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In the developing leg there are five domains of Tc ser expression (see fig.3.10E).
These five domains of Tc ser relate to the five segment boundaries in the larval leg (fig.3.10C). The coxal-2 Tc ser domain is the second ring domain of Tc ser and clearly relates to the distal part of the coxal leg segment. This is confirmed by the expression of Tc Dll in the third Tc ser domain in an earlier developing leg appendage (arrowhead in fig.3.10E) which is co-expressed with the trochanteral-3 domain that relates to the trochanter in a later stage developing leg appendage (arrowhead in fig.3.10F). The trochanter is recognizable as a one of the smallest leg segments adjacent to the coxa, the largest leg segment.
By comparing the expression of all the PD domain genes to Tc ser expression, the following conclusions can be made: Tc Dll is co-expressed with the trochanteral-3, femoral-4 and tibial-5 domains of Tc ser. The distal domain of Tc dac is co-expressed with the femoral-4 domain of Tc ser. The proximal domain of Tc dac expression is located in the coxa segment. Tc hth is co-expressed with the subcoxal-1, coxal-2 and trochanteral-3 domains of Tc ser. This is shown schematically in fig. 3.11.
The proximal domain of Tc dac expression is expressed in the coxal segment.
Prpic et al. argued that the proximal domain of Tc dac supported serial homology of the whole mandible to the coxal leg segment (Prpic et al., 2001). The expression of Tc dac relative to Tc ser indicates that the proximal domain of Tc dac is expressed in the coxa of the developing leg: the proximal domain of Tc dac is coexpressed with the coxal-2 domain of Tc ser. The proximal domain of Tc dac in other appendages is also present in the coxal segment. These results therefore support the Fig.3.11. Similarity of expression domains of Tc ser relative to the PD domain genes in different postantennal appendages suggests serial homology. All views are distal to the top. The mandible and maxilla are orientated with lateral to the right. Tc ser ring domains are marked as numbered black lines.
Based upon the similarity of the expression of the PD domain genes to particular domains of Tc ser expression in each appendage, each Tc ser domain is numbered in order from proximal to distal. Tc ser is expressed in the distal part of each segment adjacent to where the segment boundary will form. In the leg, 1 refers to the expression of Tc ser in the distal boundary of the subcoxa, 2 to the coxa, 3 to the trochanter, 4 to the femur, and 5 to the tibia. In the mandible, maxilla and labial appendages, 1 and 2 refer to the distal boundaries of the subcoxa and coxa respectively. The proximal-0 stripe domain of Tc ser in the maxilla is different to subcoxal ring domains present in other appendages and has therefore been numbered 0. (A) Representation of the expression domains of Tc dac (red) and Tc hth (blue).
Overlapping regions are purple. Tc ser ring domains are marked as numbered black lines. (B) Representation of the expression domains of Tc dac (red) and Tc Dll (blue). Overlapping regions are purple.
conclusions of Prpic et al. that the proximal domain of Tc dac is expressed in the coxa and is evidence for the serial homology of these appendages. There are, however, differences between the proximal domain of Tc dac expression in the gnathal appendages compared to the legs. More proximally to the proximal domain of Tc dac, it is apparent that there is a domain of Tc ser in all post-antennal appendages. This is the subcoxal-1 domain of Tc ser expression, and provides evidence that a subcoxal segment is present in all post-antennal appendages.
The subcoxa of the leg becomes the pleuron The subcoxal-1 domain of Tc ser expression domain provides molecular evidence for the subcoxal derivation of the pleuron in Tribolium larvae and therefore supports the subcoxal theory of the origin of the pleuron, at least for insects. The pleuron is defined as the part of the body where the legs attach to the thorax (Snodgrass, 1935; Boxshall, 2004; Grimaldi and Engel, 2005). Expression of Tc ser reveals the existence of a subcoxal segment present in the developing leg. Tc ser expression in late stage embryos shows the developing coxa with a subcoxal-1 domain of Tc ser expression more proximal to it close to the thorax, a subcoxal segment (see fig.3.10A,B,F). This segment flattens radially into the body wall and supports the base of the coxal segment of the leg. This leg derived segment is then incorporated into the body wall, although still seen to be separated from it by a segment boundary (star in fig.3.10C,D,G,H).
In the majority of insects, the coxa is attached to separate sclerites which are hypothesized to have subcoxal origin (Snodgrass, 1935; Boxshall, 2004). In the Tribolium first instar larva, the leg subcoxa does not differentiate into separate sclerites and is visible as a separate segment. The legs of Tribolium larvae are attached to a flattened subcoxal segment on thoracic segments. This condition is found in numerous larval insects (Snodgrass, 1935).
The proximal-most segment has been defined in the leg of the sawfly Athalia as a subcoxa (Oka et al., 2010). However, to date there has been no molecular evidence for the subcoxal derivation of the pleuron.
Some authors have considered the pleural sclerites to be a homologous character that unites myriapods and hexapods in the clade Tracheata (Bäcker et al., 2008). This explanation is unlikely, due to the high degree of support for monophyletic Pancrustacea. The presence of sclerites that are derived from appendage segments is more likely to be evidence of convergent evolution, possibly as an adaptation to terrestial locomotion.
Homology of the subcoxa and coxa of the mandible to the subcoxa and coxa of other post-antennal appendages Comparison of the subcoxa and coxa of the mandible reveals striking similarities in the expression of PD domain genes relative to other subcoxal and coxal segments. The subcoxa and coxa of the post-antennal appendages show numerous similarities of gene expression that suggest serial homology. This conclusion is based upon comparisons of Tc ser expression which define segment boundaries to the expression of the PD domain genes which define segment identities, especially that of Tc dac. The similarities are particularly notable between the mandible and maxilla subcoxa.
The coxal segment of the gnathal appendages is characterized by the expression of Tc prd in the developing endites (see fig.3.4) and the proximal domain of Tc dac (see fig.3.5). The subcoxal segment is characterized by exclusive expression of Tc hth (see fig.3.5).
Additional evidence for the serial homology of the coxa and subcoxa is the sequence of activation of the subcoxal-1 and coxal -2 Tc ser domains in the developing limb appendages. The first Tc ser domain that is activated is the coxal-2 ring domain.
The coxa domain is activated first in all post-antennal appendages except the mandible and is co-expressed with Tc dac (shown in fig.3.12A). The subcoxal-1 ring domain is activated next in all post-antennal appendages, including the mandible, proximal to the proximal domain of Tc dac (shown in fig.3.12B). The first ring of Tc ser expression to appear is the coxal ring of expression. The second ring of Tc ser expression to appear is the more proximal subcoxal Tc ser ring. The appearance of both of these rings so early in the development of the limb bud in all appendages suggests that these Tc ser domains are fundamental segment divisions of the developing appendages. The simultaneous activation of the subcoxal-1 domain of Tc ser suggests that the two domains share the same developmental pathway which may indicate serial homology between the gnathal appendages.
Machida hypothesized that a lateral groove present in the mandible of the jumping bristletail Pedetontus unimaculatus was serially homologous to the cardostipes segment boundary in the maxilla (Machida, 2000). The lateral groove present in Gryllus, Athalia and Tribolium is in a similar position on the mandibular and maxillary Fig.3.12. Expression of Tc ser and the proximal Tc dac domain suggests serial homology of the coxa and subcoxa. The schematic of this figure is adapted from the in situ hybridisations shown in figure 3.9.
The figure shows the onset of expression of the first and second Tc ser ring domains in the developing limb buds of the mandible, maxilla, labial and leg appendages. (A) Early limb bud formation. Expression of the coxal-2 domain of Tc ser is associated with the expression of the proximal domain of Tc dac. This occurs at the same time in all post-antennal limbs. (B) Subsequent expression of the subcoxal-1 domain of Tc ser in later developing limb buds. The distal domain of Tc dac is present in the leg appendages.
appendages and seems to represent a shared ontogenetic program between the mandibles and maxillae of these species.
The simplest explanation for the identity of the subcoxal ring domain of Tc ser in the mandible and maxilla is that the similarity of expression reflects their serial homology. Homology of the mandible subcoxa to the maxilla subcoxa depends on common ancestry of those structures in question and not just their similarity of structure or gene expression patterns although homology will often reveal itself through similarity of structure or patterning mechanism. Similarity could represent convergent evolution. Therefore care has to be taken in order not to mistake convergent evolution as homology. The diversity of arthropod appendages and the relationship of these appendages on a phylogenetic tree must be taken into account before similarities between characters can be understood as homologies or homoplasies.
Arthropod post-antennal appendages evolved from serially homologous biramous limbs. At some point in the evolution of the mandible and maxilla, the limbs were identical in structure and then diverged to evolve into the myriad forms of mandibles and maxillae that are present today. The implications of a serial homologous relationship between the subcoxa of the mandible and maxilla are that the ancestral protopodite of the gnathal appendages would have originally consisted of three segments: a subcoxa, coxa and basis (using crustacean morphological terminology). The mandibular endite is present on the mandibular coxal segment, which is more distal than the subcoxal segment. The mandibular endite could be homologous to one of the endites present on the maxillary coxal segment, the lacinia.
The segmented diplopod mandible consisting of a cardo (subcoxa) and stipes (coxa) could represent the primitive state. The basis has been lost in the insect mandible.
In the insect maxilla, it is hypothesized that maxillary stipes segment has formed from fusion of the coxa and basis which is present in more primitive hexapod maxillae (Boxshall, 2004). This fusion of the coxa and basis to form the stipes of the insect maxilla does complicate proposed homologies between the coxa of the mandible to the coxa (or stipes) of the maxilla which have been suggested in this chapter. The proposed solution to these difficult questions of serial homology of arthropod appendage segments is to acquire more data (particularly of Notch signalling pathway and PD domain gene expression) from more taxa.
The implications of the serial homology of the subcoxa between appendages will be investigated in chapter seven in more detail.
Serial homology of the gnathal appendage subcoxa to the subcoxa of the leg
Boxshall has commented that if the pleuron of the leg has a subcoxal origin then it is possible to serially homologize the leg protopodite segments to other appendages (Boxshall, 2004). The expression of Tc ser indicates that there is a subcoxal segment in the leg that develops into the pleuron in the larva confirming the subcoxa hypothesis of the origin of the pleuron. There are numerous similarities between the developing subcoxa and coxa of the leg to the subcoxa and coxa of the gnathal appendages that suggest serial homology.
The similarity of the expression of the PD domain genes relative to Tc ser domains in the subcoxa and coxa post-antennal appendages (fig.3.11) provides some evidence of homology. In addition, the coxal-2 domain of Tc ser expression is activated at the same time in the maxilla, labial and leg limb buds (fig.3.12). The proximal domain of Tc dac is activated with the coxal-2 domain of Tc ser. The subcoxal-1 domain of Tc ser is activated at the same time in all post-antennal appendages (shown in fig.3.12).