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The mandible is a notable exception in species that are lacking a palp of telopodite origin. These mandibles lack the telopodite Dll expression domain which is strong evidence for the gnathobasic nature of the mandible appendage (Niwa et al., 1997; Popadic et al., 1998; Scholtz et al., 1998).
Proximal domain of Tc dac
There are two domains of Tc dac expression, a proximal and distal expression domain. The proximal expression domain is expressed more strongly in the gnathal appendages than the trunk limbs. This proximal expression domain has been argued to be important for the development of the gnathal appendages (Prpic et al., 2001). The proximal domain of dac expression is strong in the mandible which may be indicative of its importance for mandibular development (Prpic et al., 2001). Prpic et al. have argued that the proximal domain of Tc dac is evidence of serial homology of the whole mandible to the coxa of the leg (Prpic et al., 2001).
It has also been argued that the proximal domain of dac expression is ancestral to mandibulates and is important for patterning the endite lobes on all appendages of this clade (Sewell et al., 2008).
Numerous studies have investigated the expression patterns of PD domain genes in diverse taxa, but the relationship between the PD domain genes and notch signalling has only been investigated in Drosophila. There has been no study of the Notch signalling pathway in a mandibulate arthropod with segmented gnathal larval appendages.
Aim of Chapter
By studying the expression of the segmentation marker Tc ser, it was hoped that several specific questions about the structure of the embryonic mandible and serial homology of particular mandibular structures could be addressed. Most importantly, I wanted to discover if there was any evidence of segmentation in the embryonic mandibular appendage. Machida and others have suggested through SEM studies of a bristletail that the embryonic mandible is divided into a subcoxa and coxa (Machida, 2000; Oka et al., 2010).
Also, I wanted to investigate serial homology of the arthropod mandible protopodite to other appendages. If there is evidence of segmentation in the mandible, by studying the co-expression of Tc ser and the PD domain genes it is possible to find similarities that are evidence of homology of mandibular structures to other appendage structures.
There were specific questions of serial homology that I wanted to address.
Machida and Kukalova-Peck have both hypothesized that the subcoxa of the embryonic mandible is serially homologous to the cardo of the maxilla (Machida, 2000;
Haas et al., 2001). Machida has also hypothesized that the coxa of the embryonic mandible is homologous to the stipes of the maxilla (shown in fig. 3.1A).
I also wanted to investigate the subcoxal hypothesis of the leg pleuron. The pleuron is defined as the part of the body where the legs attach to the thorax (Snodgrass, 1935; Deuve, 2001; Boxshall, 2004; Grimaldi and Engel, 2005). In the majority of insects, the coxa is attached to separate sclerites which are hypothesized to have subcoxal origin (Snodgrass, 1935; Boxshall, 2004). Boxshall has commented that if the pleuron of the leg has a subcoxal origin then it is possible to homologize protopodite segments of the leg to other appendages (Boxshall, 2004). The presence of a leg subcoxa could be revealed by the expression of Tc ser. In which case, evidence for the homology of the leg subcoxa to other segments can be shown (see fig.3.1B).
The expression of the PD domain genes, limb segmentation genes and an endite marker, Tc prd, was studied by in situ hybridization. The expression patterns of the PD domain genes was compared to the expression of the Notch signalling pathway in order to determine the precise segmental affinity of the PD domain gene expression patterns. In addition the morphology of the developing appendages will be studied using SEM of embryos and microscopy of cuticle preparations of first instar larvae. By comparing expression of these genes in different appendages, evidence for serial homology of any mandibular structures to other appendage structures for example the cardo and stipes on maxilla, or the subcoxa of leg will be considered.
3.2 Results Partial cDNA sequences of Tc ser, Tc Dll, Tc hth and Tc prd were amplified and cloned in order to synthesize antisense labelled RNA probes to detect gene expression by in situ hybridization (see chapter eight). A clone of Tc dac was given courtesy of N.
Scanning electron micrographs of Tribolium embryos SEMs were taken of Tribolium to have higher resolution of mandibular morphology, to relate the subcoxa/coxa boundary found in other embryonic taxa to expression of the PD domain genes, endite and segmentation marker genes.
A groove can be seen on the lateral side of the developing mandible (see fig.3.2). This groove is interpreted here as representing a subcoxal/coxal segment boundary that is only visible during embryogenesis. A groove is also present at a similar position in the developing maxilla (see fig.3.2A,C-D) and labial appendage (see fig.3.2D). The developing maxilla undergoes complex morphological changes during embryonic development. The orientation of the palp the protopodite changes during embryogenesis.
Tc ser expression demarcates appendage segments in all appendages
In order to mark the development of segments, the Tribolium homolog of serrate, Tc ser, expression was detected in the developing appendages of Tribolium embryos. Tc ser expression is complex, with numerous expression domains in different embryological structures (fig.3.3A,C). Tc ser is expressed in rings along the proximaldistal axis of all appendage types (fig.3.3, fig.3.4). Tc ser expression was located immediately proximal to the developing limb joints in the distal part of each limb segment (fig.3.10A,F). The situation is more complex in the gnathal appendages, as the morphology of these appendages is less uniform than the telescopic-like appearance of the leg appendages. These appendage domains of Tc ser relate to grooves revealed by SEM (fig.3.3). The orientation and morphology compounded with the relative faintness of some of the expression domains means that dissection of gnathal appendages is required to study Tc ser expression.
In the leg there are five rings of expression of Tc ser (see fig.3.3A,C). The identity of each of these domains was determined by comparison of Tc ser expression Fig.3.2. Scanning electron micrographs (SEMs) of developing gnathal appendages of Tribolium embryos showing the presence of a subcoxa/coxa boundary on the developing mandible. All views are lateral with anterior to the left unless otherwise indicated. Subcoxal/coxal division on the mandible is indicated with an arrowhead. (A) Embryo at germ band retracting stage, ventral-lateral view. Proximal division that may relate to future cardo/stipes segment boundary on the maxilla is indicated with an arrow. (B) Germ band retracting embryo at an earlier stage than A. Labial appendages have not begun to fuse, a subcoxa/coxa division is visible on the labial appendage (arrow) and the mandible. (C) Close up of head of an embryo during dorsal closure. (D) Ventral view of head of germ band fully retracted embryo.
Anterior is to the top. The labial appendages have begun to fuse to form a lower ‘lip’ below the mouth opening. A lateral groove is present on the protopodite of the mandible and maxilla.
domains with Tc prd expression (fig.3.4B-D), PD domain gene expression (fig.3.5A-K) and appendage morphology in late stage embryos (fig.3.10). The developing appendages of late stage embryos are more developed and resemble the morphology of appendages of first instar larvae. The similar identity of the subcoxal and coxal domains of Tc ser was also determined by studying the early expression of these domains with Tc dac as a marker (fig.3.8 and fig.3.9).
The five rings of expression of Tc ser correspond to the distal boundaries of the following leg segments in proximal to distal order: subcoxa-1, coxa-2, trochanteral-3, femur-4, tibia-5 (see fig.3.5I-L and fig.3.10A-C,E-F). The tarsal segment is more distal to the tibial Tc ser domain. Similarities of expression revealed by comparison of the leg Tc ser domains to the gnathal appendage Tc ser domains, particularly the leg subcoxal-1 and coxal-2 domains, has led me to use the same numerical indications to describe the gnathal appendage domains. This nomenclature will also aid in descriptions of the Fig.3.3. Tc ser expression domains mark the position of developing appendage segments and shows that there is a mandible subcoxal segment. All views are ventral with anterior to the left unless otherwise indicated. Gene expression was detected by in situ hybridisation. (A) Tc ser (blue) and Tc prd (red) expression in a fully retracted stage embryo. The five ring domains of Tc ser expression are marked by arrows. There are two ring domains in the antenna (arrowheads). Tc ser domains are present in the mandible and maxilla but are not readily distinguishable. (B) SEM of an embryo at a similar stage of embryogenesis to A. The position of the developing segmental boundaries, visible as grooves perpendicular to the P/D axis of the appendages, are in the same positions as the ring domains of Tc ser visualized in A. The segmental grooves in the legs are marked by arrows, in the antenna by arrowheads.
(C-D) Higher magnification of gnathal appendages. Numbers indicate Tc ser domains. (C) Expression of Tc ser (blue) in an embryo during dorsal closure. There are two domains of expression in the mandible limb bud. A distal spot (star) domain that is in the outer lobe and a ring domain-1 of Tc ser that marks the division of the subcoxa and coxa. The maxilla has several domains of Tc ser expression. In the protopodite, there are two ring domains numbered 1 and 2 and a smaller more proximal stripe domain, numbered 0. (D) SEM of an embryonic head at a similar stage to that of C. The subcoxa/coxa boundary is visible in both the mandible and maxilla in a similar position to the subcoxal domain-1 of Tc ser expression visible in C.
Fig.3.4. Expression of Tc prd relative to Tc ser suggests that endites develop in the distal-most segment of the mandible, maxillary and labial protopodites. All views are ventral with anterior to the left unless otherwise indicated. The subcoxal domain of Tc ser is marked with an arrowhead. Gene expression was detected by in situ hybridisation. (A) SEM of developing appendages of a fully retracted Tribolium embryo. (B-D) Tc ser (blue) and Tc prd (red) expression embryos of a similar stage to A. Tc prd is visible in the mandible, maxilla and labial endites. (B) The mandibular Tc prd domain is clearly more distal to the ring domain of Tc ser. The orientation of the maxilla and labial appendages prevent clear visualization of the relative expression of Tc ser and Tc prd. (C) Close up of gnathal appendages of an embryo at a similar stage to those in A and B. The subcoxal domain of Tc ser expression (arrow) is more proximal than the Tc prd endite domains. (D) Lateral view of gnathal appendages. Expression of Tc prd is distal to the Tc ser subcoxa ring domain in the mandible, maxilla and labial appendages.
expression domains of these genes. The evidence for the similarity of the appendage Tc ser domains between the leg and gnathal appendages is the similarities observed in Tc prd expression (see fig.3.4) PD domain gene expression (see fig.3.5) and early Tc ser expression marked by Tc dac (see fig.3.8 and fig.3.9).
The expression of Tc ser in the maxilla was the most difficult to interpret and required dissection to identify expression domains. At reasonably late stages of development, prior to dorsal closure and fusion of the labial appendages, there are two domains in the protopodite, the subcoxal-1 and coxal-2 domain, and at least two ring domains in the palp (see fig.3.5D,E and fig.3.7A,E,F). There was a fifth distal ring domain that was detected in one dissected maxilla (fig.3.5G). The expression of this ring domain was particularly faint. The maxillary subcoxal-1 domain of Tc ser is clearly associated with a cell boundary (see fig.3.7E,F). In late stages of maxilla development, there is the appearance of a stripe or spot of Tc ser at the base of the appendage proximal to all the other rings of Tc ser expression (see fig.3.7A,C). The significance of this proximal-0 Tc ser domain, and whether it relates to the future cardo/stipes is not known.