«Joshua Frederick Coulcher UCL Submitted for the Degree of Doctor of Philosophy September 2011 Declaration I, Joshua Frederick Coulcher, confirm that ...»
The introduction above has given some background to what is currently known about the evolution of the arthropod mandible. The mandible appendage has evolved from an ancestral biramous limb, which was serially homologous to all other postantennal biramous limbs. The mandible probably evolved through a maxilla-like intermediate. The mandible gnathal edge, the putative synapomorphic character of the Mandibulata, is a modified endite on the protopodite (the base of the ancestral biramous limb). The gnathal edge is hypothesized to be on the proximal segment of the protopodite. This project aims to understand the evolution of the mandible from a leg, through a maxilla-like precursor in the phylum Arthropoda.
I am interested in addressing some specific evolutionary questions regarding the arthropod mandible. Are the mandibular segment patterning mechanisms homologous between different lineages of Mandibulata? Can mandibular appendage sub-structures be homologised to structures present on other appendage types? Does the mandibular patterning mechanism reflect the evolutionary history of the mandible from the protopodite of a biramous limb through a maxilla-like precursor?
In order to answer these questions, it is necessary to understand the molecular development of the mandible in at least one mandibulate arthropod. The primary aim of this research was to investigate mandibular segment and appendage patterning genes in a mandibulate arthropod. This is a necessary first step from which comparisons with other taxa can be made. As a representative of the Mandibulata and as a mandibulate arthropod with a primitive mandible, genes in the red flour beetle Tribolium castanem were studied to investigate their role in patterning the mandible.
In particular the role of the homologue of cnc, which patterns the mandibular segment in Drosophila, was studied by RNA interference (RNAi) which is well characterized technique for Tribolium (Bucher et al., 2002).
In addition, the house spider Achaearanea tepidariorum, a chelicerate, was chosen to study mandible segment patterning genes in an outgroup to the hypothesized clade Mandibulata. From the results obtained from Tribolium and Achaearanea, it is hoped that some conclusions can be made about the conservation of mandibular segment patterning genes in mandibulates and the origin of mandible patterning functions of these genes.
I am interested in addressing several specific questions regarding the development of the arthropod mandible in Tribolium. Two structural features that are present on the developing mandible are the inner and outer lobes. The inner and outer lobes are frequently interpreted to represent respectively the developing molar and incisor processes of the mandibular gnathal edge. The incisor and molar processes have been alternatively hypothesized to be derived from one endite or from two endites.
I wanted to determine whether the outer and inner lobes correspond to the incisor and molar processes and whether these structures are derived from separate endites, or whether they derive from a single endite.
This question was investigated by detailed study of the expression of endite marker genes. The results of my investigations into questions about the mandibular endite are shown in chapter two.
I wanted to test whether the mandible gnathal edge is present on the proximal segment of the protopodite (the coxa), and whether there was any evidence for a division of the developing mandible into a subcoxa and coxa and if I could determine serial homology of these segments to segments of other gnathal appendages. I wanted to determine whether there was molecular evidence for the subcoxal derivation of the pleuron of the leg in order to determine any similarities that could suggest serial homology between mandible and leg appendage segments. The division of the mandible into subcoxa and coxa segments was studied by examining the expression of an appendage segment marker gene, the results of which are shown in chapter three.
cnc is necessary to pattern the mandibular segment of Drosophila and represses Dfd activity in this segment. To determine whether this function is conserved between the fruitfly and the beetle, the role of Tc cnc in patterning the mandibular segment and appendage in Tribolium was examined by RNAi. The effect of Tc cncRNAi on the expression of other genes was examined by in situ hybridisation. The results of these experiments are shown in Chapter four.
Tc Dfd has been shown to pattern the mandible and the base of the maxilla in Tribolium, and is therefore required to pattern the protopodite of these gnathal appendages. In chapter five I investigate this role in more detail by studying the effect of Tc Dfd knock down on other genes such as the PD domain genes, notch signalling pathway and Tc prd in the maxillary appendage which have been shown to be activated by Dfd in Drosophila.
Chapter six will outline research on mandible patterning genes in the nonmandibulate Achaearanea tepidariorum, a member of Chelicerata. This species was chosen for study as an outgroup to the Mandibulata. This chapter will also briefly review what is known about orthologues of mandible patterning genes outside of Mandibulata.
2.1 Introduction The Tribolium mandible is a modified endite attached to a protopodite, the remaining proximal part of the ancestral biramous limb. In this chapter I wanted to study the development of the mandibular endite in order to be able to compare this structure to the endites present on other appendages.
The incisor and molar processes together make up the functional biting edge of the mandible. This gnathal edge is the structure that differentiates the mandible from all other arthropod appendages. The mandibular gnathal edge is considered to be homologous between insects, crustaceans and myriapods (Kraus, 2001; Edgecombe et al., 2003).
The developing embryonic mandibular appendage is a relatively undifferentiated lobe-like structure with few morphological landmarks. Two structural features that are present on the mandibular lobe are the inner and outer lobes. The inner and outer lobes are frequently interpreted to represent the developing molar and incisor processes respectively. It has not been demonstrated whether the mandibular incisor and molar processes derive from the outer and inner lobes respectively. In addition, it has been suggested, but not convincingly demonstrated, that the inner and outer lobe are derived from two separate endites, as opposed to being derived from one endite which is the more conventional interpretation.
Therefore a detailed examination of the expression of genes that are expressed in the mandible endite was undertaken to answer these questions. The expression of appendage patterning genes that are expressed in the endites such as the PD domain gene Tc dac and an endite marker, Tc prd, was studied by in situ hybridization. The expression patterns of these genes was compared and related to morphological features of the developing mandible as determined by scanning electron microscopy.
The inner and outer lobes have been revealed by SEM studies of various hexapods (Machida, 2000; Liu et al., 2010; Oka et al., 2010). The inner and outer lobes are also present in the embryonic mandible of the millipede Glomeris marginata (Prpic and Tautz, 2003).
Expression of genes in the endites The genes that control endite development are not known. As the mandible is a modified endite, these genes are very likely to be important for patterning the mandible. Drosophila does not possess endites in the larval appendages, but has a proximal region of the gnathal lobes that is interpreted to be homologous to the endites of other species (Jurgens et al., 1986). There are several genes which are known to be expressed in developing endites such as prd (Vanario-Alonso et al., 1995;
Aranda et al., 2008), dac (Prpic et al., 2001; Prpic and Tautz, 2003; Ronco et al., 2008;
Sewell et al., 2008) and Dll (Rogers et al., 2002). In addition to the role that notch plays in patterning appendage segments, preliminary results from the crustacean Triops indicate that notch is also involved in defining the endite boundaries in the phyllopodous limb (Sewell et al., 2008). The expression of Tc ser (a component of the notch signalling pathway) will be investigated in chapter 3.
The PD domain genes, Dll, dac and hth are expressed in endites but not in the manner in which they are expressed along the P/D axis of developing appendages.
Therefore endite development does not involve a reiteration of the PD axis (Jockusch et al., 2004). A proximal domain of Dll, distinct from the distal telopodite domain is expressed in some endites. It has been suggested that Dll is involved in sensory organ development, such as chemoreceptors and mechanoreceptors (Mittmann and Scholtz, 2001; Prpic and Tautz, 2003). Dll expression is lacking from the insect mandible endite.
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. The proximal domain of Tc dac expression is strong in the mandible which may be indicative of its importance for mandibular development (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).
dac is expressed in the distal half of endites of the maxilla and labial appendages of numerous insect species (Prpic et al., 2001; Ronco et al., 2008) and the five endites of phyllopodous limbs of Triops and Thamnocephalus (Sewell et al., 2008).
dac is associated with sclerotization in these species, the mandible as has been noted is a highly sclerotized endite (Sewell et al., 2008).
Aim of Chapter This chapter will compare endite development in the mandibular and maxillary appendages of Tribolium by comparing the expression of genes in the mandibular and maxillary endites. The purpose of such an investigation is to determine whether the inner and outer lobes form the incisor and molar processes and whether the inner and outer lobes are formed from separate endites or from a single endite.
The expression of genes such as the PD domain gene Tc dac, the endite marker gene Tc prd and the segment polarity gene Tc wg will be studied in the developing endites. The morphology of the inner and outer lobes of the mandible will be studied by using scanning electron microscopy (SEM). The pars incisiva and pars molaris will be examined by microscopy of cuticle preparations of first instar larvae.
The question of whether the mandible gnathal edge (consisting of the incisor process and molar process) is formed from one or two endites will be investigated by comparing genes that are expressed in the mandibular endite to the maxillary endites.
This question has implications regarding the serial homology of the mandibular endite to the maxillary endites. Machida hypothesized that the maxillary lacinia and galea are serially homologous to the incisor and molar processes respectively (see fig.2.1).
Implicit in his hypothesis is that the incisor and molar processes develop from separate endites on the mandibular limb bud. If the mandible is derived from one endite, this would contradict any hypothesis that sought to homologize the incisor and molar processes to two separate endites.
Fig.2.1. Hypothesis of the serial homology of the mandibular inner and outer lobes to the maxillary lacinia and galea endites after Machida (2000). The inner (yellow) and outer (blue) lobes of the mandible embryonic appendage are hypothesized to be serially homologous to the lacinia and galea by Machida. This hypothesis assumes that the molar and incisor processes are derived from the inner and outer lobes and the molar and incisor processes are derived from separate endites. If the molar and incisor processes are derived from one endite, then they can only be serially homologous to one of the maxillary endites. Figure is adapted from Machida (2000).
2.2 Results Partial cDNA sequences of Tc prd and Tc wg 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. PrpicSchäper.