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Graham E Budd

Publications and source records attributed to Graham E Budd.

8 recordsLinked to original sources
PubMed2006

On the origin and evolution of major morphological characters.

Although the mathematical principles underpinning population-level evolution are now well studied, the origin and evolution of morphological novelties has received far less attention. Here, a broad but general theory for how this sort of change takes place is outlined, relying on functional continuity, least-constrained components of morphology, redundancy and preadaptation. At least four distinct sorts of redundancy are identified: (i) redundancy arising through duplication (amplification); (ii) redundancy arising through regionalisation (parcellation); (iii) redundancy arising through functional convergence; and (iv) redundancy arising from shared function (functional degeneracy). Although organisms are here recognised to be functionally constrained ("burdened", in Riedl's terminology), these constraints can be overcome through the combination of the four principles given above. Contrary to its common treatment, functional constraint is neither an ever-increasing restriction on the scope of evolution, nor does it require drastic events to overcome or "break" it. Rather, it is an evolutionary quantity, subject to selection at some level. The rules that govern morphological evolution are the primary controls on what is allowed to happen in the evolution of the overall genotype-phenotype system, suggesting strong controls on the sorts of developmental changes that might be associated with macroevolution. This model, then, sees organism functionality as the primary control on evolvability, with exact genetic make-up being of secondary importance. It should prove possible to recast traditional notions of body-plan evolution into the formulations of complex system analysis, which in the future may prove a unifying discipline for fields as disparate as palaeontology and gene regulatory networks. In particular, understanding how morphology can evolve may provide the critical link between the ecological and morphological networks that are currently the intense focus of evolutionary investigations.

Adaptation, Physiological↗
PubMed2005

Expression of engrailed in the developing brain and appendages of the onychophoran euperipatoides kanangrensis (Reid).

We have cloned an engrailed-class gene in the onychophoran Euperipatoides kanangrensis and investigated its expression using in situ hybridisation. The expression pattern was found to differ drastically from that previously described for another onychophoran species. In the present investigation, engrailed transcripts were detected in a subset of developing neurons in the brain anlage, and in the mesoderm as well as ectoderm of the developing limb buds. The engrailed positive cells of the brain are of differing developmental maturity, ranging from subepidermal neuronal precursors to neurons located basally in the embryo with developing axons. The lack of the traditional expression in the posterior compartment of segments reported earlier in onychophorans is discussed, and we suggest that onychophorans may have acquired two copies of engrailed with different functions.

Amino Acid Sequence↗
PubMed2005

Eggs and embryos in Xenoturbella (phylum uncertain) are not ingested prey.

Xenoturbella is an enigmatic animal that has puzzled science for almost a century. The eggs and embryos found in Xenoturbella have recently been interpreted as ingested prey. However, PCR on individual eggs as well as in situ hybridisation and in situ PCR unambiguously show that they are Xenoturbella's own. The eggs and embryos are individually enclosed within follicles with the same ultrastructure. The cortical granules in oocytes and eggs from Xenoturbella but not Nucula stained positively with an antiserum against Reissner's substance. The embryos incorporated 5-bromodeoxyuridine in vivo, i.e. they replicate their genome and are living.

Animals↗
PubMed2003

Head development in the onychophoran Euperipatoides kanangrensis with particular reference to the central nervous system.

The neuroectoderm of the Euperipatoides kanangrensis embryo becomes distinguishable during germ band formation when the antennal segment is evident externally. During later stages of development, the neuroectoderm proliferates extensively and, at the anterior part of the head, newly-formed neuron precursor cells occupy most of the volume. The antenna forms from the dorsolateral side of the anterior somite. The antenna has no neuroectoderm of its own at the onset of its formation, but instead, neurons migrate out to the appendage from the nearby region of the developing brain. When the antennal tract is formed it is positioned horizontally in the brain, in line with the antennal commissure. Only later, and coincidentally with the anterior repositioning of the antenna, is the tract's distal part bent anteriorly and positioned laterally. The eye starts to develop posteriorly to the antenna and the antennal commissure. This suggests that the segment(s) associated with the onychophoran eye and antenna are not serially homologous with segments carrying equivalent structures within the Euarthropoda. Evidence is presented to further support the presence of a terminal mouth in the ground plan of the Onychophora and, hence, an acron may not exist in the arthropod clade.

Animals↗
PubMed2003

The place of phylogeny and cladistics in Evo-Devo research.

Here we review the various uses to which phylogenetic trees may be put when analysing the evolution of organisms and of the genotypic and phenotypic characteristics of these organisms. We briefly discuss the cladistic method and its application in the inference of phylogenetictrees. Next we consider the uses to which phylogenetic trees can be put: in particular for determining the homology or otherwise of characters distributed on those trees and for estimating the likely characteristics of ancestral taxa. Finally we show the application of this information for deepening our understanding of the processes of evolution. All of these forms of inference are fundamental for comparative biology and of immediate importance to the practice of evolutionary developmental biology.

Animals↗
PubMed2002

A palaeontological solution to the arthropod head problem.

The composition of the arthropod head has been one of the most controversial topics in zoology, with a large number of theories being proposed to account for it over the last century. Although fossils have been recognized as being of potential importance in resolving the issue, a lack of consensus over their systematics has obscured their contribution. Here, I show that a group of previously problematic Cambrian arthropods from the Burgess Shale and Chengjiang faunas form a clade close to crown-group euarthropods, the group containing myriapods, chelicerates, insects and crustaceans. They are characterized by modified or even absent endopods, and two pre-oral appendages. Comparison with reconstructions of the crown-group euarthropod ground plan and recent investigations into onychophorans demonstrates that these two appendages are the first antenna (of extant crustaceans) and a more anterior appendage associated with an ocular segment. The latter appendage has been reduced in all crown-group euarthropods. Its most likely relic is as a component of the labrum. These fossils thus tie together results from disparate living groups (onychophorans and euarthropods).

Animals↗