The 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.
Our 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.
Tc-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.
The arthropod mandible is an appendage adapted for biting and chewing and is present in three arthropod groups, the insects and crustaceans (collectively the Pancrustacea) and the myriapods (millipedes and centipedes). The mandibulate arthropods, commonly grouped together in the monophyletic Mandibulata, constitute the majority of animals both in terms of numbers of species and biomass on this planet. The mandible is therefore an evolutionary novelty of particular interest.
There are many different types of mandible, but the characteristic that most mandibles share, and which differentiates it from other arthropod appendages, is the presence of a functional biting edge made up of the incisor and molar processes. This gnathal edge is widely considered to be a homologous structure within the Mandibulata[1–3].
Other arthropod groups, the chelicerates and trilobites, do not have mandibles and instead have a walking leg on the homologous segment to the mandibular segment[4, 5]. An unsegmented appendage, or lobopod, is present in closely related outgroups to the arthropods, such as the onychophorans and tardigrades.
An alternative phylogenetic hypothesis to the monophyletic Mandibulata is the Myriochelata hypothesis, which groups the myriapods with the chelicerates. Accepting this hypothesis would suggest that the mandible evolved independently in the Myriapoda and Pancrustacea or that it has reverted to a walking leg in the Chelicerata. While still controversial, recent molecular phylogenies including evidence from unique microRNAs favour Mandibulata over Myriochelata. This phylogeny is also strongly supported on morphological grounds[8–11].
The mandible is serially homologous with other arthropod post-antennal appendages all of which are thought to have evolved from a segmented biramous limb. The archetypal biramous limb consists of a protopodite (the base of the limb) to which are attached two branches: the telopodite (or palp) and an exopodite[12–14]. Structures called endites, often involved in food processing, are also present on the protopodite. The gnathal edge of the mandible is thought to have evolved from the proximal most endite on the protopodite of this ancestral biramous limb[2, 11]. The mandible is thought to be a gnathobasic structure and this interpretation is supported by expression data: the distal limb expression domain of Distal-less (Dll) is missing from the embryonic mandibular limb bud in diverse mandibulate arthropods[15–17].
All arthropod mandibles appear to be gnathobasic and are restricted to a monophyletic group implying that the mandible has a unique origin and is a homologous structure between mandibulate arthropods. We might therefore expect significant similarities in the embryonic patterning of the mandible between diverse mandibulate taxa. Finding the identity of the genes that pattern the mandible and showing how they function in diverse arthropod taxa could support the view that the mandible is homologous across the Mandibulata and, through comparisons with non-mandibulate sister groups, could give an insight into how the mandible evolved from a primitive arthropod limb.
We have undertaken a functional study of some of the genes that pattern the mandible in a model organism with a typical insect mandible to compare its development with the development of mandibles in other taxa. We chose to study the red flour beetle Tribolium castaneum that, unlike Drosophila melanogaster, has a canonical mandible in which the gnathal edge is made up of the incisor and molar processes.
Mandibular segment patterning in Drosophila
The majority of research into the function of genes patterning arthropod gnathal appendages has focused on insects with very derived mouthparts, in particular the involuted larval head and non-biting adult proboscis of the dipteran D. melanogaster[18–25] and the stylet of the hemipteran Oncopeltus fasciatus[26, 27].
Although developing Drosophila embryos possess gnathal lobes (structures from which the gnathal appendages are formed in other less derived insects), following head involution, Drosophila larvae do not have any gnathal appendages[28–31] and both larval and adult Drosophila lack an appendage on the mandibular segment.
In Drosophila, the gene Deformed (Dfd) is required for the specification of both mandibular and maxillary identities[23–25, 32, 33]. Dfd does not differentiate the mandibular segment from the maxillary segment; for this function another gene, cap’n’collar (cnc), is required[22–24]. cnc is a basic leucine zipper family gene (bZIP) that is expressed in an anterior ‘cap’ domain in the labrum and a posterior ‘collar’ domain in the mandibular segment and is necessary for the development of both labral and mandibular derived structures. It is likely that cnc achieves its mandible patterning function in part indirectly by repressing the maxilla patterning function of Dfd: Dfd expression is repressed by cnc in the anterior of the mandibular gnathal lobe and the activity of the Dfd protein is also repressed by cnc in the mandibular segment. cnc null mutants lose both labral and mandibular segment derived structures and have a duplication of maxillary structures[22–24, 34].
Previous work in Tribolium
In Tribolium, Brown et al. have demonstrated that the homolog of Dfd, Tc-Dfd, is necessary for patterning the mandibular and maxillary segments and that Tc-Dfd expression is progressively downregulated in the mandibular limb buds as in Drosophila[35, 36]. In Tc-Dfd mutants there is a homeotic transformation of the mandible to an antenna and a loss of the maxillary endites. Dfd, although required for mandible development, does not differentiate the mandibular segment from the maxillary segment in Drosophila or Tribolium. The role of Tc-cnc in Tribolium is not known, however, it is expressed in a very similar pattern to that seen in Drosophila and this is also true of cnc in other mandibulate arthropods[38–40] suggesting it may have a conserved function. Embryonic expression in non-mandibulate arthropods is not known.
With the ultimate aim of understanding the origin of the mandible, we were interested in the role that Tc-cnc might play in patterning the mandibular segment of Tribolium castaneum, a mandible-bearing insect. In order to test its function in Tribolium, Tc-cnc was knocked down using parental RNA interference (RNAi) by injecting double-stranded RNA (dsRNA) into female Tribolium pupae. The knockdown phenotype was detected both in embryos and in the first instar larvae of offspring of injected parents. The effect of Tc-cnc knockdown on downstream genes was studied by in situ hybridization in Tribolium embryos.
Tribolium castaneum culture
Wild-type T. castaneum (San Bernardino strain) were kindly provided by Dr Gregor Bucher (Department of Developmental Biology, Georg-August-University Göttingen, Göttingen, Germany) and raised at 32°C in organic wholemeal flour supplemented with 5% brewer’s yeast.
Cloning of Tribolium orthologs
Tc-cnc, Tc-Dfd, Maxillopedia the Tribolium ortholog of Proboscipedia (Pb) (Tc-mxp), the Tribolium ortholog of paired (Tc-prd) and Tc-Dll were amplified from mixed stage cDNA by polymerase chain reaction (PCR) amplification using the following primers: Tc-cnc, a 2,612 bp clone for hapten-labelled RNA probe synthesis (forward: 5′-GCAACAGTGGGCCCTATTTA-3′ and reverse: 5′-GTGGTGGCTCCTTGTGTTCT-3′). Tc-cnc, a 633 bp clone for dsRNA synthesis (forward: 5′-GATTACAGCTATACGAGTCGG-3′ and reverse: 5′-GTCAGCCAGACTCAAAATCTG-3′). Tc-Dfd (forward: 5′-CCAAGTGAGGAGTACAACCAG-3′ and reverse: 5′-TACAAGGCCGTGAGTCCGTAA-3′), Tc-mxp (forward: 5′-ATAGCTGCTTCGCTAGACCTTA-3′ and reverse: 5′-TCGCAGGTGGGGTCATTAT-3′), Tc-Dll (forward: 5′-CAGCAGGTGCTCAATGTGTT-3′ and reverse: 5′-ATTAAACAGCTGGCCACACC-3′), Tc-prd (forward: 5′-ATGCACAGACATTGCTTTGG-3′ and reverse: 5′-GGATCGTCACAGTGTTGGTG-3′). Accession numbers are as follows: Tc-cnc (GenBank: NM_001170642.1), Tc-Dfd (GenBank: NM_001039421), Tc-mxp (GenBank: NM_001114335), Tc-Dll (GenBank: NM_001039439), Tc-prd (GenBank: NM_001077622).
Parental RNAi was performed as previously described: 0.25 to 0.4 μl of Tc-cnc dsRNA (dissolved in distilled water at a concentration of 0.36 to 3 μg/μl) was injected into female pupae. Then, 633 bp of Tc-cnc dsRNA (positions 1,389 to 2,021, including part of the bZIP domain which starts at position 1,932) was injected. Embryos were either fixed 24 to 48 h after egg laying or left to develop into first instar larvae for cuticle preparation. In total, 1,736 female beetle pupae were injected for collecting embryos for in situ hybridization.
In order to characterize the Tc-cnc phenotype, 218 female pupae were injected with 1 to 2 μg/μl dsRNA and the cuticles of first instar larvae were analyzed. Of these 218 injected pupae, 195 successfully eclosed. At 20 days after injection a further 117 beetles (60%) had died. Parental injection of Tc-cnc dsRNA resulted in the mortality of a much greater number of injected females compared to the numbers killed in other RNAi experiments, in which typically 10% of injected female beetles die by day 20 (data not shown). The higher mortality rate may be a consequence of the effects of Tc-cnc knockdown. Only one phenotype was detected in first instar larvae: transformation of the mandible to maxillary identity and loss of the labrum.
In order to obtain partial phenotypes (incomplete transformations of the mandible to maxillary identity) we tried injecting lower concentrations of Tc-cnc dsRNA (360 to 750 ng/μl). However, only wild-type larvae or those with fully transformed mandibles were obtained, and no partial phenotypes were observed. Similar rates of mortality were observed even at lower concentrations.
To obtain Tc-DfdRNAi embryos, 1,142 bp (positions 491 to 1,632) Tc-Dfd dsRNA was injected into female pupae and embryos were fixed for in situ hybridization. The Tc-DfdRNAi phenotype was confirmed by comparing cuticle preparations of first instar larvae to previously described phenotypes[36, 42].
Cuticles from first instar larvae were prepared in Hoyer’s medium and lactic acid as previously described. The cuticle preparations were observed using differential interference contrast (DIC) and confocal fluorescent microscopy (larval cuticle autofluoresces at visible wavelengths). Cuticle preparations were observed using confocal microscopy with an excitation frequency of 488 nm using an upright Leica TCS SPE confocal microscope (Leica microsystems, Wetzlar, Germany). Images were obtained and edited using Leica application suite advanced fluorescence software, LAS-AF (Leica microsystems, Wetzlar, Germany).
Whole mount in situ hybridization
Embryos were fixed in 9% formaldehyde. Both single stainings (nitro blue tetrazolium/5-bromo-4-chloro-3-indolyl phosphate (NBT/BCIP)) and double stainings (NBT/BCIP and FastRed) were performed as previously described. Some modifications, for example in the frequency and duration of washes, were incorporated from alternative in situ hybridization protocols.
Stained embryos were dissected from their yolk and mounted in glycerol. Embryos (and cuticle preparations) were observed using differential interference contrast (DIC) microscopy with an Imager M1 microscope (Carl Zeiss Ltd., Cambridge, UK). Images were taken with Axiocam HRC (Carl Zeiss Ltd., Cambridge, UK) and processed using Axiovision product suite software release 4.8.2 (Carl Zeiss Ltd., Cambridge, UK). Images were edited with GIMP (release 2.6.10.).
Scanning electron microscopy
Embryos were fixed as described for the whole mount in situ hybridization protocol. Fixed embryos were rinsed in ethanol and immersed in hexamethyldisilazane (HMDS), air dried and sputter coated with gold. Images were taken in a JEOL JSM-5410LV scanning microscope ( JEOL Ltd., Tokyo, Japan) at a magnification of 100 to 350 fold and processed with DigitalMicrograph (Gatan Inc., Pleasanton, California, USA).
Tc-cnc is expressed in two distinct domains, an anterior cap that includes the developing labrum and around the stomodeum and a posterior collar domain in the mandibular segment (see Figure1A). Tc-cnc expression remains constant in these two domains from their first appearance during germ band elongation and through late embryogenesis (see Figure1D,F,H) and is expressed in regions of the mandibular limb bud where Tc-Dfd expression becomes repressed (see star in Figure1E-I). In the mandibular limb bud, Tc-cnc is expressed predominantly in the ectoderm, with weaker expression (or no discernable expression) in the mesoderm of the limb bud (see asterisk in Figure1G,J).
Tc-Dfd expression retracts from the developing mandible
Tc-Dfd is expressed throughout the mandibular and maxillary segments in the early developing Tribolium embryo (see Figure1B). As the mandibular limb buds start to form, Tc-Dfd expression progressively retracts from the ventral-proximal region of the mandibular limb bud (see Figure1E-J). Tc-Dfd continues to retract from this ventral-proximal region (star in Figure1E).
In the developing maxillae, Tc-Dfd expression is continually expressed in the protopodite (see Figure1E-J). Mandibular Tc-Dfd is increasingly repressed until only weak expression remains on the lateral side of the mandible (see Figure1H,I and Figure2A,C,D).
The mandibular limb bud has two lobes, the inner and the outer (see Figure2A). The distal-most part of the mandibular limb bud becomes the outer lobe of the mandible and develops into the future incisor process. Tc-Dfd is not present in this most distal region, which is more clearly noticeable in lateral orientations of dissected Tribolium embryos (see Figure2C).
We found that Tc-prd, in addition to its function as a secondary pair-rule gene, is expressed in the predicted location of the developing endites of the embryonic mandibular, maxillary and labial limb buds (see Figure2B). We therefore used Tc-prd expression as a marker for endite development. Tc-prd expression reveals that the ventral-medial region of the mandibular limb bud, where Tc-Dfd expression is lost, encompasses the mandibular endite and the immediate surrounding tissue. Tc-Dfd expression is retained in the lateral part of the mandibular limb bud, but fades throughout embryogenesis (Figure2D). Tc-Dfd expression is absent (or considerably weaker) in the distal part of the maxillary palps throughout embryogenesis (see arrow in Figure2E).
Tc-cnc RNAi phenotype
In order to test the role Tc-cnc might play in patterning the mandibular segment, the gene was knocked down in developing embryos by injecting Tc-cnc dsRNA into female pupae. The knockdown phenotype was determined in the offspring of injected parents using cuticle preparations of their first instar larvae (see Figure3).
Injection of Tc-cnc dsRNA produces phenotypes that relate to both the cap domain and the collar domain of Tc-cnc expression. The effect in the collar domain is the homeotic transformation of the mandibular appendage into a maxillary identity showing that the posterior collar domain of Tc-cnc expression differentiates the mandible from the maxillary appendage. This is shown in Figure3D,F, where Tc-cncRNAi larvae can be seen to possess an additional pair of maxillae. The mandibular appendages are transformed into a maxillary identity, in possession of a maxillary palp, and maxillary endites (which in wild-type first instar Tribolium larvae are fused to form the ventral branch; see Figure3A,C). Knockdown of the cap domain results in a dramatic deletion of the labrum showing Tc-cnc is necessary to pattern this structure (see Figure4B). There are also abdominal defects visible in some embryos, although it is possible that this aspect of the phenotype was an artifact of the cuticle preparation procedure.
Tc-cnc represses Tc-Dll and modifies Tc-prd expression in the Mandibular segment
To investigate the transformed mandibular appendage in Tc-cnc knockdown embryos, the expression patterns of the homeobox genes Tc-prd and Tc-Dll were studied as genetic markers of the developing endites and telopodites respectively (see Figure5).
In wild-type embryos, Tc-prd is expressed in the developing endites of all three pairs of gnathal appendages (mandibles, maxillae and labia; see Figure2B and Figure5A,C). There are two distinct domains of Tc-prd expression in the maxilla, which we assume correspond to the developing lacinia and galea. There is a single domain of Tc-prd in the labial appendage and a larger single domain of expression in the mandibular appendage.
Tc-Dll is expressed in the distal part of all appendages of wild-type Tribolium embryos except the mandible. In the developing maxilla, there are two domains of Tc-Dll expression, a distal domain in the developing palp and a proximal domain in the lacinia endite. Tc-cnc RNAi results in homeotic transformation of the mandibular appendage into maxillary identity. The solitary domain of Tc-prd expression in the mandible is transformed into two domains of Tc-prd expression that relate to the maxillary endites (see Figure5B,D-F). Tc-Dll is de-repressed resulting in expression in the palp and in a proximal endite that appears on the transformed mandible.
The transformed mandibular appendage develops more slowly than the adjacent true maxillary appendages at several stages of embryogenesis resembling the maxillary appendage of an earlier stage (see Figure5E). By late embryogenesis, there is no evident morphological difference between the maxillae and the ectopic maxillary appendages on the mandibular segment.
Asymmetry of different appendages in Tc-cnc RNAi embryos is often evident in germ band extending stage embryos and occurs left or right at random (see Figure5E). This does not appear to be an artifact of the RNAi procedure or the in situ hybridization process as appendages other than the mandible can be affected and parental RNAi experiments of other genes in Tribolium have not yielded a similar result (data not shown). Instead this may be related to a loss of the role that cnc has been shown to have in Drosophila in protecting the embryo from oxidative stress.
Tc-Dfd and Tc-mxp are expressed in a maxilla-like manner in the transformed mandibular limb bud of Tc-cncRNAi embryos
The Hox genes Tc-Dfd and maxillopedia (Tc-mxp), the Tribolium ortholog of pb, pattern the maxillary appendage in an additive fashion. Tc-Dfd is expressed in the proximal part of the maxilla (the protopodite), and Tc-mxp is expressed in the palp and is excluded from the proximal part of the protopodite, although it is expressed in the distal protopodite and galea endite (see Figure2B). Tc-Dfd patterns the protopodite: the proximal part of the appendage including the endite. Tc-mxp patterns the telopodite (the palp) and mutants of Tc-mxp possess legs instead of palps in both the maxillary and labial segments. These transformed appendages are attached to a protopodite that is unaffected by the loss of Tc-mxp[50, 51].
As Tc-cnc RNAi results in a homeotic transformation of the mandible into a maxilla, we predicted that both the Hox genes responsible for patterning the maxillary appendage will be expressed in the maxillary pattern in the homeotically transformed appendage. It was found that this is indeed the case (see Figure6).
In wild-type embryos, Tc-Dfd expression retracts from the mandibular limb bud (see Figure6C,F). In Tc-cncRNAi embryos, the mandible is transformed to maxillary identity and Tc-Dfd expression is retained in the protopodite of this transformed appendage (see Figure6D,G-I).
Tc-mxp is expressed in the maxillary and labial palps in wild-type embryos (see Figure6A,C,F). In the maxillae, Tc-mxp is expressed in the distal part of the protopodite, including the galea endite. In the transformed mandibular appendage of Tc-cncRNAi embryos, Tc-mxp is expressed in the ectoderm of the ectopic palp as it is in the maxillary palp and also includes expression in the galea endite and distal protopodite (see Figure6B,D,E,G,I).
Tc-mxp is expressed in the mesoderm of the mandibular appendages of wild-type embryos (white arrow in Figure6A,F). Interestingly, this mesodermal expression of Tc-mxp is seen in Tc-cnc knockdown embryos (white arrow in Figure6E,H). This suggests that there is cnc independent regulation of Tc-mxp in the mandibular limb bud. Tc-cnc is expressed in the ectoderm of the mandibular limb bud, and expression is weaker (or absent) in the mesoderm.
Tc-Dfd activates the posterior ‘collar’ domain of Tc-cnc in the mandibular segment
Experiments performed on Drosophila have shown that Dfd does not activate cnc expression. In order to investigate whether Tc-Dfd has any role in regulating Tc-cnc expression in Tribolium, we knocked down Tc-Dfd by parental RNAi and detected Tc-cnc expression via in situ hybridization.
Surprisingly, we found that in Tc-DfdRNAi embryos the posterior collar domain of Tc-cnc expression is completely missing from all stages of embryo investigated, from germ band extending embryos through to stages where embryos are undergoing dorsal closure (Figure7). The anterior cap domain of expression is unaffected. This shows that, unlike in Drosophila, Tc-Dfd is necessary for the activation of the posterior domain of Tc-cnc in the mandibular segment of Tribolium.
Tc-Dfd activates Tc-prd expression in the mandible and maxillary segments
Brown et al. have shown that Tc-Dfd is required to pattern the mandible and the proximal part of the maxillary appendages In Tc-Dfd mutants, the mandible is transformed to antennal identity and the maxillae lose the endites whilst retaining the palp.
In order to further investigate the role of Tc-Dfd in patterning the gnathal appendages, we studied Tc-prd expression in Tc-DfdRNAi knockdown embryos. In Tc-DfdRNAi knockdown embryos, Tc-prd expression is lacking in both the transformed mandible (ectopic antennae) and the affected maxillary appendages (see Figure8C,D). Tc-prd is still expressed in the developing labial endite. This result shows that Tc-Dfd is necessary for the activation of Tc-prd expression in the mandibular and maxillary segments and is further evidence that Tc-Dfd is required for development of the endites on these segments.
The role of Tc-cnc in patterning the mandible of Tribolium
We sought to understand mandible patterning in a model arthropod that has a mandible with primitive characteristics. Our results show that Tc-cnc is required for specification of the identity of the mandibular segment of Tribolium and differentiates the mandible from a maxilla.
Knockdown of Tc-cnc transcripts by parental RNAi results in a homeotic transformation of the mandible into maxillary identity in Tribolium embryos and first instar larvae. The homeotic transformation is also evident in the changed expression of the genes Tc-Dll and Tc-prd (markers for the developing telopodite and endite of the maxilla) in knockdown embryos.
The Hox genes Tc-mxp and Tc-Dfd are required to pattern the maxillary appendage and do so in an additive manner, Tc-Dfd patterns the base of the appendage and Tc-mxp patterns the palp[36, 51]. We show that in Tc-cnc knockdown embryos, Tc-Dfd and Tc-mxp are expressed in a maxilla like pattern in the transformed mandibular appendage.
We show that the ‘collar’ domain of Tc-cnc in the mandibular segment is activated by Tc-Dfd in Tribolium. The mandibular segment collar domain of cnc is not activated or regulated by Dfd or by any other Hox gene in Drosophila. We also show that Tc-Dfd is necessary for the expression of Tc-prd in both the mandible and the maxilla.
Based upon the results of this and previous studies we present a model for the roles of these genes in mandible patterning in Tribolium (see Figure9).
The role of Tc-cnc in patterning the labrum of Tribolium
The deletion of the labrum in Tc-cncRNAi embryos is consistent with the loss of the cap domain of Tc-cnc expression in the labrum. The labrum is a structure of considerable interest as it is shared by all extant groups of euarthropods whilst its evolution and development remain controversial. The labrum has appendage-like characteristics and may have evolved from a fused pair of appendages, for example from structures homologous to the anterior antennae of lobopods. However, unlike all other paired arthropod appendages, the labrum is not associated with a segment and may have a different origin.
Comparisons with Drosophila
There are many similarities between Tribolium and Drosophila in the expression patterns of genes in the mandibular and maxillary segments and in how these segments are patterned. In both insects Dfd and cnc are both required to pattern the mandibular segment. cnc is required for the patterning of labral derived structures and the differentiation of the mandible from maxillary identity. cnc represses Dll expression in the mandibular segment.
The Hox genes Dfd and pb/Tc-mxp are also expressed in similar proximal and distal domains respectively in the maxillary segment limb bud or gnathal lobe as are prd and Dll. Dfd patterns proximal structures that are derived from the maxillary lobe or limb buds. In both species, Dfd activates the proximal domain of Dll[24, 25, 36]. Tc-Dfd activates the maxillary prd domain in both Drosophila, and also, as we have shown in this study, in Tribolium[55, 56].
There are nevertheless differences in the patterning of the mandibular and maxillary segments between Tribolium and Drosophila. In Drosophila, loss of cnc function does not result in a full homeotic transformation of the mandibular gnathal lobe to maxillary identity, rather, the mandibular gnathal lobe is transformed into just the proximal part of the maxillary gnathal lobe. This is in contrast to Tribolium where loss of Tc-cnc function results in a complete transformation of mandible to maxillary identity.
In addition to the activation of Tc-cnc in the mandibular segment by Tc-Dfd, another difference between Drosophila and Tribolium is the regulation of collier (col) by cnc. The anterior mandibular expression of cnc is upstream of col in Drosophila and both genes are required to pattern the hypopharyngeal lobes[57–59]. In Tribolium, which does not have hypopharyngeal lobes, it has been recently shown that Tc-cnc is not activated by Tc-col.
The role of cnc as a repressor of maxilla patterning Hox genes
While we have shown that Tc-cnc patterns the mandible and differentiates the mandible from a maxilla, the precise role that it has in patterning the mandibular segment is not clear. The many similarities in the patterning function of cnc in Tribolium and Drosophila suggest that the molecular functions of Cnc protein revealed by experiments in Drosophila may be similar in Tribolium.
Research in Drosophila has demonstrated the role of cnc as a repressor of Hox gene function in the mandibular segment[23, 24]. cnc has been shown to repress Dfd transcription and Dfd protein activity in the anterior mandibular segment in Drosophila[23, 24]. There is co-expression of cnc and Dfd in the posterior of the mandibular segment, indicating that some mandibular expression of Dfd is not affected by the presence of cnc[23, 24]. Dfd has also been shown to repress pb in the ectoderm of the mandibular segment in Drosophila.
As the dynamics of Tc-Dfd expression in Tribolium resemble the dynamics of Dfd expression in Drosophila, with initial coexpression followed by subsequent repression of Tc-Dfd in a part of the mandibular segment, it seems likely that a similar situation is occurring in Tribolium.
We have shown that Tc-cnc is necessary for both the repression of Tc-Dfd expression in the mandibular limb bud and the repression of the ectodermal palp domain of Tc-mxp in the developing mandibular limb bud. However, further research is needed to determine whether Tc-cnc has a direct functional role in the repression of these Hox genes.
The possible role of Tc-cnc as a direct activator of mandible patterning genes
In Drosophila, several lines of evidence suggest that Cnc functions as an activator, activating mandibular segment specific patterning genes and thereby indirectly repressing Hox genes. cnc also patterns some mandibular segment derived structures independently of Dfd. Ectopic activation of cnc in Drosophila embryos results in ectopic hypopharyngeal lobe derived structures. Although the hypopharyngeal lobes have been thought to derive from the intercalary segment, it has recently been shown that they are in fact derived from the mandibular segment. This result indicates that cnc is in fact necessary and sufficient to pattern some mandibular segment derived structures suggesting that Tc-cnc may directly activate mandible patterning genes in Tribolium.
Conserved expression of cnc, Dfd and pb in mandibulate arthropods
Comparison of the expression of cnc homologs in mandibulates suggests that both functions of the labral patterning anterior ‘cap’ domain and the mandible patterning posterior ‘collar’ domain are conserved in mandibulate arthropods. Species that have been studied in addition to Drosophila and Tribolium include the cricket Acheta domestica, the milkweed bug Oncopeltus fasciatus, and the firebrat Thermobia domestica[39, 40]. Outside insects, only one species has been studied to date, the myriapod Glomeris marginata, which also shows expression in a cap and a mandibular collar.
The expression patterns of orthologs of Dfd and pb are also conserved in other mandibulates suggesting that patterning of the maxilla may also be conserved. Dfd is expressed in the mandible and maxilla bearing segments in the majority of mandibulates and expression is stronger in the protopodite than in the palps of maxillary appendages[36, 39, 62–66]. There is loss of Dfd expression in the mandibular limb bud across mandibulates, as in Tribolium and Drosophila[24, 35, 65, 66]. Expression of pb is conserved in the telopodites of these maxillary appendages[39, 65, 67].
In an onychophoran, the closest extant outgroup to the Arthropoda, a homolog of Dfd is expressed in the proximal region of each walking limb bud suggesting that Dfd expression in the base of the mandibular and maxillary limbs may be the primitive condition in the Arthropoda.
cnc and the evolution of the mandible from a maxilla-like precursor
The manner in which cnc differentiates the mandible from maxillary identity may ultimately provide clues about how the mandible has evolved from a maxilla-like precursor in the stem lineage of mandibulate arthropods.
A study of the fossil record shows that the mandible has evolved from a particular type of jointed appendage, the biramous limb (see Figure10A). In the ancestor to the arthropods, the primitive post-antennal limbs were similar in structure. As stem lineage arthropods diverged during the Cambrian, post-antennal biramous limbs diverged from the primitive biramous limb structure. The likely precursor to the mandible was a maxilla-like appendage, with numerous well-defined endites similar to those present on other post-antennal segments (see Figure10G). Such a maxilla-like second post-antennal limb is present in numerous ‘crustaceamorph’ stem lineage mandibulate arthropods like Martinssonia elongata and the Phospatocopida[2, 11, 69, 70].
We hypothesize that, in the stem lineage to the mandibulate arthropods, Dfd patterned the base of the ancestral monopodial limb (see Figure10B) and the protopodite of the primitive biramous gnathal appendages (see Figure10C). At some point in the stem-lineage leading to the mandibulate arthropods, cnc acquired a new role patterning the mandibular segment: differentiating the mandibular endite and protopodite from those of the maxilla resulting in the mandibular gnathal edge (see Figure10D).
The mandible has probably evolved from a biramous maxilla-like precursor by modification of the most proximal endite to form the characteristic mandibular gnathal edge whilst, at least primitively, retaining both the telopodite palp and the exopodite (see Figure10G).
The role of cnc homologs in chelicerates and onychophorans
To test the idea that the function of cnc evolved to pattern the mandible in the lineage leading to the mandibulates, it is necessary to study cnc homologs in outgroups to the Mandibulata with the prediction that it does not have a comparable role in patterning the segment homologous to the mandibular segment (the first leg segment in chelicerates).
The homologous segment to the mandibular segment in the chelicerates and the onychophorans is the first leg segment and homologs of Dfd are expressed in this segment (see Figure10E,F)[4, 5, 68]. In these groups there is no obvious differentiation between the first leg appendage and the second leg appendage (maxilla homolog). It is therefore not obvious what role a ‘collar’ domain of cnc would perform in chelicerates or onychophorans.
Although the expression of cnc is not known in non-mandibulate arthropods, expression of chelicerate anterior Hox genes such as Dfd and pb are different in several respects to the conserved expression of these genes in mandibulate arthropods. This suggests that the conserved expression of Hox genes in the mouthparts of the mandibulate arthropods is a synapomorphy for the Mandibulata[5, 71, 72].
The closest related outgroup of the Mandibulata in which a cnc homolog has been investigated is the nematode Caenorhabditis elegans. The C. elegans cnc homolog, Skn1, has been shown to have developmental role in patterning mesoderm and endoderm derived structures[73, 74]. One important, non-developmental role of cnc (and its homologs across Bilateria) that has been studied in some detail is its role in xenobiotic and oxidative stress responses[49, 75–77]. This role has been discovered in diverse organisms and is likely to be present both in mandibulates and in closely related outgroups to the Mandibulata such as the chelicerates.
Our study is the first functional investigation of some of the genes necessary specifically to pattern the mandible of an arthropod species with a canonical mandible in which the gnathal edge is made up of the incisor and molar processes.
Using parental RNAi to knockdown gene transcripts in Tribolium, we show that Tc-cnc is required for specification of the identity of the mandibular appendage and differentiates it from maxillary identity. Analysis of gene expression by in situ hybridization shows that Tc-cnc is required for the repression of the maxillary expression domains of the Hox genes Tc-mxp and Tc-Dfd, which pattern the maxilla. We also show that Tc-cnc is necessary for the formation of the labrum. The mandible differentiating function of Tc-cnc is similar to the role of cnc in Drosophila in patterning the mandibular segment; in both beetle and fly, cnc and Dfd cooperate to specify mandibular identity. One significant difference is that Tc-cnc is activated by Tc-Dfd in the mandibular segment in Tribolium whereas cnc is activated independently of Dfd in Drosophila.
Similar expression patterns of cnc, Dfd and pb homologs in other mandibulate arthropods suggests that the functions of these genes are conserved, that cnc also differentiates the mandible from the maxilla in these species and that cnc evolved a mandible patterning function in the lineage leading to the mandibulates and possibly acts in conjunction with Dfd to achieve this.
To show that cnc has a conserved role in patterning the mandible across Mandibulata requires study of the function of cnc, or at the very least additional expression data, in more representatives of the mandibulate arthropods. In particular, expression data are lacking from any crustacean species.
The authors would like to thank Gregor Bucher (Göttingen) for providing a culture of Tribolium castaneum beetles. This work was supported by funding from the Biotechnology and Biological Sciences Research Council (BBSRC).
Department of Genetics, Environment and Evolution, University College London
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