«Joshua Frederick Coulcher UCL Submitted for the Degree of Doctor of Philosophy September 2011 Declaration I, Joshua Frederick Coulcher, confirm that ...»
With consideration of the above, there were three specific hypotheses that I wanted to test to examine the serial homology of the mandible to other appendage types. Firstly I wanted to determine if the mandible gnathal edge derived from two endites as Machida has suggested, or whether it is derived from one endite. If it was shown that the mandible is derived from one endite, this would contradict Machida’s hypothesis of serial homology between the mandible and maxillary endites. Secondly I wanted to determine if there was any molecular evidence for the division of the insect mandible into a subcoxa and coxa. Evidence of a more proximal mandibular subcoxa could support serial homology of the mandibular subcoxa to the maxillary cardo as Machida has hypothesized. And finally, I wanted to provide molecular evidence for the subcoxal origin of the pleuron which could provide evidence for the serial homology of the leg subcoxa to the gnathal appendages.
The question of appendage segment homology of the gnathal appendages is a difficult problem to resolve with comparative morphology alone. The problem is exacerbated by the lack of certainty regarding the higher level phylogeny of some arthropod groups, and the difficulty in reconstructing the primitive protopodite from the diversity of arthropod appendage forms. By studying genes that are involved in patterning the protopodite, it might become easier to determine serial homology of different segments and endites, and homology of protopodite segments between different arthropod taxa.
1.7 Molecular development of the mandible
If the mandible gnathal edge, and the mandible appendage itself, is a homologous character across Mandibulata, it would suggest that there may be significant similarities between the mandible developmental pathway of diverse taxa of Mandibulata. Hox genes have a conserved role in patterning segments of arthropods. In addition to Hox genes, research in Drosophila, a non-mandibulate arthropod, has shown that another gene cap’n’collar (cnc) functions to modify Hox gene function and is required to pattern the mandibular segment.
The appendages of arthropods have evolved and diversified since the last common ancestor, which possessed identical serially homologous biramous limbs.
Arthropod appendages are present in pairs on individual segments. The segments on which the anterior appendages are found have maintained their identity across arthropod lineages. The evidence for this is from several sources, one of the most powerful is the conserved segmental expression (especially the anterior limit of expression) of a class of genes known as Hox genes.
There are several additional genes that have a conserved role in patterning the arthropod limb which can be used to study the developing mandibular appendage of Tribolium. These are the genes that pattern the proximal-distal axis of the limb (Angelini and Kaufman, 2005) and the notch signalling pathway which has a role in segmenting the limb (Rauskolb, 2001; Prpic and Damen, 2009). As these genes have a conserved role in patterning the arthropod limb, they can be used to study the homology of different limb segments and endites. The relative position of the PD domain genes to the notch signalling pathway, which demarcates appendage segments, may reveal the precise identity of particular segments as they have evolved throughout the Arthropoda and determine whether there are any similarities that are suggestive of serial homology.
PD domain genes
A common mechanism of gene regulation in embryonic development is the division of regions along an axis by genes that are expressed in broadly overlapping domains that activate or repress downstream genes according to their position along the axis. Such a situation occurs in along the Drosophila anterior/posterior (A/P) axis as mediated by both Gap genes and Hox genes but a similar mechanism patterns the Proximo-Distal (PD) axis of the appendages.
Several genes in Drosophila have been discovered that define the PD axis of leg appendages. Four of the most important genes are the transcription factors Distal-less (Dll), dachshund (dac), homothorax (hth) and Extradenticle (Exd). These genes and the role they perform in patterning the proximal-distal axis of the limb has been well studied in the leg imaginal discs of Drosophila (Panganiban et al., 1994; de Celis et al., 1998; Wu and Cohen, 1999; Dong et al., 2001; Rauskolb, 2001; Schram and Koenemann, 2001; Hao et al., 2003; Kojima, 2004).
These four PD domain genes, Dll, dac, hth and Exd, set up five overlapping domains within the PD axis which define particular regions of the limb: 1) hth, 2) hth + dac+ dll, 3) dac, 4) dac + dll, 5) dll. Exd and hth are cofactors that pattern the base of an appendage (Jaw et al., 2000; Prpic and Telford, 2008). Dac patterns the medial portion of an appendage whilst Dll is responsible for both limb outgrowth and patterning the distal tip of an appendage (Rauskolb, 2001; Inoue et al., 2002).
Dll and dac function in a manner analogous to Gap genes, embryos that are mutant for either of these two genes have deletions of tissue that relate to their expression domain, there is no transformation of tissue type. Dll is expressed distally in the tibia, tarsi and pretarsi and in a proximal domain between the trochanter and femur (Gonzalez-Crespo and Morata, 1996). dac is expressed in a medial intermediate domain in the imaginal disc (Mardon et al., 1994; Rauskolb, 2001).
Consistent with these gene expression patterns, hypomorphic Dll mutant flies lack the tibia, tarsi and pretarsi segments (Cohen and Jurgens, 1989; Panganiban, 2000). While flies that are mutant for Dac have the femur, tibia and proximal three tarsus segments fused and condensed but not segments proximal or distal to its domain are unaffected (Mardon et al., 1994).
Two genes that pattern the base of the legs are homothorax and Extradenticle.
Exd and Hth act as a gene pair. Exd requires Hth as a cofactor for nuclear localization in order to become active. Nuclear Exd is referred to as nExd. Hth and Exd do not function like gap genes but rather mutants of these two cofactors results in a failure of the leg segments to form (Abu-Shaar and Mann, 1998; Jaw et al., 2000; Casares and Mann, 2001; Prpic and Telford, 2008).
Comparison of the PD domain genes across Arthropoda
Comparisons of gene expression patterns and of gene function have shown the expression of the PD domain genes to be conserved in leg appendages across Arthropoda in all studied organisms (see fig. 1.10). Homologues of Dll are expressed in the distal parts of all appendages (except the mandible) (Panganiban et al., 1997), dac is expressed in the medial portion. hth and Exd co-expression, which is necessary for their function, is conserved in the proximal part of arthropod appendages (Jaw et al., 2000; Prpic et al., 2003; Angelini and Kaufman, 2005; Prpic and Telford, 2008).
Species that have been studied include the insects: Tribolium castaneum (Beermann et al., 2001; Prpic et al., 2001; Prpic et al., 2003), Oncopeltus fasciatus (Angelini and Kaufman, 2004), Gryllus bimaculatus (Inoue et al., 2002), Acheta domesticus (Abzhanov and Kaufman, 2000b) and Schistocerca americana (Giorgianni and Patel, 2004; Jockusch et al., 2004). Crustaceans that have been studied include Porcellio scaber (Abzhanov and Kaufman, 2000b) and Parhyale hawaiensis (Prpic and Telford, 2008; Liubicich et al., 2009). There has only been analysis of one myriapod, the Diplopod Glomeris marginata (Prpic and Tautz, 2003). Several Chelicerates have been studied and include Steatoda triangulosa (Abzhanov and Kaufman, 2000b), Cupiennius salei (Prpic and Damen, 2004), and Acanthoscurria geniculata (Pechmann and Prpic, 2009).7 Interestingly, PD domain genes have been studied in an onychophoran Euperipatoides kanangrensis that possesses non-segmented lobopodial limbs. The PD domain genes are expressed in a similar proximal-distal order to those of arthropods indicating that the PD axis specifying function of the PD domain genes probably Expression of Dll has been investigated in significantly more diverse taxa in addition to those mentioned above. Dll expression has been investigated in several insects: Manduca Tanaka, K. and Truman, J. W.
(2007) 'Molecular patterning mechanism underlying metamorphosis of the thoracic leg in Manduca sexta', Dev Biol 305(2):
539-50., Precis, Athalia, Thermobia, Lepisma, Folsomia, Xenylla. In several crustaceans Artemia, Mysidopsis, Daphnia, Nebalia, Triops, Sacculina, Thamnocephalus. And also in the chelicerates Achaearanea, Arachae Argiope, Xiphosuran Limulus and an onychophoran Peripatopsis Angelini, D. R. and Kaufman, T. C. (2005) 'Insect appendages and comparative ontogenetics', Dev Biol 286(1): 57-77.
Fig.1.10. Conservation of PD domain gene expression across Panarthropoda. Figure is adapted from Angelini and Kaufman (2005). Expression of the PD domain genes Dll, dac, hth and exd is shown schematically for numerous arthropods and an onychophoran. Dll is expressed in the distal part of each limb. dac is expressed in the medial region. There is more variation of hth and exd expression, however, co-expression of hth and Exd necessary for function is conserved at the base of the limb. References to the expression patterns of these genes in different taxa are indicated in the text.
predates the formation of jointed arthropod appendages (Panganiban et al., 1997;
Janssen et al., 2010).
Wherever examined, the function of the PD domain genes has been shown to be conserved. The homologue of Dll was investigated in Tribolium where there is truncation of leg segments distal of femur in Tc Dll mutants (Beermann et al., 2001). Dll knock down by RNAi in Oncopeltus resulted in deletion of segments distal of femur (Angelini and Kaufman, 2004). In Daphnia magna, RNAi resulted in truncation of the second antenna (Kato et al., 2011).
Outside of mandibulates, Dll function has been examined in the spider Cupiennius (Schoppmeier and Damen, 2001) and Achaearanea (Prpic, personal communication) and the two-spotted spider mite Tetranychus urticae (Khila and Grbic,
2007) where distal appendage regions were missing when the gene’s function was disrupted. dac function has been investigated in Oncopeltus and knockdowns affected the medial region of limbs (Angelini and Kaufman, 2004). hth/Exd function has been investigated in the cricket Gryllus (Mito et al., 2008; Ronco et al., 2008) and the milkweed bug Oncopeltus (Angelini and Kaufman, 2004).
The protopodite-telopodite boundary from the molecular perspective
The protopodite is defined morphologically as the proximal part of the limb to which the telopodite and endopodite attach. It is of interest as to whether the PD domain genes are seen to define the protopodite-telopodite boundary from a molecular perspective, and whether this would agree with morphological analyses.
Gonzalez-Crespo and Morata hypothesised that exd expression related specifically to the protopodite and dll expression to the telopodite (Gonzalez-Crespo and Morata, 1996). Whilst the broad outline of this is true, at different stages of leg development during embryogenesis, the PD domain genes can have different regulatory interactions (Rauskolb, 2001). For example, early in limb development Dll is expressed in cells that will form the protopodite (McKay et al., 2009).
Initial research into the division of the protopodite and telopodite looked at the expression of genes as markers for these fundamental leg divisions. Dll and Dac expression domains were used as markers for the telopodite (Gonzalez-Crespo and Morata, 1996; Estella and Mann, 2008; Estella et al., 2008). There are no obvious markers for the protopodite. nExd is expressed in both the coxa and trochanter. The trochanter is not considered to be part of the protopodite8 and therefore nullifies nExd’s use as a protopodite marker (McKay et al., 2009). In addition, during the earliest stage of leg primordia formation, Hth-nExd are co-expressed with Dll. Later in leg development, Hth-nExd and Dll are mostly mutually exclusive. One region that does co-express Hth-nExd and Dll gives rise to the trochanter (Abu-Shaar and Mann, 1998;
Dll enhancers and the protopodite-telopodite boundary