«Joshua Frederick Coulcher UCL Submitted for the Degree of Doctor of Philosophy September 2011 Declaration I, Joshua Frederick Coulcher, confirm that ...»
Fig.7.6 Post-antennal expression patterns of cnc, Dfd and pb in a hypothetical nonmandibulate ancestor to Mandibulata and a hypothetical ancestral mandibulate arthropod.............................................................. 203
Table A.7 Numbers of female Tribolium pupae injected with concentrations of Tc cnc dsRNA used for collection of embryos for in situ hybridization................... 246 Table A.8 Numbers of female Tribolium pupae injected with Tc Dfd dsRNA........ 247 Table A.9 Additional Tribolium parental RNAi experiments..................... 248
1.1 General introduction The arthropod mandible is an appendage adapted for biting and chewing and is present in three arthropod groups, the insects, crustaceans and myriapods (millipedes and centipedes). The mandibulate arthropods constitute the majority of animals both in terms of species diversity and biomass on this planet. The evolution of the mandible is therefore of particular interest as it is an evolutionary novelty of such an important and diverse group of animals.
There are many different types of mandible, but the characteristic that is shared by most mandibles is the presence of a functional biting edge made up of an incisor and a molar process (see fig.1.1). A study of the fossil record shows that the mandible, along with most appendages, has evolved from a particular type of limb, which is called the biramous limb. The biting edge of the mandible has evolved from the base of the leg (more specifically from a structure called an endite). This fact is most obvious in some types of crustacean mandibles which have a leg-like palp attached to the segment that carries the biting edge (see fig.1.1F).
If the mandible evolved once in the ancestor to the insects, crustaceans and myriapods, it might be expected that there would be some similar genes involved in development that would be shared by these different groups. Understanding what these genes are, how they function and whether they are shared by diverse mandibulates could tell us about mandible evolution: it could tell us the mandible of insects, crustaceans and myriapods had a common origin and it could also tell us about how the mandible has evolved from a leg-like appendage.
The overall aim of my research thesis is to understand the evolution of the mandible by studying its embryonic development from a molecular perspective. An understanding of the development of the mandible is best achieved initially through functional study of patterning genes in a model organism. From this position it is easier to study the role of genes in more diverse taxa that are more difficult to study.
Fig.1.1. The arthropod mandible. The gnathal edge is comprised of an incisor (ip) and molar (mp) process forming the gnathal edge. A movable plate-like structure, the lacinia mobilis (lm) is present on some mandibles. (A-B) Tribolium larva are in possession of a characteristic mandibular gnathal edge (A) Close up of a first instar Tribolium mandible visualized by fluorescence microscopy (B) Close up of a cuticle preparation of a later stage instar Tribolium larval mandible. (C) Cuticle preparation of Tribolium larval head with highlighted mouthpart and labral structures: Labrum (yellow), mandible (blue), maxilla (green), labial appendages (red). (D) SEM of the gnathal edge of a symphylan myriapod Hanseniella (Edgecombe et al., 2003) (E) SEM of the gnathal edge of a peracarid crustacean Gnathophausia gracilis (Richter, 2004). (F) Mandible of Gammarus, an amphipod crustacean, with a telopodite derived palp (palp)(Browne and Patel, 2000).
The majority of research into the function of genes patterning arthropod gnathal appendages has focused on insects that possess highly derived, mouthparts in which the mandible has been converted to a proboscis in the fruitfly Drosophila melanogaster and a stylet in the milkweed bug Oncopeltus fasciatus. There has been no functional study of genes necessary to pattern the mandible in a mandibulate arthropod. To achieve this I therefore chose a model organism, the red flour beetle Tribolium castaneum, which possesses a typical mandible with primitive characteristics. By understanding important mandible patterning genes in this species and comparing them to other taxa across Arthropoda, it is hoped that similarities to other mandibulate arthropods can be discovered to show that the mandible evolved once in the ancestor to the mandibulate arthropods. Also, it is hoped that the manner in which the developmental genes function in Tribolium will provide some clues about how the mandible has evolved, say for example from the starting point of another type of appendage and whether there are structures still present on the mandible which are similar to other parts of other appendages.
To complement embryological, morphological and palaeontological studies, I wanted to investigate how the developing Tribolium mandible is constructed in the embryo by studying the expression of genetic markers. By dividing the developing mandibular lobe into sub-structures (like the molar and incisor processes) and comparing them to similar structures on other appendages, any similarities observed could be suggestive of serial homology. Another important goal was to identify the genes which are necessary to pattern the mandible of Tribolium. By comparing the expression of these genes to their homologs in other mandibulate arthropods and nonmandibulate arthropods, the manner in which the mandible is patterned in Tribolium could demonstrate that the mandible is a homologous structure of mandibulate arthropods and could provide clues as to how the mandible has evolved from another type of appendage, for example from a leg or another type of mouthpart through changes to the genetic pathways that pattern them.
The first half of this introduction will present conclusions from subjects outside of molecular development. The purpose of this is to present ideas that form the basis of many assumptions that have informed the hypotheses and the design of the experiments revealed in later chapters.
Study of fossils of the likely ancestors to the arthropod lineage has shown that all post-antennal arthropod appendages have evolved from a particular type of limb known as a biramous limb. This is a particular limb structure present in the earliest representatives of true arthropods. The evidence for the origin of the biramous limb will be discussed.
As with all arthropod appendages, mandibles vary greatly according to the natural history of the particular species it is attached to; I provide an overview of the diversity of mandibular structures, such as the presence of palps, segmentation, attachment points (condyles) and the gnathal edge will be presented. Despite this diversity, the defining character of the mandible that distinguishes it from other appendages is the gnathal edge which is thought to be a homologous structure in insects, crustaceans and myriapods. The gnathal edge is thought to be derived from the endite of a biramous limb.
The insects, crustaceans and myriapods are hypothesized to form a monophyletic group called Mandibulata. Support for a monophyletic grouping of the arthropods and for grouping the insects with the crustaceans is strong. The position of the myriapods is more problematic and has implications regarding the evolution of the mandible. The phylogenetic evidence both for and against acceptance of Mandibulata will be summarized.
A hypothetical outline of mandible and maxilla evolution from an ancestral two-segmented protopodite will be presented. In this evolutionary scenario, the primitive mandibular gnathal edge is hypothesized to reside on the proximal-most segment of the mandible, the coxa. An alternative hypothesis will also be presented that suggests the primitive mandible possesses a subcoxal segment.
In order to study the developing embryonic mandible, it is necessary to use genetic markers. The second half of this introduction will present relevant background information from molecular development of embryos from diverse arthropod taxa.
One conserved set of genes, the proximal-distal (PD) domain genes, are required to pattern the PD axis of all arthropod appendages and are useful to study the developing PD axis. The Notch signalling pathway is involved in patterning arthropod appendage segment boundaries.
Another class of genes, the Hox genes, are required to pattern segments along the anterior-posterior axis of the body. The conserved expression domains of Hox genes have shown that the homologous segment to the mandibular segment is the first leg segment of chelicerates.
Whilst Drosophila genetics is a useful resource for mining of candidate developmental genes, it is not suitable to study as a model organism for mandible development as it is lacking mandibular appendages. One gene, however - cap’n’collar
- is required to pattern mandibular segment derived structures in Drosophila embryos.
The evidence for this will be summarized.
Tribolium is a useful model organism to study embryonic mandible development as, unlike fruit flies, it possesses the primitive character of the mandible that distinguishes the mandible from other appendages, the gnathal edge. A summary of what is known about the mandible patterning genes in Tribolium will be presented.
Having presented a summary of the themes introduced above, specific questions that will be addressed experimentally in each chapter will be outlined.
1.2 Evolution of the biramous limb
In order to account for diversity of mandibles and other arthropod appendages and make sense of their evolutionary history, it is necessary to study the fossil record.
The study of fossils is important for understanding ancestral character states and how these characters have evolved as it provides examples of appendages that actually existed in ancestral arthropods. Study of Cambrian fossils has shown that there were two types of limbs present in the ancestor to all arthropods which have subsequently evolved into all other types. They are the articulated antenna and, more posteriorly, the biramous limb. These serially homologous biramous limbs were present on each segment posterior to the antenna in numerous arthropods during the Cambrian. It is therefore clear that every post-antennal appendage has evolved from a biramous limb (Boxshall, 2004; Waloszek et al., 2007; Chen, 2009).
The articulated antenna is the most anterior appendage, homologous to the jaw of onychophorans, and is present in trilobites, myriapods and pancrustaceans. In the chelicerate lineage the antennae have probably been modified to form the chelicerae. By contrast the biramous limb is a complex appendage comprised of three parts: the protopodite, exopodite and the telopodite. The term biramous means that the limb has two axis or branches: the telopodite and exopodite. These two branches, or rami, are attached to a proximal structure called the protopodite. Structures called endites are often present on the medial/ventral side of the protopodite (see fig. 1.2D).
The protopodite may also possess unsegmented outgrowths/branches called exites on the dorsal part of the protopodite.
Endites are convex structures that possess spines or bristles that have either a sensory function or are involved in the manipulation and processing of food. If the endite covers the entire medial side of the segment then it is called a gnathobase. The gnathobase is derived from the most proximal endite situated adjacent to a food groove leading to the mouth with a role in feeding. The mandible gnathal edge is derived from an endite. It is not hard to imagine derivation of the mandible from an existing structure on the proximal part of the limb that is already involved in feeding (Boxshall, 2004).
Fig.1.2. Evolution of the biramous limb in the stem lineage of Euarthropoda from a monopodial limb.