Cap'n'collar differentiates the mandible from the maxilla in the beetle Tribolium castaneum

Abstract

BackgroundThe biting mandible of the arthropods is thought to have evolved in the ancestor of the insects, crustaceans and myriapods: the Mandibulata. A unique origin suggests a common set of developmental genes will be required to pattern the mandible in different arthropods. To date we have functional studies on patterning of the mandibular segment of Drosophila melanogaster showing in particular the effects of the gene cap’n’collar (cnc), however, the dipteran head is far from representative of insects or of more distantly related mandibulates; Drosophila does not even possess a mandibular appendage. To study the development of a more representative insect mandible, we chose the red flour beetle Tribolium castaneum and investigated the function of the Tribolium orthologs of cap’n’collar (Tc-cnc) and the Hox gene Deformed (Tc-Dfd). In order to determine the function of Tc-cnc and Tc-Dfd, transcripts were knocked down by maternal RNA interference (RNAi). The effects of gene knockdown were examined in the developing embryos and larvae. The effect of Tc-cnc and Tc-Dfd knockdown on the expression of other genes was determined by using in situ hybridization on Tribolium embryos.ResultsOur analyses show that Tc-cnc is required for specification of the identity of the mandibular segment of Tribolium and differentiates the mandible from maxillary identity. Loss of Tc-cnc function results in a transformation of the mandible to maxillary identity as well as deletion of the labrum. Tc-Dfd and the Tribolium homolog of proboscipedia (Tc-mxp = maxillopedia), Hox genes that are required to pattern the maxillary appendage, are expressed in a maxilla-like manner in the transformed mandible. Tribolium homologs of paired (Tc-prd) and Distal-less (Tc-Dll) that are expressed in the endites and telopodites of embryonic appendages are also expressed in a maxilla-like manner in the transformed mandible.We also show that Tc-Dfd is required to activate the collar of Tc-cnc expression in the mandibular segment but not the cap expression in the labrum. Tc-Dfd is also required for the activation of Tc-prd in the endites of the mandible and maxillary appendages.ConclusionsTc-cnc is necessary for patterning the mandibular segment of Tribolium. Together, Tc-cnc and Tc-Dfd cooperate to specify mandibular identity, as in Drosophila. Expression patterns of the homologs of cnc and Dfd are conserved in mandibulate arthropods suggesting that the mandible specifying function of cnc is likely to be conserved across the mandibulate arthropods.

Hox genes require homothorax and extradenticle for body wall identity specification but not for appendage identity specification during metamorphosis of Tribolium castaneum

“…Rather, legs adopted a mixed leg identity, exhibiting characteristics of both the T1 and T2 legs. Moreover, during embryogenesis in T. castaneum, loss of Tc-Dfd function results in transformation of the mandibles to antennae (Brown et al, 1999Coulcher and Telford, 2012), but targeting Tc-Dfd with RNAi during metamorphosis did not affect mandible identity (Fig. 1C).…”

The function of appendage patterning genes in mandible development of the sexually dimorphic stag beetle

“…However, the insect mandible is thought to have lost expression of the distal portion of the appendage pattering pathway, including Dll, resulting in outgrowths that are homologous with the proximal, or basal, regions of other appendages (i.e., insect mandibles are gnathobasal; Popadic et al 1998, Scholtz et al 1998, Coulcher & Telford 2012. Most of the recent expression and functional analyses on Dll's role in mandible development collectively suggest that Dll does not play a significant role in the mandibular development of insects (Panganiban et al 1994, Popadic et al 1998, Beermann et al 2001, Angelini & Kaufman 2004, Angelini & Kaufman 2005, Simonnet & Moczek 2011, Coulcher & Telford 2013, and our a priori assumption was that Dll would not be not expressed in stag beetle mandibles.…”

Evolutionary Developmental Biology of Invertebrates 5

“…Developing mandibles are the only appendages that lack Dll expression (Popadić et al 1998 ). In both Tribolium and Drosophila , Dll expression in the mandibular segment is repressed by the transcription factor Cap-n-collar (Cnc;McGinnis et al 1998 ;Coulcher and Telford 2012 ). This supports an evolutionary scenario in which the mandible evolved by truncation of an ancestral appendage, which, developmentally, was caused by the loss of Dll expression in the network patterning this appendage (Popadić et al 1998 ).…”

“…This raises the question of how endites are specifi ed and patterned during development. This endite expression is regulated downstream of the Hox gene Deformed in the maxilla (Coulcher and Telford 2012 ). One such feature in Tribolium is that the gnathal endites specifi cally express the transcription factor-encoding gene paired (Fig.…”

“…This supports an evolutionary scenario in which the mandible evolved by truncation of an ancestral appendage, which, developmentally, was caused by the loss of Dll expression in the network patterning this appendage (Popadić et al 1998 ). In Tribolium , its expression is regulated downstream of the Hox gene Dfd (Coulcher and Telford 2012 ), while in Drosophila it is regulated by more upstream components of the segmentation cascade (Mohler 1993 ). cnc expression distinguishes the mandible from neighboring segments in non-chelicerate arthropods (Sharma et al 2014 ).…”