Nonlinear growth dynamics and the origin of fluctuating asymmetry

Relating Fluctuating Asymmetries and Mean Values and Discordances of Asymmetries in a Set of Morphological Traits

This study addresses the problem of concordance in fluctuating asymmetry (FA) across traits by analyzing the relationship between FAs and the mean values of character measurements in a set of morphological traits. Regression slopes vary in natural populations, thus, revealing discordance in FA across traits among these populations. Hence, commonly accepted techniques for measuring developmental instability with FA result in uncertainties. Here, I relate FA to mean as a two-dimensional complex to demonstrate the uniformly negative slopes of standardized FA vs. mean value regressions for sets of morphological traits from eighteen distinct natural marine and aquatic populations. Comprehensive analysis of the FA–mean complex cannot be recommended for wide use in assessing stress and fitness, but it offers promise to improve FA measuring methodologies and to better understand the nature of developmental instability.

Craniofacial fluctuating asymmetry in gorillas, chimpanzees, and macaques

OBJECTIVES

Craniofacial fluctuating asymmetry (FA) refers to the random deviations from symmetry exhibited across the craniofacial complex and can be used as a measure of developmental instability for organisms with bilateral symmetry. This article addresses the lack of data on craniofacial FA in nonhuman primates by analyzing FA magnitude and variation in chimpanzees, gorillas, and macaques. We offer a preliminary investigation into how FA, as a proxy for developmental instability, varies within and among nonhuman primates.

MATERIALS AND METHODS

We generated 3D surface models of 121 crania from Pan troglodytes troglodytes, Gorilla gorilla gorilla, and Macaca fascicularis fascicularis. Using geometric morphometric techniques, the magnitude of observed FA was calculated and compared for each individual, sex, and taxon, along with the variation of FA across cranial regions and for each bilateral landmark.

RESULTS

Gorillas and macaques exhibited higher and more similar magnitudes of FA to each other than either taxon did to chimpanzees; variation in magnitude of FA followed this same trend. No significant differences were detected between sexes using pooled data across species, but sex did influence FA magnitude within taxa in gorillas. Further, variation in FA variance across cranial regions and by landmark was not distributed in any particular pattern.

CONCLUSION

Possible environmentally induced causes for these patterns of FA magnitude include differences in growth rate and physiological stress experienced during life. Developmental stability may be greatest in chimpanzees in this sample. Additionally, these results point to appropriate landmarks for future FA analyses and may help suggest more urgent candidate taxa for conservation efforts.

Nature, Nurture, and Noise: Developmental Instability, Fluctuating Asymmetry, and the Causes of Phenotypic Variation

Phenotypic variation arises from genetic and environmental variation, as well as random aspects of development. The genetic (nature) and environmental (nurture) components of this variation have been appreciated since at least 1900. The random developmental component (noise) has taken longer for quantitative geneticists to appreciate. Here, I sketch the historical development of the concepts of random developmental noise and developmental instability, and its quantification via fluctuating asymmetry. The unsung pioneers in this story are Hugo DeVries (fluctuating variation, 1909), C. H. Danforth (random variation between monozygotic twins, 1919), and Sewall Wright (random developmental variation in piebald guinea pigs, 1920). The first pioneering study of fluctuating asymmetry, by Sumner and Huestis in 1921, is seldom mentioned, possibly because it failed to connect the observed random asymmetry with random developmental variation. This early work was then synthesized by Boris Astaurov in 1930 and Wilhelm Ludwig in 1932, and then popularized by Drosophila geneticists beginning with Kenneth Mather in 1953. Population phenogeneticists are still trying to understand the origins and behavior of random developmental variation. Some of the developmental noise represents true stochastic behavior of molecules and cells, while some represents deterministic chaos, nonlinear feedback, and symmetry breaking.

Size and shape regulation during larval growth in the lepidopteran Pieris brassicae

By adopting a longitudinal study design and through geometric morphometrics methods, we investigated size and shape regulation in the head capsule during the larval development of the cabbage butterfly Pieris brassicae under laboratory conditions. We found evidence of size regulation by compensatory growth, although not equally effective in all larval stages. Size compensation is not attained through the regulation of developmental timing, but rather through the modulation of per-time growth rate. As for the shape, neither the variance of the symmetric component of shape, nor the level of fluctuating asymmetry show any evidence of increase across stages, either at the population or individual level, which is interpreted as a mark of ontogenetic shape regulation. In addition, also the geometry of individual asymmetry is basically conserved across stages. While providing specific documentation on the ontogeny of size and shape variation in this insect, this study may contribute to a more general understanding of developmental regulation and its influence on phenotypic evolution.

Filter feeding, deviations from bilateral symmetry, developmental noise, and heterochrony of hemichordate and cephalochordate gills

We measured gill slit fluctuating asymmetry (FA), a measure of developmental noise, in adults of three invertebrate deuterostomes with different feeding modes: the cephalochordate Branchiostoma floridae (an obligate filter feeder), the enteropneusts Protoglossus graveolens (a facultative filter feeder/deposit feeder) and Saccoglossus bromophenolosus (a deposit feeder). FA was substantially and significantly low in B. floridae and P. graveolens and high in S. bromophenolosus. Our results suggest that the gills of species that have experienced a relaxation of the filter feeding trait exhibit elevated FA. We found that the timing of development of the secondary collagenous gill bars, compared to the primary gill bars, was highly variable in P. graveolens but not the other two species, demonstrating an independence of gill FA from gill bar heterochrony. We also discovered the occasional ectopic expression of a second set of paired gills posterior to the first set of gills in the enteropneusts and that these were more common in S. bromophenolosus. Moreover, our finding that gill slits in enteropneusts exhibit bilateral symmetry suggests that the left-sidedness of larval cephalochordate gills, and the directional asymmetry of Cambrian stylophoran echinoderm fossil gills, evolved independently from a bilaterally symmetrical ancestor.

North Atlantic Oscillation drives the annual occurrence of an isolated, peripheral population of the brown seaweed Fucus guiryi in the Western Mediterranean Sea

The canopy-forming, intertidal brown (Phaeophyceae) seaweed Fucus guiryi is distributed along the cold-temperate and warm-temperate coasts of Europe and North Africa. Curiously, an isolated population develops at Punta Calaburras (Alboran Sea, Western Mediterranean) but thalli are not present in midsummer every year, unlike the closest (ca. 80 km), perennial populations at the Strait of Gibraltar. The persistence of the alga at Punta Calaburras could be due to the growth of resilient, microscopic stages as well as the arrival of few–celled stages originating from neighbouring localities, and transported by the permanent Atlantic Jet flowing from the Atlantic Ocean into the Mediterranean. A twenty-six year time series (from 1990 to 2015) of midsummer occurrence of F. guiryi thalli at Punta Calaburras has been analysed by correlating with oceanographic (sea surface temperature, an estimator of the Atlantic Jet power) and climatic factors (air temperature, rainfall, and North Atlantic Oscillation –NAO-, and Arctic Oscillation –AO- indexes). The midsummer occurrence of thalli clustered from 1990–1994 and 1999–2004, with sporadic occurrences in 2006 and 2011. Binary logistic regression showed that the occurrence of thalli at Punta Calaburras in midsummer is favoured under positive NAO index from April to June. It has been hypothesized that isolated population of F. guiryi should show greater stress than their congeners of permanent populations, and to this end, two approaches were used to evaluate stress: one based on the integrated response during ontogeny (developmental instability, based on measurements of the fractal branching pattern of algal thalli) and another based on the photosynthetic response. Although significant differences were detected in photosynthetic quantum yield and water loss under emersion conditions, with thalli from Punta Calaburras being more affected by emersion than those from Tarifa, the developmental instability showed that the population from Tarifa suffers higher stress during ontogeny than that from Punta Calaburras. In conclusion, this study demonstrates the teleconnection between atmospheric oscillations and survival and proliferation of marine macroalgae.

The ontogeny of developmental buffering in lizard head shape

Canalization and developmental stability (DS) are important organismal properties involved in determining the level of phenotypic variation. Ontogenetic patterns of phenotypic variance components can shed light on the mechanistic basis of developmental buffering (DB). Here, we analyze how individual FA and among-individual variation in head shape change in ontogenetic series of three lizard species raised in laboratory. The degree of asymmetry increased slightly with size, suggesting that developmental mechanisms hypothesized to correct for deviations either do not exist, or that their efficiency is truncated with increasing size. Alternatively, they may need the disturbance as a trigger. The relationship between asymmetry and age was complex, with asymmetry being stable across the age range in two species but increased with age in the third. Lack of congruence in ontogenetic patterns of asymmetry might be due to intrinsic differences in buffering mechanisms or a result of species-specific growth patterns. Head shape was shown to be equally canalized across both size and age range in all species, probably as a result of a balance between the buffering mechanisms and mechanisms generating variance. The patterns of symmetric and asymmetric head shape variation were highly correlated across species meaning that DS and canalization may rely on similar mechanisms.

GENETICS OF FLUCTUATING ASYMMETRY: A DEVELOPMENTAL MODEL OF DEVELOPMENTAL INSTABILITY

Although numerous studies have found that fluctuating asymmetry (FA) can have a heritable component, the genetic and developmental basis of FA is poorly understood. We used a developmental model of a trait, according to a diffusion-threshold process, whose parameters are under genetic control. We added a small amount of random variation to the parameter values of this model to simulate developmental noise. As a result of the nonlinearity of the model, different genotypes differed in their sensitivity to developmental noise, even though the noise is completely random and independent of the genotype. The heritable component of FA can thus be understood as genetically modulated expression of variation that is itself entirely nongenetic. The loci responsible for this genetic variation of FA are the same that affect the left/right mean of the trait, showing that genetic variation for FA does not require genes that specifically control FA. Furthermore, the model offers alternative explanations for phenomena widely discussed in the literature on FA, for instance, the correlations between FA and heterozygosity and between FA and trait size. The model underscores the importance of dominance and epistasis, and therefore unites the study of FA with the classical theory of quantitative genetics.

FLUCTUATING ASYMMETRY IN A SALIX HYBRID SYSTEM: THE IMPORTANCE OF GENETIC VERSUS ENVIRONMENTAL CAUSES

To examine the effects of hybridization and environmental stress on developmental instability, we examined fluctuating asymmetry (FA), the variance in random deviations from perfect symmetry in bilaterally symmetrical traits, for leaf symmetry in a Salix hybrid system. An abiotic environmental stress (water stress), an interspecific biotic stress (pathogen attack), and an intraspecific biotic stress (competition) were examined to determine which factors increase developmental instability. None of these three environmental stressors significantly increased FA. However, genetic stress through hybridization was detected; hybrid plants showed significantly higher levels of FA than parental species. In contrast to hybridization providing greater developmental stability through heterozygosity, these results suggest that complex, nonadditive interactions provided developmental stability and that developmental instability increased when coadapted gene complexes were disrupted through hybridization. In addition, plant biomass was significantly, negatively correlated with FA, suggesting that those individuals that were more able to buffer themselves against the disruptive effects of environmental stress may have a selective advantage over those that are less able to buffer themselves against these disruptive effects.

Climatic Conditions and Herbivory Effects on Morphological Plasticity of Argania spinosa

The main objective of this paper was to look into the morphological differentiation patterns and phenotypic plasticity in four populations of Argania spinosa with environmentally contrasted conditions. Mean response, magnitude and pattern of morphological intra- and inter-population plasticity indexes were measured and analyzed in order to identify which characters contribute the most to the acclimation of this species. Populations growing in the ecological optimum of the species presented the lowest plasticity, while those growing in the most stressed habitats showed an increased morphological variability. The study of four populations showed that human pressure seems to play an important function in the regulation of morphological characters. However, climatic conditions seem to play a significant role in the increase of morphological plasticity.

How accurate is the phenotype? - an analysis of developmental noise in a cotton aphid clone

Background

The accuracy by which phenotype can be reproduced by genotype potentially is important in determining the stability, environmental sensitivity, and evolvability of morphology and other phenotypic traits. Because two sides of an individual represent independent development of the phenotype under identical genetic and environmental conditions, average body asymmetry (or "fluctuating asymmetry") can estimate the developmental instability of the population. The component of developmental instability not explained by intrapopulational differences in gene or environment (or their interaction) can be further defined as internal developmental noise. Surprisingly, developmental noise remains largely unexplored despite its potential influence on our interpretations of developmental stability, canalization, and evolvability. Proponents of fluctuating asymmetry as a bioindicator of environmental or genetic stress, often make the assumption that developmental noise is minimal and, therefore, that phenotype can respond sensitively to the environment. However, biologists still have not measured whether developmental noise actually comprises a significant fraction of the overall environmental response of fluctuating asymmetry observed within a population.

Results

In a morphometric study designed to partition developmental noise from fluctuating asymmetry in the wing morphology of a monoclonal culture of cotton aphid, Aphis gossipyii, it was discovered that fluctuating asymmetry in the aphid wing was nearly four times higher than in other insect species. Also, developmental noise comprised a surprisingly large fraction (≈ 50%) of the overall response of fluctuating asymmetry to a controlled graded temperature environment. Fluctuating asymmetry also correlated negatively with temperature, indicating that environmentally-stimulated changes in developmental instability are mediated mostly by changes in the development time of individuals.

Conclusion

The amount of developmental noise revealed in this trait potentially does interfere with a substantial amount of the sensitivity of fluctuating asymmetry to change in temperature. Assuming that some genetic-based variation in individual buffering of developmental instability exists in natural aphid populations, the amount of internal developmental noise determined in this study could also substantially reduce evolvability of the aphid wing. The overall findings here suggest that individual response to the seemingly high cost of stabilizing some aspects of the phenotype may account for the frequent observation of trait and species specificity in levels of fluctuating asymmetry.

Microcontaminant accumulation, physiological condition and bilateral asymmetry in zebra mussels (Dreissena polymorpha) from clean and contaminated surface waters

Chemical and biological monitoring of pollution in the aquatic environment is essential to assess the quality of surface waters. Zebra mussels (Dreissena polymorpha) have been used extensively to monitor pollution in freshwater environments, especially in bioaccumulation studies, whereby pollutant levels in tissues have been used as a measure of exposure. However, there is a need for good biomarkers that reflect the impact of exposure to pollutants. Bilateral asymmetry, commonly used as a measure of developmental instability, has a high potential as a biomarker to monitor stress caused by pollution. Nevertheless, until recently, no studies have evaluated bilateral asymmetry as a biomarker in zebra mussels. Biomarkers related to the energy metabolism may give a good indication of the physiological cost of exposure to pollution. In this study, we investigated whether the physiological condition (energy reserves and condition indices) and bilateral asymmetry of shells of zebra mussels are potentially useful biomarkers to monitor the impact of micropollution, such as trace metals, polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and di(p-chlorophenyl) dichloroethylene (p,p'-DDE) in the freshwater environment. Bilateral asymmetry of the zebra mussel shells was examined with respect to levels of pollutants accumulated in the mussels and compared to the physiological condition of the mussels. Levels of PCBs and several trace metals (especially Cd, Cu and Zn) were very high in four of the six sampling locations and in some locations the physiological condition of the mussels was significantly depressed. Nevertheless we did not find any relation (on individual or population level) with bilateral asymmetry of zebra mussel shells. Therefore our results suggest that bilateral asymmetry of zebra mussel shells is not a good measure for the impact of pollution in freshwater ecosystems. The energy reserves and condition indices, on the other hand, gave a valuable indication of the physiological condition of zebra mussels and are useful to monitor the impact of pollution if physiological and environmental factors are taken into account.

Inbreeding reduces power-law scaling in the distribution of fluctuating asymmetry: an explanation of the basis of developmental instability

The study of fluctuating asymmetry has been controversial because of conflicting results found in much of the primary literature. It has been suggested that the source of this conflict is the fact that the basis of fluctuating asymmetry is poorly understood and that, as a consequence, methodology of fluctuating asymmetry studies may be flawed. A new model for the phenomenological basis of fluctuating asymmetry, that variation in fluctuating asymmetry is in large part due to the random exponential growth of cell populations (geometric Brownian motion) that are terminated randomly around a genetically programmed development time, is presented here. If termination of development has a genetic component, then scaling effects and kurtosis in the distribution of fluctuating asymmetry should increase with genetic redundancy of the population. This model prediction was tested by comparing the distribution of multivariate size and shape fluctuating asymmetry in large samples collected from both wild populations and four moderately inbred lines of Drosophila simulans. It was found that while wild populations were best described by a lognormal distribution with power-law scaled tails, the inbred lines derived from the wild stock were dramatically normalized (half-normal) in three of four cases. As predicted, the scaling exponent of the upper tail of the distribution of fluctuating asymmetry increased with inbreeding while the kurtosis and mean fluctuating asymmetry decreased with inbreeding. The model suggests an additional explanation of leptokurtosis in fluctuating asymmetry. Kurtosis and scaling of the statistical distribution of fluctuating asymmetry in a population is related directly to genetic differences between individuals and these differences affect their ability to buffer the process of development against random perturbations.

Patterns of asymmetry in the twining vine Dalechampia scandens (Euphorbiaceae): ontogenetic and hierarchical perspectives

We studied patterns of fluctuating asymmetry (FA) in leaves of four populations of the neotropical vine Dalechampia scandens to obtain insight into the origin of leaf FA and the level at which it is controlled. We analysed correlations in signed and unsigned asymmetry at different organizational levels. We also analysed the ontogeny of FA during leaf expansion to test whether asymmetry is regulated during cell expansion, and whether fast-expanding leaves are more or less asymmetrical. Signed asymmetry was negatively correlated between successive leaves, that is, when the right side of a leaf was larger than the left side, the next leaf on the shoot tended to show the opposite pattern. The magnitude of FA, however, was very weakly correlated among successive leaves or among leaves measured on different shoots. The direction of asymmetry did not change during leaf expansion, but the relative asymmetry, that is, asymmetry corrected for difference in trait size, decreased during expansion. We found a weak negative relationship between leaf expansion rate and relative asymmetry on the fully expanded leaves. These results suggest that leaf asymmetry in Dalechampia originates from perturbations in cell proliferation in the stem, creating asymmetries in opposite directions in successive leaves. These asymmetries persist during leaf expansion, but tend to be reduced by unknown mechanisms.

Assessing developmental errors in branchiostegal rays as indicators of chronic stress in two species of Pacific salmon

Stress during ontogeny reduces homeostasis, increasing the formation of developmental errors. Fused and partial branchiostegal rays were assessed as indicators of stress throughout embryogenesis in coho salmon (Oncorhy nchus kisutch Walbaum) and chinook salmon (Oncorhynchus tshawytscha Walbaum). Error frequencies and the proportion of fish possessing them should increase when development is stressed. Coho parr were examined from reciprocal crosses between two hatchery stocks experiencing artificially fluctuated (between 7 and 12 °C) or ambient (natural) temperatures throughout embryogenesis. Temperature means and ranges were equivalent between treatments, allowing patterns of thermal variance to be compared. Chinook presmolts, having high versus low levels of infection with bacterial kidney disease owing to similar levels of parental infection, also were examined. Sampled fish were cleared and stained and the errors analyzed using categorical linear models and observations of positional distributions. Although partial rays in coho were genetically influenced, fusion frequency increased significantly under fluctuating (551) versus ambient temperatures (163), as did the number of fish with fusions (207 versus 104, respectively). No significant difference between bacterial kidney disease groups was observed, indicating the disease had little influence over error development. Positional distributions of fused rays were bimodal in both species, showing anterior and posterior zones of weaker developmental control. Partial rays occurred at initial and terminal positions in the series, suggesting evolutionary vestigialization in both species. Branchiostegal ray fusions are shown to be useful traits for further investigation of thermal stress during embryogenesis in salmon.

Translational asymmetry as a sensitive indicator of cadmium stress in plants: a laboratory test with wild-type and mutant Arabidopsis thaliana

• Translational and bilateral asymmetry have been proposed as sensitive measures of stress in plants, but few studies have addressed the asymmetry-stress association for individuals grown under strictly defined conditions. Here, we assess the impact of cadmium (Cd) stress on various asymmetry measures in a wild-type and mutant strain of Arabidopsis thaliana. • Fitness measures (fresh weight, pod count and shoot length) and developmental stability (DS) measures (bilateral asymmetry and translational asymmetry (TA)) were compared between plants grown under different cadmium concentrations. • Cadmium stress sharply increased TA in both strains but had inconsistent effects on bilateral asymmetry. The TA effects were detected at a Cd concentration when effects on growth and reproduction were not yet evident. • Translational asymmetry, but not bilateral asymmetry, may therefore act as a sensitive indicator of cadmium stress and could be used to assess soil contamination in transplanted A. thaliana.

The ontogeny of fluctuating asymmetry

We tested seven hypotheses regarding the mechanisms by which fluctuating asymmetry (FA) originates. We did this by analyzing data on four bilateral characters measured repeatedly during the development of individual domestic fowl. Immediately posthatching, there was substantial directional asymmetry, which rapidly decreased. We detected FA at significant levels in all characters in the majority of our measurements over the remainder of development. We also examined the effects of known environmental stressors (food and density stress) on levels of FA. At the levels we examined, changes in these stressors did not alter the degree of asymmetry we found in fowl. Time series of asymmetry for individuals did not exhibit regular oscillations, as much of the relevant literature predicts. Asymmetry levels reflected the combined effects of developmental noise, which was random in degree and direction, and feedback processes, which decreased asymmetry by altering growth rates on both sides of the body. Our findings best fit the predictions of the residual asymmetry and compensatory growth hypotheses, which suggest that levels of asymmetry reflect only recent growth history.

Ontogeny of individual asymmetries in several traits of larvalChironomus ripariusMeigen, 1804 (Diptera, Chironomidae)

In recent years considerable effort has been dedicated to the analysis of alterations in the morphology of individuals, mainly fluctuating asymmetry, as bioindicators of stress. However, inconsistent results and a general lack of knowledge about the processes involved in the generation and development of asymmetries have discouraged some researchers from using this tool. We present a study of the ontogeny of individual asymmetries in several traits of Chironomus riparius larvae. Development of asymmetries through consecutive instars showed no evidence of sidedness and was in accordance with a compensatory growth model, since growth of the smaller side was typically greater than growth of the larger side during one moult. Surprisingly, however, although growth was compensatory, right–left (R-L) values were size-dependent and increased throughout larval development. Once R-L values were corrected for size dependence, asymmetry levels proved to be similar in all instars except for instar I in one case. We discuss our findings in relation to the developmental patterns and regulation mechanisms described and proposed in similar studies. Specifically, the coincidence of our results with those of another study on insects may indicate the existence of similar regulatory systems throughout this group of animals.

Visualizing patterns of craniofacial shape variation in Homo sapiens

The geometric morphometric analysis of shape variation in complex biological structures such as the human skull poses a number of specific challenges: the registration of homologous morphologies, the treatment of bilateral symmetry, the graphical representation of form variability in three dimensions and the interpretation of the results in terms of differential growth processes. To visualize complex patterns of shape change, we propose an alternative to classical Cartesian deformation grids in the style of D'Arcy W. Thompson. Reference to the surface structures of the organism under investigation permits a comprehensive visual grasp of shape change and its tentative interpretation in terms of differential growth. The application of this method to the analysis of human craniofacial shape variation reveals distinct modes of growth and development of the neurocranial and viscerocranial regions of the skull. Our data further indicate that variations in the orientation of the viscerocranium relative to the neurocranium impinge on the shapes of the face and the cranial vault.

'Vertition' of integumental organs in mites revisited: a case of fluctuating asymmetry

Vertition in mites is defined as a meristic variation for a bilateral integumental organ with a separate genetic control for each body side. A prominent hypothesis expressed by Grandjean is the role of vertition in the evolutionary trend towards a reduced number of hair-like organs (mechano- and/or chemo-receptors) known to have occurred in many mite groups. Observations on leg setae in the two-spotted spider mite Tetranychus urticae do not support this hypothesis. Meristic variation for leg setae rather conforms to the notion of fluctuating asymmetry: the difference between the number of leg setae on the right and left sides of the body had a unimodal distribution with a mean of zero. Moreover, lack of heritability for left/right absences of leg setae in an inbred laboratory strain suggests that vertition could be purely environmental. It is therefore argued that meristic variation for hair-like organs in mites is caused by random developmental accidents not corrected by homeostatic mechanisms normally resulting in a perfect bilateral symmetry.

Idiopathic scoliosis and spondylolysis in the female athlete. Tips for treatment

Idiopathic scoliosis and spondylolysis can be common back problems in female athletes. Diagnosis and treatment can be difficult. With the notable trend toward increasing participation of women and girls in organized sports, it is necessary to know which sports carry additional risks for participants to have these two conditions develop and to determine treatment modalities. In general, idiopathic scoliosis is more prevalent in females and even may be higher in the athletes. Treatment options may include observation, the use of a brace, and surgery. In determining treatment, the type of sport and caliber of athlete must be considered in conjunction with the severity of the curve. Spondylolysis or a stress fracture of the posterior vertebral elements can be a common cause of back pain in an athlete. In many sports that are dominated by females (gymnastics, dancing, figure skating), the athletes carry a high risk of having spondylolysis or a stress fracture. Knowing the risk factors permits precise diagnosis and appropriate treatment. Treatment options include the use of a brace and surgery. In the current study, an extensive review of the literature in conjunction with the extensive experience of a well-established sports medicine clinic at the authors' institution is presented.

Developmental Instability as a Means of Assessing Stress in Plants: A Case Study Using Electromagnetic Fields and Soybeans

Developmental instability is often assessed using deviations from perfect bilateral symmetry. Here, we review the literature describing previous studies, suggest mechanisms that may account for both the generation and disruption of bilateral symmetry, and examine the influence of electromagnetic fields on the asymmetry of soybean leaves. Leaves from plants under high-voltage power lines generating pulsed magnetic fields of <3 to >50 mG were more asymmetrical for two parameters (the terminal leaflet widths and lateral rachilla lengths) than leaves of plants even 50 or 100 m away from power lines. This asymmetry could not be attributed to either size scaling or measurement error.

Scoliosis and developmental theory: adolescent idiopathic scoliosis

STUDY DESIGN

Statistical analysis of maturity and asymmetry criteria in adolescent idiopathic scoliosis.

OBJECTIVES

To explore the hypothesis that scoliosis is a manifestation of developmental destabilization under physiologic stress.

BACKGROUND

Advances in genetics and theoretical biology have broadened the understanding of morphogenesis and the controlling mechanisms for the development of the adult form. The morphologic genome can be viewed as a cybernetic control system, and the homeobox genes can be viewed as the master controls for the specific subroutines. Deformity will occur from a faulty "program" or a disturbance in the running of that program. An early and subtle indication of such an occurrence is failure of bilateral symmetry.

METHODS

An analysis of variance in Cobb angle, age at diagnosis, and apical site in 327 girls with spinal curves 5 degrees or greater according to prospectively maintained scoliosis screening records was performed. Dermatoglyphics were compared in 114 female control individuals and 164 female patients with adolescent idiopathic scoliosis (minimum Cobb angle, 10 degrees), and then by subdivision into school screening (n = 86) and general clinic referrals (n = 78).

RESULTS

Girls from the screening program (from the years 1979-1990) had statistically significant associations between age and apical vertebra, suggesting an age-deformity relationship independent of maturation and growth spurt. Girls with adolescent idiopathic scoliosis from a later cohort, both screened and unscreened, had increased directional asymmetry, as previously reported. There was a statistically significant increase in fluctuating asymmetry in general clinic referrals when compared with school screening referrals.

CONCLUSIONS

Developmental instability can explain adolescent idiopathic scoliosis as part of wider developmental theory without the necessity of a disease process in the etiology. The differences between screening and general clinic referrals suggest the need for natural history studies, and treatment protocols also should consider the provenance of the individuals described.

On the ontogeny of developmental stability in a stabilized trait

Developmental stability is most accurately assessed by measuring the small asymmetries between left and right elements of bilateral traits, i.e. fluctuating asymmetry. Although there has been much recent interest in fluctuating asymmetry, as low asymmetry may reflect high fitness, relatively little is known concerning the developmental origins of these minor discrepancies. Understanding the mechanisms that determine fluctuating asymmetry is crucial to interpreting much of the recent literature, for example, it has often been claimed that asymmetry reflects properties of an individuals genome. Therefore, in this study, we have examined the ontogeny of fluctuating asymmetry in the primary feathers of European starlings (Sturnus vulgaris ) and compared our data with six published hypotheses of the mechanisms of asymmetry development. We found that signed asymmetries were not consistently biased toward either the left or the right side among feathers on the same individual; growth increments (measured every two days) were also not sided; and both absolute and relative asymmetry decrease as the feathers develop. These data are most consistent with a developmental regulatory system that involves some elements of feedback between left and right sides and episodes of compensational growth to correct large asymmetries. We discuss the possible differences in developmental mechanisms that determine asymmetry in traits of varying functional importance.

Epigenetic control of sexual phenotype in a dioecious plant, Melandrium album

Melandrium album (syn. Silene latifolia) is a model dioecious species in which the Y chromosome, present only in heterogametic males, plays both a male-determining and a strict female-suppressing role. We showed that treatment with 5-azacytidine (5-azaC) induces a sex change to androhermaphroditism (an-dromonoecy) in about 21% of male plants, while no apparent phenotypic effect was observed in females. All of these bisexual androhermaphrodites (with the standard male 24, AA + XY karyotype) were mosaics possessing both male and hermaphrodite flowers and, moreover, the hermaphrodite flowers displayed various degrees of gynoecium development and seed setting. Southern hybridization analysis with a repetitive DNA probe showed that the 5-azacytidine-treated plants were significantly hypomethylated in CG doubles, but only to a minor degree in CNG triplets. The bisexual trait was transmitted to two successive generations, but only when androhermaphrodite plants were used as pollen donors. The sex reversal was inherited with incomplete penetrance and varying expressivity. Based on the uniparental inheritance pattern of androhermaphroditism we conclude that it originated either by 5-azaC induced inhibition of Y-linked female-suppressing genes or by a heritable activation of autosomal female-determining/promoting genes which can be reversed, on passage through female meiosis, by a genomic imprinting mechanism. The data presented indicate that female sex suppression in M. album XY males is dependent on methylation of specific DNA sequences and can be heritably modified by hypomethylating drugs.

Fluctuating asymmetry and fractal dimension of the sagittal suture as indicators of inbreeding depression in dama and dorcas gazelles

The effects of inbreeding on the developmental instability of skulls of dorcas (Gazella dorcas) and dama (G. dama) gazelles were investigated. In total, 132 dorcas gazelle skulls and 74 dama gazelle skulls from the Estación Experimental de Zonas Aridas in Almera, Spain, were measured. The fluctuating asymmetry of 9 meristic characters, consisting of the numbers of foramina on the two sides of the skull and mandible, was calculated. Although only the foramen infraorbitalis showed a significant increase in asymmetry with inbreeding in dorcas gazelles, the sum of the foramina in 5 of the skull regions clearly indicates an increase in asymmetry with inbreeding in both dorcas and dama gazelles. The fractal dimension of the sagittal suture was calculated by means of the coastline method. A greater effect of inbreeding on the sagittal suture in dama than in dorcas gazelle was observed, in concordance with the more evident deleterious effects of inbreeding depression in dama than in dorcas gazelles.