«Joshua Frederick Coulcher UCL Submitted for the Degree of Doctor of Philosophy September 2011 Declaration I, Joshua Frederick Coulcher, confirm that ...»
Fig.3.9. Timing of early expression domains of Tc ser suggest serial homology of subcoxa and coxa between different appendages. Images show higher magnification of the expression of Tc ser (blue) and Tc dac (red) from Fig.3.8. Gene expression was detected by in situ hybridisation. 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. The first and second Tc ser ring domains are interpreted to represent the distal segment boundaries of the subcoxa and the coxa respectively. Successive germ band extending stage embryos are shown in A-G and germ band retracted stage embryos are shown in H-J. All views are ventral with anterior to the left unless otherwise indicated. (A-C) There are proximal domains of Tc dac in each appendage. The maxilla, labial and leg limb buds have a coxal-2 domain of Tc ser expression. There is no domain of Tc ser in the mandibular segment at this stage. (A) Mandible and maxilla limb buds (B) Labial limb buds. (C) First leg segment. (D) Mandibular, maxillary, labial and first leg segments of a later germ band extending embryo. (E-G) Germ band retracting stage embryos. The proximal domain of Tc dac in the leg appendage appears to be broadly expressed between the subcoxaland coxal-2 domains of Tc ser. (E) Mandible. (F) Maxilla and labial appendages. (G) First leg appendage. Distal is to the top. (H-J) Germ band retracted stage embryos.
(J) Leg. Ventral view.
Analysis of expression of Tc ser and Tc dac during early stages of appendage development in Tribolium embryos shows the identity and the order in which particular rings of Tc ser expression appear. The first ring of Tc ser expression to appear in the developing limb appendages is the coxal-2 Tc ser ring domain in the maxilla, labial and leg limb buds (see arrowhead in fig.3.8B and fig.3.9A-C). The next proximal domain of Tc ser to appear is the subcoxal-1 Tc ser ring domain present in the mandibular, maxillary, and leg appendages (see fig.3.8C-D and fig.2.9D-G). Subsequent unidentified telopodite ring domains of Tc ser appear (though probably the tibial-5 domain) at a very similar time to the subcoxal domain (see fig.3.9D).
The faintness of the expression of these distal domains as they appear makes conclusions on the precise order of onset of Tc ser expression for the entire limb very difficult. For the purposes of this study, I was more interested in the onset of expression of the proximal domains of Tc ser that relate specifically to the developing protopodite. For these two domains the results are clear. The coxal-2 ring domain of Tc ser appears first in all appendages where it is present (not the mandible) and is coexpressed with Tc dac in all segments. The subcoxal-1 domain expressed after the coxal-2 domain and is proximal to the expression of Tc dac.
The expression of Tc dac is slightly different in the leg, it is associated with the coxal-2 domain throughout embryogenesis (see fig.3.9C,G,J). The proximal Tc dac domain in the gnathal appendages are initially co-expressed with the coxal-2 domain (except the mandible) (see fig.3.9A,B). It is then expressed in the coxa segment, migrating proximally towards the subcoxal-1 domain as the endites develop (see fig.3.9D-F, H,I).
The pleuron is derived from the subcoxal segment of the leg 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 Fig.3.10. The pleuron is subcoxal in origin. By comparing the expression of Tc ser and Tc Dll, in late stage embryos to both larval legs and earlier embryonic legs reveals that four of the Tc ser domains relate to the coxa, trochanter, femur and tibia segments. The proximal Tc ser ring domain relates to a subcoxal segment which forms the pleuron in the larva. Anterior is to the left unless otherwise indicated. Arrows mark the position of the subcoxa segment distal boundary. Stars mark the subcoxa segment in the developing embryonic leg and the pleuron in the larvae. Asterisks mark the coxa. The numbers relate to the distal part of the leg segments as follows: 1) subcoxa 2) coxa 3) trochanter 4) femur 5) tibia. Gene expression was detected by in situ hybridisation. (A) Expression of Tc ser (blue) and Tc Dll (red) in a very late stage Tribolium embryo undergoing dorsal closure. Lateral view. Tc ser is expressed immediately proximal to where the segment boundary forms (white arrow) (B) Tc ser expression in a late stage Tribolium embryo. (C) Cuticle preparation of a first instar larval leg with corresponding leg segments to A and B indicated. (D) SEM of germ band retracting stage embryo. The subcoxal/coxal segment boundary of the leg is clearly visible as a lateral groove at the base of the leg (arrow). (E) Expression of Tc ser (blue) and Tc Dll (red) in germ band retracting stage. Distal is to the top and lateral is to the right. The five Tc ser ring domains are serially positioned along the P/D axis. The proximal limit of Tc Dll expression (arrowhead) is the third ring domain which relates to the trochanter. (F) Expression of Tc ser (blue) and Tc Dll (red) in a very late stage leg shown in A. The proximal limit of Tc Dll expression (arrowhead) is adjacent to the coxa. (G) Tribolium first instar larva showing the relative position of the subcoxa (pleuron) to the rest of the larva. (H) Lateral view of the attachment of the larval leg to the body. The pleuron/subcoxa is easily distinguishable as a separated segment.
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 (see fig.3.10C,D,G,H).
Comparison of the morphology of the larval leg (see fig.3.10C,G-H) to that of the morphology of late stage embryos (see fig.3.10A,F) enables the identity of the Tc ser domains of expression to be related to leg segments with confidence. The coxal-2 domain of Tc ser expression clearly relates to the coxal segment (white arrow in fig.3.10A). The subcoxal-1 domain of Tc ser expression (black arrow in fig.3.10A) and is clearly related to the distal boundary of the developing subcoxal segment of the leg (star in fig.3.10A,B,D-F) that forms part of the pleuron (star in fig.3.10C,G-H).
Study of Tc ser expression, as a marker for the position of future appendage segments and Tc prd expression as a marker for endite development, shows that the mandible is divided into a coxa and subcoxa. The mandibular endite is attached to the coxa, a more distal segment than the proximal subcoxal segment. This result therefore supports Machida’s hypothesis that the embryonic mandible is divided into a subcoxal and coxal segment.
In addition, subcoxal and coxal segments are present in the protopodites of all post-antennal appendages as defined by the expression of Tc ser and the position of expression of Tc prd and the PD domain genes Tc hth, Tc dac and Tc Dll. There are significant similarities between the developing subcoxa and coxa of the mandible and the developing maxilla as defined by the expression of these genes. These similarities presumably indicate serial homology of the subcoxa and coxa segments of the mandible and maxilla as hypothesized by Machida. If this is the case, the ancestral protopodite of the gnathal appendages would have likely originally consisted of three segments: a subcoxa, coxa and basis (using crustacean terminology), with the mandibular gnathal edge located on a more distal segment, the coxa, which is not the most proximal segment as Boxshall and others have hypothesized.
There is strong evidence for the existence of a subcoxal leg segment that forms the pleuron in the larva. This result therefore supports the subcoxal theory of the origin of the pleuron. There are some similarities between the subcoxa and coxa of the gnathal appendages to the subcoxa and coxa of the developing legs. The similarities between the gnathal appendages and the legs may indicate possible homology of the leg subcoxa to the subcoxa of the gnathal appendages, although the similarities are less significant than those observed between the mandible and maxillary appendages.
Mandible limb bud is divided into subcoxa and coxa
The most significant result of this chapter was the discovery of the expression of a ring of Tc ser in the developing mandible. This expression domain is interpreted to represent the distal segment boundary of a subcoxa. The Tribolium mandible is an unsegmented appendage in both the larva and adult like all hexapods. However, a lateral groove is present in the mandibular limb bud in the developing embryo as visualized by SEM and clearly relates to the expression of Tc ser. The expression of Tc ser is in a similar position on the proximal distal axis to the groove visible in the maxillary segment (shown in fig.3.2, fig.3.3).
This lateral groove on the mandible has been detected in other hexapods.
Machida hypothesized that a lateral groove present in the jumping bristletail Pedetontus unimaculatus was evidence of vestigial segment boundary and was serially homologous to the cardo/stipes segment boundary in the maxilla (Machida, 2000). The lateral groove is present in both the cricket Gryllus (Liu et al., 2010) and the sawfly Athalia rosae (Oka et al., 2010). The authors of the study of Athalia define the segment more proximal to this groove as the subcoxa segment. However, there was no genetic evidence for the subdivision of the mandible into subcoxa and coxa.
The ring domain of expression in the mandible could be evidence of an ancestral segment boundary, or some other functional developmental boundary that doesn’t reveal itself through the formation of an actual segment joint. Notch signalling is often involved in demarcating different populations of cells (Bray, 2006). The ring of Tc ser expression could represent some particular boundary of cells, such as those present in the developing endites as the subcoxal ring domain is expressed proximal to all developing endites. This interpretation seems unlikely given the similarities of the subcoxa domain of expression to other segment forming domains. The similarites of the proximal Tc ser domain in the mandible to the maxilla, labial and leg appendages instead suggests that the mandible subcoxa is serially homologous to the subcoxa present in other appendages.
No apparent expression of Tc ser in the gnathal appendage endites
Sewell et al. suggested from evidence in Triops that notch signalling is associated with endites11 (Sewell et al., 2008). There is no evidence of any endite specific Tc ser domains in the mandible, maxilla or labial appendages in Tribolium, and therefore the Notch signalling pathway does not appear to have a role in endite development, at least in Tribolium.
Although, there was no description of the observed expression pattern of any members of the Notch signalling pathway, so it is difficult to make a fair comparison.
Segmental identity of the Tc ser expression domains in the leg.
By comparing the expression of the PD domain genes to Tc ser expression, and comparing the expression of Tc ser to the morphology of the developing legs in late stage embryos it is possible to define the segmental identity of each Tc ser domain.
The largest segment of the larval leg, and the most recognizable, is the coxa. It forms a cone-like shape at the base of the leg. The characteristic morphology of the coxal segment is recognizable in late embryogenesis, as is the morphology of the other segments (compare fig.3.10A-B,F to fig 3.10C,G-H). There is clearly a coxal-2 Tc ser domain associated with the distal part of the coxa next to the coxal-trochanter boundary (white arrow in fig.3.10A).