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Wallace Arthur

Publications and source records attributed to Wallace Arthur.

10 recordsLinked to original sources
PubMed2006

D'Arcy Thompson and the theory of transformations.

D'Arcy Thompson was a biologist, a mathematician and a classicist. His writing was great literature as well as great science. He is primarily known for a single book--On Growth and Form--and indeed for a single chapter within it, on his 'theory of transformations', which shows how the differences between the forms of related species can be represented geometrically. This theory cries out for causal explanation, which is something the great man eschewed. Perhaps the time is close when comparative developmental genetics will be able to provide such an explanation.

Biological Evolution↗
PubMed2005

The centipede Strigamia maritima: what it can tell us about the development and evolution of segmentation.

One of the most fundamental features of the body plan of arthropods is its segmental design. There is considerable variation in segment number among arthropod groups (about 20-fold); yet, paradoxically, the vast majority of arthropod species have a fixed number of segments, thus providing no variation in this character for natural selection to act upon. However, the 1000-species-strong centipede order Geophilomorpha provides an exception to the general rule of intraspecific invariance in segment number. Members of this group, and especially our favourite animal Strigamia maritima, may thus help us to understand the evolution of segment number in arthropods. Evolution must act by modifying the formation of segments during embryogenesis. So, how this developmental process operates, in a variable-segment-number species, is of considerable interest. Strigamia maritima turns out to be a tractable system both at the ecological level of investigating differences in mean segment number between populations and at the molecular level of studying the expression patterns of developmental genes. Here we report the current state of play in our work on this fascinating animal, including our recent finding of a double-segment periodicity in the expression of two Strigamia segmentation genes, and its possible implications for our understanding of arthropod segmentation mechanisms in general.

Animals↗
PubMed2004

A double segment periodicity underlies segment generation in centipede development.

The number of leg-bearing segments in centipedes varies extensively, between 15 and 191, and yet it is always odd. This suggests that segment generation in centipedes involves a stage with double segment periodicity and that evolutionary variation in segment number reflects the generation of these double segmental units. However, previous studies have revealed no trace of this. Here we report the expression of two genes, an odd-skipped related gene (odr1) and a caudal homolog, that serve as markers for early steps of segment formation in the geophilomorph centipede, Strigamia maritima. Dynamic expression of odr1 around the proctodaeum resolves into a series of concentric rings, revealing a pattern of double segment periodicity in overtly unsegmented tissue. Initially, the expression of the caudal homolog mirrors this double segment periodicity, but shortly before engrailed expression and overt segmentation, the intercalation of additional stripes generates a repeat with single segment periodicity. Our results provide the first clues about the causality of the unique and fascinating "all-odd" pattern of variation in centipede segment numbers and have implications for the evolution of the mechanisms of arthropod segmentation.

Animals↗
PubMed2002

The emerging conceptual framework of evolutionary developmental biology.

Over the last twenty years, there has been rapid growth of a new approach to understanding the evolution of organismic form. This evolutionary developmental biology, or 'evo-devo', is focused on the developmental genetic machinery that lies behind embryological phenotypes, which were all that could be studied in the past. Are there any general concepts emerging from this new approach, and if so, how do they impact on the conceptual structure of traditional evolutionary biology? In providing answers to these questions, this review assesses whether evo-devo is merely filling in some missing details, or whether it will cause a large-scale change in our thinking about the evolutionary process.

Anatomy, Comparative↗
PubMedDate not supplied

The pattern of segment formation, as revealed by engrailed expression, in a centipede with a variable number of segments.

Arthropods vary enormously in segment number, from less than 20 to more than 200. This between-species variation must have originated, in evolution, through divergent selection operating in ancestral arthropod species with variable segment numbers. Although most present-day arthropod species are invariant in this respect, some are variable and so can serve as model systems. Here, we describe a study based on one such species, the coastal geophilomorph centipede Strigamia maritima. We investigate the way in which segments are formed using in situ hybridization to demonstrate the expression pattern of the engrailed gene during embryogenesis. We also analyze segment number data in mother-offspring broods and thereby demonstrate a significant heritable component of the variation. We consider how natural selection might act on this intraspecific developmental variation, and we discuss the similarities and differences in segment formation between the geophilomorphs and their phylogenetic sister-group.

Animals↗
PubMedDate not supplied

Early development and segment formation in the centipede, Strigamia maritima (Geophilomorpha).

Geophilomorph centipedes exhibit a number of unique characteristics that make them of particular developmental and evolutionary interest. Segment numbers in geophilomorphs are higher than in any other centipedes, ranging from 27 to 191. They may be constant within a species, presenting in extreme form the "counting" problem in development, or they may vary--a situation that provides us with the opportunity to study naturally occurring variation in segment numbers. All their segments are generated during embryogenesis, a situation unlike that in the more basal centipede orders, which generate only a fraction of their 15 trunk segments in the embryo and develop the rest postembryonically. Here we provide a foundation for further developmental studies of the Geophilomorpha, building on the one study that has been conducted to date, on the coastal species Strigamia maritima. Development begins with the migration of nuclei to the surface of the egg, which then condense to form an embryonic rudiment of more than 20,000 cells, covering an entire hemisphere. During early development, the embryo can be divided into two distinct areas: a large terminal disc of apparently undifferentiated tissue and the germ-band, which has a clear anteroposterior axis and differentiated segments. The germ-band forms from the anterior of the terminal disc and extends anteriorly as the disc contracts. New segments are formed at the posterior margin of the germ-band. Once the process of segmentation ends, the germ-band folds and sinks into the yolk. We note that the classic description of centipede development, by Heymons more than a century ago, contains a fundamental error in the identification of the axes and hence in the interpretation of early segmentation.

Animals↗
PubMedDate not supplied

The effect of development on the direction of evolution: toward a twenty-first century consensus.

One of the most important questions in evolutionary biology is: what orients the evolutionary process? That is, what causes evolution to proceed toward certain developmental trajectories, and hence phenotypes, rather than others? In particular, there has been prolonged controversy over whether the direction of evolution is determined solely by external factors or whether the nature of the ontogenetic process, and the ways in which it can be altered by mutations in developmental genes, may also play a major role. Here, I examine this issue, concentrating on the following: the possible evolutionary orienting role of "developmental bias;" the question of whether selection can and/or will break bias; the extent to which bias is already incorporated in quantitative genetic studies; and ways of approaching the possible role of bias in the origin of evolutionary novelties. Finally, I suggest that developmental bias may provide a focal point for the coming together of conceptual and practical approaches to evo-devo.

Biological Evolution↗
PubMedDate not supplied

How does arthropod segment number evolve?--Some clues from centipedes.

Studies of intraspecific variation in the number of trunk segments of geophilomorph centipedes provide clues as to how different species of arthropods, and whole clades in some cases, come to be characterized by different segment numbers. However, although previous work in this area has revealed an interesting geographical pattern-a latitudinal cline in which segment number decreases with increasing latitude-the causality of the cline remains obscure. Is it because of selection on genetically based variation, or is it a result of a form of phenotypic plasticity in which the segmentation process is directly affected by a latitude-correlated factor such as temperature? Here, we provide some indirect evidence for plasticity. If the cline is indeed a plastic one, a paradox arises, because the cline mirrors interspecific variation-geophilomorph species with more northern ranges typically have fewer segments than those from further south-but interspecific differences cannot arise from nonheritable variation. We propose a resolution of this apparent paradox via a model in which genetic and environmental factors interact through selection acting on developmental reaction norms.

Animals↗