«Joshua Frederick Coulcher UCL Submitted for the Degree of Doctor of Philosophy September 2011 Declaration I, Joshua Frederick Coulcher, confirm that ...»
cnc patterns the mandible by differentiating the mandible from maxillary appendage identity. As the expression of the genes cnc, Dfd and pb are conserved, there is good reason to suspect that the additive patterning role of these genes in the gnathal appendages of Tribolium is conserved in other mandibulates. Dfd is expressed in the protopodite of the mandible and maxillae of numerous species. pb expression is conserved in the maxillary palps of numerous species. Both cnc expression and the repression of Dfd expression from the mandibular limb bud are conserved. This strongly suggests that the ancestral role of Dfd was to pattern the protopodite, the role of pb was to pattern the palp and the role of cnc was to differentiate the mandibular protopodite, in particular, the mandibular endite, from maxillary identity.
With consideration of the evidence outlined above, I present a hypothesis of mandible evolution from a molecular perspective. In the ancestor to all mandibulates Dfd patterns the base of appendages. Dfd expression is conserved in numerous mandibulate arthropods in the mandibular and maxillary protopodites. There is evidence that this condition is primitive for all arthropods, as Dfd is expressed at the base of each monopodial limb in an onychophoran (see fig.7.6A).
In the stem lineage mandibulate lacking a characteristic mandibular gnathal edge, pb patterns the telopodite and Dfd patterns the protopodite of the ancestral second post-antennal limb (the maxilla-like precursor to the mandible). In the ancestor to mandibulate arthropods, the appendages would have likely been serially homologous biramous appendages. There would have been a biramous limb present Fig.7.6 Post-antennal expression patterns of cnc, Dfd and pb in a hypothetical non-mandibulate ancestor to Mandibulata and a hypothetical ancestral mandibulate arthropod. Expression of Dfd in an onychophoran (A) may represent the primitive state present in lobopods, the likely ancestral group to all arthropods. (A) Expression of Dfd in an onychophoran (B-C) Post-antennal limbs are serially homologous biramous limbs. Whether cnc, Dfd or pb is expressed in the exopodite is unknown. (B) hypothetical expression of cnc, Dfd and pb in a hypothetical non-mandibulate ancestor to Mandibulata (C) expression of cnc, Dfd and pb in a hypothetical ancestral mandibulate arthropod which is ancestral to all mandibulate arthropods. The mandibular appendage endite is differentiated from maxillary endite identity by cnc.
on the second antennal segment. Considering the ancestral anterior boundary of pb is likely to be the first post-antennal segment (homologous to the intercalary, second antennal and pedipalp segment) based on expression data from onychophorans and chelicerates, it is not unreasonable to hypothesize that pb may pattern the palp of the second antennal appendage and the mandibular palp (compare to the expression of pb in a chelicerate shown in fig.7.5B). This hypothetical expression pattern in a nonmandibulate ancestor to Mandibulata is shown in fig. 7.6B. The expression of Hox genes in the exopodite derived palp are unknown.
I hypothesize that cnc acquired a new role to pattern the mandibular segment, to differentiate the mandibular endite (and protopodite) from the maxilla to pattern the mandibular gnathal edge in the ancestor to mandibulate arthropods (shown in fig.
7.6C). Based on the results from chapter four, where cnc differentiates the mandible from maxillary identity in part probably by the repression of maxillary patterning Hox genes. In this case, the ancestral mandibular appendage is only differentiated from the maxillary appendage by modification of the maxilla-like proximal endite to form a characteristic mandibular gnathal edge comprised of incisor and molar processes.
The primitive mandible most likely possessed both a telopodite and an exopodite. Therefore the ancestral function of cnc would have repressed the Hox gene Dfd but not pb in the mandible, as pb in this scenario is required to pattern the mandibular palp.
In the insect lineage, cnc has acquired the role of repressing pb to repress palp development. It is possible that cnc has convergently evolved to repress pb in the myriapods which also do not have mandibular palps.
One experiment that could test one aspect of the hypothesis above is to study the expression pattern of the Hox gene pb in arthropods that have mandibles with palps. If it was found that mandibles with palps expressed pb in the mandibular telopodite it would provide some support for this hypothesis. It is important to state however, that to convincingly demonstrate a likely ancestral state (in terms of gene expression and function), numerous diverse taxa have to be studied, and the different expression patterns plotted onto a phylogeny to evaluate whether each expression pattern represents an ancestral or derived character state.
However, the number of taxa sampled, especially within the crustaceans is quite small. The only studied crustaceans have maxillae that are derived in some respects as well, having missing or greatly reduced telopodites. It therefore remains to be seen whether cnc represses the maxilla patterning Hox genes Dfd and pb in other mandibulates and whether the protopodite patterning function of Dfd and the telopodite patterning function of pb are conserved.
7.3 General Conclusions
I sought to understand mandible patterning in a model mandibulate arthropod.
It was found that Tc cnc, like in Drosophila, is important for mandible patterning in Tribolium. The manner in which Tc cnc differentiates the mandible from a maxilla reflects the likely way in which the mandible has evolved in the ancestor to all mandibulates from a maxilla-like precursor that was present in ‘crustaceamorph’ species like Martinssonia. Conserved expression patterns of cnc, Dfd, and pb suggest that the mandibular and maxillary patterning mechanisms are conserved in mandibulate arthropods.
However, primitive mandibles were in possession of both a telopodite and an exopodite. The mandible therefore evolved from a biramous limb (a biramous maxillalike precursor) by modification of the proximal endite to form the incisor and molar processes. Both mandibular palps have been lost in the lineage leading to Tribolium, as in many other taxa. The maxilla to mandible differentiating role of Tc cnc recapitulates the evolutionary history of the appendage but the role of Tc cnc to repress palp development in the mandibular appendage is certainly derived.
There has not been a study of the expression of Hox genes in a crustacean with a mandibular palp. The ancestral mandibular appendage undoubtedly had a palp (and was probably a biramous limb). If cnc differentiated the mandible from the maxilla in the ancestor to all mandibulate arthropods, the ancestral function of cnc would not have been to repress palp development.
As pb expression is conserved in the palps of the maxillae of numerous mandibulates, it would be interesting to see the expression pattern of pb homologues of crustacean species that have mandibles with palps to see if expression of pb is present in the developing mandibular palp telopodite.
As this was the first functional study to examine the mandible patterning genes in a mandibulate arthropod, it is necessary to test function of cnc in more mandibulate arthropod taxa, such as the myriapods and crustaceans, to examine whether cnc has a conserved role in patterning the mandibular segment.
I also sought to compare mandibular structures to other appendages by studying the expression of genetic markers of appendage segments and the PD domain genes in order to evaluate possible serial homology relationships. Significantly, I discovered genetic evidence of a subcoxal mandibular segment. I also gave molecular evidence for the subcoxal leg segment derivation of the pleuron. Both the subcoxa and coxa of the mandible display significant similarities to the subcoxa of other gnathal appendages, and even the leg appendages. To test that the subcoxal segment is primitive, and that these similarities are evidence of serial homology not convergent evolution, it is important to investigate the expression of the PD domain genes and the Notch signalling pathway in more taxa. Particular appendage types that would be of interest to study would include crustacean mandibles with palps, biramous limbs and the segmented diplopod mandible. By examining the expression of the PD domain genes and Notch signalling, it may be possible to determine if the cardo segment of the segmented diplopod mandible is homologous to the subcoxa of the Tribolium mandible. It may also be possible to determine if the mandibular subcoxa is homologous to the maxillary cardo and the subcoxa of the leg.
The attempt to homologize arthropod appendage segments between taxa and serially homologize appendage segments to other appendage segments is a difficult enterprise. However, it may be possible to determine serial homology between appendage segments by comparison of PD domain gene expression and notch signalling pathway expression across numerous arthropod taxa.
It may also be possible to define the protopodite genetically by comparing Notch signalling and PD domain gene expression. The identification of the protopodite in uniramous limbs is more difficult morphologically as the protopodite is defined by the podite to which the telopodite and exopodite are attached, and the exopodite is missing in uniramous limbs. nExd-hth expression was originally used to define the protopodite (Gonzalez-Crespo and Morata, 1996). This was shown to be invalid when it was discovered that hth is co-expressed with Dll in the trochanter and the discovery that hth expressing cells can contribute to the telopodite. Most current analyses exclude the trochanter of the leg from the protopodite. Tc hth expression domain extends to the trochanter in the leg, and the first segment of the palp in the maxilla.
hth is expressed in the first three segments of the uniramous peraeopods (trunk limbs) of Paryhyale (Prpic and Telford, 2008). Maybe nExd-hth expression marks the position of the ancestral protopodite.
The expression of PD domain genes together with the expression of the Notch signalling pathway in biramous limbs may be informative in relating the expression of PD domain genes unambiguously to the protopodite. The protopodite is clearly defined morphologically as where the segments to which the exopodite and the telopodite attach.
In order for genetic expression and functional data to contribute towards understanding of the serial homology of limb segments (such as the putative mandible subcoxa), more data from relevant taxonomic groups are required, such as from sister taxa, particularly those with biramous limbs. There has been little attempt to locate the expression of PD domain genes directly to appendage segments in order to homologize different appendage segments across Arthropoda. By studying the coexpression of PD domain genes and the Notch signalling pathway, this may be possible and may provide new evidence that can resolve long running disputes of segment homology.
Tribolium castaneum culture The strain of Tribolium castaneum cultured was the San Bernadino strain (SB), given courtesy of Gregor Bucher (GGNB, Göttingen). Stock cultures of beetles were reared at 25oC (Sanyo incubator, MIR-253) on organic wholemeal flour (Doves farm) supplemented with 5% brewer’s yeast (MP Biomedicals, 903312). Egg producing Tribolium stocks, pupae producing Tribolium stocks and dsRNA injected Tribolium beetles were reared at 32 oC to accelerate development. Flour was replaced every 2-4 weeks.
Achaearanea tepidariorum culture Spiders were given courtesy of Dr. Angelika Stollewerk (Queen Mary, University of London) and Matthias Pechman (GGNB, Göttingen). Spiders were reared at 25oC in Drosophila culture bottles on coconut husk fibre which was maintained at constant moderate level of humidity. Spiders were fed twice a week to once a fortnight.
Recently hatched spiderlings were fed wild type Drosophila melanogaster. More mature spiderlings were fed Drosophila pseudoobscura. Adult spiders were fed Gryllus assimilis larval instars. Cocoons were produced by introducing male spiders to mate with female spiders. A cocoon containing about 100-150 embryos would be produced roughly every 5 days.
8.2 Molecular biology techniques.