Aberrant forebrain signaling during early development underlies the generation of holoprosencephaly and coloboma

In this review, we highlight recent literature concerning the signaling mechanisms underlying the development of two neural birth defects, holoprosencephaly and coloboma. Holoprosencephaly, the most common forebrain defect, occurs when the cerebral hemispheres fail to separate and is typically associated with mispatterning of embryonic midline tissue. Coloboma results when the choroid fissure in the eye fails to close. It is clear that Sonic hedgehog (Shh) signaling regulates both forebrain and eye development, with defects in Shh, or components of the Shh signaling cascade leading to the generation of both birth defects. In addition, other intercellular signaling pathways are known factors in the incidence of holoprosencephaly and coloboma. This review will outline recent advances in our understanding of forebrain and eye embryonic pattern formation, with a focus on zebrafish studies of Shh and retinoic acid pathways. Given the clear overlap in the mechanisms that generate both diseases, we propose that holoprosencephaly and coloboma can represent mild and severe aspects of single phenotypic spectrum resulting from aberrant forebrain development. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.     

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

SUMMARY 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.     

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

SUMMARY 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.     

Ontogenetic development underlies population response to mortality

Understanding demographic responses to mortality is crucial to predictive ecology. While classic ecological theory posits reductions in population biomass in response to extrinsic mortality, models containing realistic developmental change predict the potential for counterintuitive increase in stage-specific biomass, i.e. biomass overcompensation. Patterns of biomass overcompensation should be predictable based on differences in the relative energetic efficiencies of juvenile maturation and adult reproduction. Specifically, in populations where reproduction is the limiting process, adult-specific mortality should enhance total reproduction and thus juvenile biomass. We tested this prediction by inducing an array of stage-specific harvesting treatments across replicate populations of Daphnia pulex. In accordance with reproductive regulation, the greatest biomass response occurred in the juvenile Daphnia stage and this response occurred most strongly in response to adult mortality. Nevertheless, we failed to detect significant biomass overcompensation and instead report largely compensatory effects. In total, our work demonstrates that knowledge of population structure is necessary to accurately predict population dynamics, but cautions that further research is needed to illuminate the factors generating over-compensatory versus compensatory responses across natural populations.     

A hydrogen exchange study of the open segment in a DNA double helix

The kinetics of the hydrogen-deuterium exchange reactions of double-helical poly[d(A-T)]·poly[d(A-T)], poly(dA)·poly(dT), and constituent nucleosides (deoxyadenosine and thymidine) have been examined at various temperatures by stopped-flow ultraviolet spectrophotometry, in the spectral region 240–300 nm. The results were interpreted on the basis of a mechanism of the hydrogen exchange reaction of a helical polynucleotide, proposed by Englander and colleagues as well as by the Tsuboi and Nakanishi group. It was concluded that the rates of the base-pair opening reactions are nearly equal to one another in double-helical DNAs, irrespective of the base sequence. On the other hand, the free energy required for bringing the open segment at a particular base-pair was found to be much greater for poly(dA)·poly(dT) than for poly[d(A-T)]· poly[d(A-T)].     

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.     

Influence of forest development phase on centipede diversity in managed beech forests in Slovenia

Biodiversity in managed forests depends on management practices that determine the forest structure. The influence of stand structure on centipede diversity was investigated by comparing centipede communities from different forest development phases. Using the quadrat counts method, soil samples were taken thrice during 2000 (in spring, summer and autumn) from a deforested area and three evenly aged beech stands (juvenile, pole and timber phases) in managed beech forests in Slovenia. The characteristics of centipede communities and the similarities among them were estimated. The collection comprised 1150 centipedes representing 32 species. At a single site, 10–24 species were found. The average density was between 22 and 808 individuals per m². Differences among communities from different sites were found. The highest species number and abundance were found in the juvenile phase and the lowest in the deforested area. The occurrence of species differed among phases, but none of the species found with a high incidence in one development phase was restricted to that phase. The composition of the centipede community was more sensitive to the successional status of the forest than to the season. A comparison with differently structured forest was made, which suggests that homogenisation leads to reduced centipede diversity. To preserve biodiversity and healthy forest functioning, the maintenance of heterogeneity within a forest stand should be an important focus of management operations.     

Variation in body segment number within and between populations of the centipede Strigamia maritima (Chilopoda: Geophilomorpha)

Although most arthropod species have a fixed number of body segments, one order of centipedes – the Geophilomorpha – provides an unusual opportunity to study the variation and microevolution of segment number. This is because all species in all but one family exhibit variation in the number of leg‐bearing segments (LBS) within and between natural populations. One species in particular, the coastal geophilomorph Strigamia maritima, has become a ‘model system’ for these studies, because of its high population densities and the consequent ease of collecting large samples. Previous studies on this species have examined various aspects of segment number variation. However, most studies have characterized each population by an LBS distribution and a mean LBS number that are based on data from all life‐stages. Here, we dissect the variation within as well as between populations and show that different cohorts within a population often have significantly different LBS number distributions. This is almost certainly due to developmental plasticity, probably related to the prevailing microhabitat temperature within brood chambers, but possibly related to other environmental factors too. Although we found no evidence of selection, the fact that different species of geophilomorphs have different LBS distributions suggests that, in the long term, selection may act on the developmental reaction norm of LBS number. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107 , 678–685.     

Transition metals: A double edge sward in ROS generation and signaling

Transition metals such as Iron (Fe) and Copper (Cu) are essential for plant cell development. At the same time, due their capability to generate hydroxyl radicals they can be potentially toxic to plant metabolism. Recent works on hydroxyl-radical activation of ion transporters suggest that hydroxyl radicals generated by transition metals could play an important role in plant growth and adaptation to imbalanced environments. In this mini-review, the relation between transition metals uptake and utilization and oxidative stress-activated ion transport in plant cells is analyzed, and a new model depicting both apoplastic and cytosolic mode of ROS signaling to plasma membrane transporters is suggested.     

Fluctuation periodicity,generation separation,and the expression of larval competition

A suite of models has been formulated to investigate the dynamic consequences of the various routes by which uniform larval competition for food can find demographic expression. It is found that while delayed expression through the vital rates of later age classes gives rise to limit cycles containing multiple overlapping generations, immediate expression via changes in the death or growth rates of the larvae themselves leads to self-sustaining single generation limit cycles. When immediate expression of competition is combined with high adult fecundity and short reproductive lifespan the amplitude of the single generation cycles is so large that they constitute a series of evenly spaced discrete generations, which is maintained indefinitely even in the absence of external cues.     

A measure of DNA periodicity

A Fourier transform g(n) of a sequence of bases along a given stretch of DNA is defined. The transform is invariant to the labelling of the bases and can therefore be used as a measure of periodicity for segments of DNA with differing base content. It can also be conveniently used to search for base periodicities within large DNA data bases.     

Evolutionary trends and patterns in centipede segment number based on a cladistic analysis of Mecistocephalidae (Chilopoda: Geophilomorpha)

Abstract.  Evolutionary changes in segment number during the radiation of Mecistocephalidae, a group of geophilomorph centipedes with segment number usually invariant at the species level, were explored based on a cladistic analysis of forty-six mecistocephalid species, representative of the extant diversity in segment number. The data matrix included 118 morphological characters. Trends were recognized in the evolution of segment number and discussed in relation to the underlying ontogenetic mechanisms of segmentation. The basic trend was towards an increasingly higher number of leg-bearing segments, from (most probably) forty-one to sixty-five (101 in one exceptional case). Changes always involved even sets of segments. Additions of two, four or eight segments usually occurred, but a case of overall duplication of the whole number was also documented. Most changes occurred starting from values belonging to the arithmetical series forty-one, forty-five, forty-nine, whereas the intermediate values forty-three, forty-seven, fifty-one were often evolutionary dead-ends. This evidence suggests a multiplicative mechanism of segmentation involving one or more final run of duplication, as well as a precise control of the final number of segments which produces absolute number stability, except for a single, highly derived species with an exceptionally high number of segments. These ideas contribute to a more general model of arthropod segmentation recently developed by Minelli. A taxonomic revision of mecistocephalids is presented: three subfamilies are proposed (Arrupinae, Dicellophilinae and Mecistocephalinae) and Sundarrup is recognized as a junior synonym of Anarrup .