Variation In Number Of Segments Research Articles

An early temperature‐sensitive period for the plasticity of segment number in the centipede Strigamia maritima

Geophilomorph centipedes show variation in segment number (a) between closely related species and (b) within and between populations of the same species. We have previously shown for a Scottish population of the coastal centipede Strigamia maritima that the temperature of embryonic development is one of the factors that affects the segment number of hatchlings, and hence of adults, as these animals grow epimorphically--that is, without postembryonic addition of segments. Here, we show, using temperature-shift experiments, that the main developmental period during which embryos are sensitive to environmental temperature is surprisingly early, during blastoderm formation and before, or very shortly after, the onset of segmentation.

Demonstration of a heritable component of the variation in segment number in the centipede Strigamia maritima

Here we address the question of how arthropod segment number may evolve by reporting the results of further work on the model system Strigamia maritima. Recently, we showed that there was a plastic component of the variation in segment number within this species; now we demonstrate that there is also a heritable component. This is important because it enables a connection to be made between the known latitudinal trend among species of geophilomorph centipedes (more segments at lower latitudes) and the parallel trend within them. This latter trend is best documented in S. maritima but is also known in several other species. However, while a general connection between the inter- and intraspecific trends can now be made, deciding upon a specific hypothesis of the nature of the selection involved is still problematic. We provide two alternative hypotheses, one based on the temperature-related plasticity in segment number being adaptive, the other based on it being nonadaptive.

Temperature‐dependent plasticity of segment number in an arthropod species: the centipede Strigamia maritima

The evolution of arthropod segment number provides us with a paradox, because, whereas there is more than 20-fold variation in this character overall, most classes and orders of arthropods are composed of species that lack any variation in the number of segments. So, what is the origin of the higher-level variation? The centipede order Geophilomorpha is unusual because, with the exception of one of its families, all species exhibit intraspecific variation in segment number. Hence it provides an opportunity to investigate how segment number may change in a microevolutionary context. Here, we show that segment number can be directly altered by an environmental factor (temperature)-this is the first such demonstration for any arthropod. The direction of the effect is such that higher temperature during embryogenesis produces more segments. This potentially explains an intraspecific cline in the species concerned, Strigamia maritima, but it does not explain how such a cline is translated into the parallel interspecific pattern of lower-latitude species having more segments. Given the plastic nature of the intraspecific variation, its link with interspecific differences may lie in selection acting on developmental reaction norms.

Variation in Consonant Cluster Production by Pediatric Cochlear Implant Users

To provide an account of variation in the production of consonant clusters by pediatric cochlear implant users. Productions of consonant clusters by pediatric cochlear implant users were analyzed for variation in number of segments, reduction patterns, and segmental substitutions within the framework of Optimality Theory. A finite set of faithfulness and markedness constraints and variable constraint ranking can account for much of the variation in the production of consonant clusters by pediatric cochlear implant users. Variation in consonant cluster production by pediatric cochlear implant users is not structurally random but subject to the same principles of phonological organization that govern other linguistic systems.

Intraspecific variation in segment number in Pachymerium ferrugineum (C. L. Koch, 1835) (Chilopoda: Geophilomorpha) in the south Aegean Archipelago (north-east Mediterranean, Greece)

Geophilomorph centipedes have numerous leg-bearing segments, always an odd number. Previous research has shown the significant role of latitudinal clines and temperature on intraspecific variation, supporting the view that species of geophilomorphs from cold and temperate regions tend to have fewer segments than those from warmer regions. Here, data are presented on segment numbers of Pachymerium ferrugineum (C. L. Koch, 1835) from the south Aegean Archipelago. Within the framework of a study of the centipede fauna of the south Aegean Islands, three adjacent geographical areas with discrete geological histories were studied (Crete, Cyclades, and Dodecanese). There is no evidence to indicate that a latitudinal trend is present across these three main geographical regions. Apart from Crete, there is no demonstration of a cline along the longitudinal axis of the Aegean. However, the insular characteristics of the central and south-east Cyclades could possibly support higher modal segment numbers. In addition, examination of males and females of P. ferrugineum confirmed the existence of latitudinal clines in certain cases among the islands of Crete, Cyclades, and Dodecanese. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 88, 533–539.

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.

Analysis of segment number and enzyme variation in a centipede reveals a cryptic species, Geophilus easoni sp. nov., and raises questions about speciation

Geophilomorph centipedes provide an unusual opportunity to examine patterns of variation in segment number within extant species. (Most other arthropod species are invariant in this respect.) Recent work on several geophilomorph species has revealed latitudinal clines in segment number, but has generally not called into question the identities of the species themselves. However, analysis of a unique pattern of variation in British populations of Geophilus carpophagus Leach, coupled with other morphological and enzyme data, reveals a cryptic species, Geophilus easoni sp. nov. This species has not yet been detected on the European mainland. Four distinct hypotheses are advanced regarding the speciation event through whichG. easoni arose and the phylogeographic processes that resulted in the present distributions of the two species in Britain. Further study will be necessary to test these hypotheses. In any event, G. carpophagus and G. easoni provide something that has until now been absent: a case study of the genetic differences between closely-related geophilomorph species.

Analysis of segment number and enzyme variation in a centipede reveals a cryptic species, Geophilus easoni sp. nov., and raises questions about speciation

Geophilomorph centipedes provide an unusual opportunity to examine patterns of variation in segment number within extant species. (Most other arthropod species are invariant in this respect.) Recent work on several geophilomorph species has revealed latitudinal clines in segment number, but has generally not called into question the identities of the species themselves. However, analysis of a unique pattern of variation in British populations of Geophilus carpophagus Leach, coupled with other morphological and enzyme data, reveals a cryptic species, Geophilus easoni sp. nov. This species has not yet been detected on the European mainland. Four distinct hypotheses are advanced regarding the speciation event through whichG. easoni arose and the phylogeographic processes that resulted in the present distributions of the two species in Britain. Further study will be necessary to test these hypotheses. In any event, G. carpophagus and G. easoni provide something that has until now been absent: a case study of the genetic differences between closely-related geophilomorph species.

Geographic patterning of variation in segment number in geophilomorph centipedes: clines and speciation.

Since their origin as a metameric group, arthropods have diversified considerably in their number of segments. Present-day geophilomorph centipedes provide a model system for investigating the evolutionary origins of this diversification, because they exhibit intraspecific variation in segment number. (This variation is, however, derived; it is not a plesiomorphic condition within the Chilopoda.) Previous studies have shown that there are significant differences in segment number between populations within several geophilomorph species. In one (arguably two) species, it has been demonstrated that there is a particular form of geographic patterning of the variation, namely a latitudinal cline, with the segment number decreasing with increasing distance north. Here, we provide additional data on four more species, all of which show evidence of a latitudinal cline in either one or both sexes. It is therefore becoming clear that this is a general phenomenon, applying widely (perhaps universally) across the Geophilomorpha, a group consisting of some 1,000 known species. It may be that latitudinal clines are a frequent part of the speciation cycle in this group.

Latitudinal cline in segment number in an arthropod species, Strigamia maritima.

Arthropods vary more than 30-fold in segment number. The evolutionary origins of differences in segment number among species must ultimately lie in intraspecific variation. Yet paradoxically, in most groups of arthropods, the number of segments is fixed for each species and shows no intra- or interpopulation variation at all. Geophilomorph centipedes are an exception to this general rule, and exhibit intraspecific variation in segment number, with differences between individuals being determined during embryonic development and hence independent of population age structure. Significant differences in segment number between different geographical populations of the same species have been previously reported, but insufficient sampling has been conducted to reveal any particular geographical pattern. Here, we reveal a latitudinal cline in segment number in the geophilomorph species Strigamia maritima: segment number in British populations decreases with distance north. This is the first such cline to be reported for any centipede species; indeed as far as we are aware it is the first such cline reported for any arthropod species. In vertebrates, fish are known to exhibit a latitudinal cline in segment number, but interestingly, this is in the opposite direction; fish add segments with increasing latitude, centipedes subtract them.

The Pattern of Variation in Centipede Segment Number as an Example of Developmental Constraint in Evolution

The range of animal morphologies observed in nature is partly determined by natural selection. However, there is no agreement yet regarding whether it is also partly determined by developmental constraint. Testing for the effects of constraint has been difficult due to the lack of both an appropriate null model and a sufficiently simple system capable of yielding unambiguous results regarding the model's plausibility. Here we examine the case of variation in segment number in geophilomorph centipedes. Curiously, while this ranges between 29 and 191, there are no species in which an even number of segments is observed, in contrast to about 1000 species with odd numbers of segments. It seems unlikely that this distribution of character values is determined by selection alone. Using an approach based on Bayesian inference, we attempt to quantify the probability of obtaining the observed distribution of values given a null model in which developmental constraint is absent. Since this probability is in the region of 10−20, we conclude that constraint must be involved. We discuss various implications of this conclusion, and comment on the unexpected absence of neoteny and progenesis in centipede evolution.

The epimorphosis of Strigamia maritima (Leach, 1817) (Chilopoda: Geophilidae)

AbstractThe epimorphic status of geophilomorphs has been questioned several times during the last decade. The variation in number of segments for Strigamia maritima (Leach) from a population in Western Norway is described. The total material shows a significant increase in the number of segments from the foetus to the maturus senior stadium. However, divided into males and females this increase is clearly shown to be caused by a shift from males dominating in the younger stadia to females dominating in the older stadia. The epimorphic status of the geophilids is not violated by our investigation on S. maritima.

Variation and its causation in earthworm regeneration.

AbstractThe coefficient of variation for the number of segments regenerated posteriorly by earthworms (Eisenia foetida) rarely exceeds 10 but the range may be over 100% of the mean. The manner of wound healing after transection (open or enteroparietal vs. closed or cicatricial) can not be responsible for the variation since under the conditions of these experiments all worms healed in the open manner. The angle of transection can be varied widely without any measurable effect on the number of segments regenerated. However, when the cut leaves a piece of ventral body wall and nerve cord extending posteriorly, the mortality is greatly increased and the number of segments formed by the survivors greatly reduced.Three successive regenerations in the same individuals revealed statistically highly significant consistencies indicating a genetic factor behind variation in segment number. Data from wild populations in similar but widely separated sites and also the large number of segments regenerated by individuals with initially very large numbers of segments all support the view that these worms are genetically heterogeneous regarding segment number. However, the effect of these genetic factors cannot be great.Bilateral asymmetry in number of segments regenerated indicates that the counting mechanism underlying variation can hardly be humoral because both blood and coelomic fluid are common to both sides of the body.

VARIATION IN ARENICOLA MARINA (L.) AND THE STATUS OF ARENICOLA GLACIALIS MURDOCH (POLYCHAETA)

Summary. An account is given of ninety‐four anomalous specimens of A. marina, collected from the Firth of Clyde (sixty‐six specimens) and the Firth of Forth (twenty‐eight specimens). More than half of the specimens show anomalies of segment number. The commonest is the presence of an incomplete extra (twentieth) segment at the base of the tail. The highest segment number is twenty with an incomplete twenty‐first; the lowest is eighteen of which the last is incomplete. About 7 per cent of the natural population shows anomalies of segment number. Variation in segment number may be caused in more than one way. It is not generally correlated with other anomalies, but there is a group of worms with eighteen segments and several other peculiarities, suggesting a genetic condition with manifold effects. The collection also includes the following anomalies: vestigial gills on segment vi associated with headward displacement of the hearts through one segment; a nephridium opening on segment iii; a nephridium opening on segment x; absence of the usual hindmost pair of nephridia opening on ix; three oesophageal glands. Fourteen specimens of A. glacialis from Point Barrow, Alaska, were examined. Two had vestigial gills on segment vi with headward displacement of the hearts. The more representative specimens differed from typical A. marina only (a) in lacking segments xviii and xix, and (b) in a forward displacement of the level at which the inner end of the dorsal septal vessel (branchial efferent) moves from the dorsal to the subintestinal vessel. The latter displacement occasionally occurs in A. marina. Several of the anomalous A. marina showed departures from the typical condition that were at least as striking at those of A. glacialis. It is suggested that A. glacialis should be given subspecific status as A. marina glacialis Murdoch. A. marina then includes three subspecies differing in segment number and geographical distribution. These are: A. m. marina (L.) Nineteen trunk segments. North Atlantic, north of about 40°, the neighbouring parts of the Arctic Ocean, Hudson Bay, the northern part of the western Mediterranean; probably also in cold water on the Pacific Coast of North America. A. m. schantarica Zachs. Eighteen trunk segments. Sea of Okhotsk. A. m. glacialis Murdoch. Seventeen trunk segments. Point Barrow, Bernard Harbour (N.W. Territories), Bering Strait. There is evidence that A.m. marina tends to separate into much smaller local populations with some degree of genetic differentiation.