks the most anterior neurogenic territory in animals (Li et al, 1996;Lowe et al, 2003;Posnien et al, 2009;Schinko et al, 2008;Schröder, 2003;Seo et al, 1999). This investigation corroborates the earlier ones in that six3 is expressed in the extreme anterior region of the neuroectoderm of the brain anlage.…”

“…In Drosophila and Tribolium, six3 is expressed in and essential for the development of the clypeolabrum and labrum respectively (Posnien et al, 2009;Seo et al, 1999). Indeed, the labrum has been much debated and it appear to posses appendicular characters homologous to segmental appendages but on the other hand displays unique features that have lead researchers attribute it to an unsegmental region of the arthropod head (Haas et al, 2001b;Posnien et al, 2009;Rempel, 1975;Scholtz and Edgecombe, 2005).…”

“…Indeed, the labrum has been much debated and it appear to posses appendicular characters homologous to segmental appendages but on the other hand displays unique features that have lead researchers attribute it to an unsegmental region of the arthropod head (Haas et al, 2001b;Posnien et al, 2009;Rempel, 1975;Scholtz and Edgecombe, 2005). It is certainly premature to homologize the onychophoran antenna with the arthropod labrum on the grounds of the common expression of one gene and on them being positioned on the homologous, otd-six3 defined zone on the most anterior neuromere, territory, but future molecular characterization might prove enlightening.…”

“…Is it an appendage or structure of a pre ocular segment (Posnien et al, 2009;Schmidt-Ott et al, 1994;Schmidt-Ott and Technau, 1992), the ocular segment (Budd, 2002;Eriksson and Budd, 2000;Eriksson et al, 2003) and homologous to the great appendage or primary antenna of certain fossil stem-group arthropods (Budd, 2002;Scholtz and Edgecombe, 2005) or a limb belonging to a segment placed further posterior (Haas et al, 2001a;Haas et al, 2001b). There have been suggestions that the antenna of the onychophorans corresponds to the labrum of arthropods (Budd, 2002;Eriksson and Budd, 2000;Eriksson et al, 2003).…”

“…It has been shown that the anterior expression borders of some anterior Hox-genes are conserved among arthropods and workers have aligned head segments between different arthropod groups (Abzhanov and Kaufman, 1999;Damen et al, 1998;Jager et al, 2006;Mittmann and Scholtz, 2003;Telford and Thomas, 1998) and the head patterning genes six3 and otd have been used as markers for the most anterior region of bilaterians (Li et al, 1996;Lowe et al, 2003;Posnien et al, 2009;Schinko et al, 2008;Schröder, 2003;Seo et al, 1999).…”