Stefan Van Dongen

Fluctuating asymmetry as an indicator of fitness: can we bridge the gap between studies?

There is growing evidence from both experimental and non‐experimental studies that fluctuating asymmetry does not consistently index stress or fitness. The widely held – yet poorly substantiated ‐ belief that fluctuating asymmetry can act as a universal measure of developmental stability and predictor of stress‐mediated changes in fitness, therefore staggers. Yet attempts to understand why the reported relationships between fluctuating asymmetry, stress and fitness are so heterogeneous – i.e. whether the associations are truly weak or non‐existent or whether they become confounded during different stages of the analytical pathways – remain surprisingly scarce. Hence, we attempt to disentangle these causes, by reviewing the various statistical and conceptual factors that are suspected to confound potential relationships between fluctuating asymmetry, stress and fitness. Two main categories of factors are discerned: those associated with the estimation of developmental stability through fluctuating asymmetry, and those associated with the effects of genotype and environment on developmental stability. Next, we describe a series of statistical tools that have recently been developed to help reduce this noise. We argue that the current lack of a theoretical framework that predicts if and when relationships with developmental stability can be expected, urges for further theoretical and empirical research, such as on the genetic architecture of developmental stability in stressed populations. If the underlying developmental mechanisms are better understood, statistical patterns of asymmetry variation may become a biologically meaningful tool.

Synchronization of hatching date with budburst of individual host trees (Quercus robur) in the winter moth (Operophtera brumata) and its fitness consequences

1. Due to varying selection pressures among host individuals, herbivorous insects may show local adaptation at the individual level. In the winter moth, synchrony of egg hatching and host tree (Quercus robur L.) budburst may have important fitness consequences and, therefore, may result in a local adaptation to the hosts phenology. However, relatively high gene flow levels may disrupt such a fine-scale adaption. 2. We determined hatching dates of clutches laid by females collected during copulation, and variation in budburst dates of the trees on which these couples were collected. 3. In our study area, budburst date showed substantial variation within years and areas, as the first and last tree to start leafing were separated by up to 26 days. Relative budburst dates of individual trees were constant between years. Average hatching dates differed significantly among clutches and varied over 30 days, a range comparable to that of budburst. 4. Hatching dates were positively associated with budburst of the respective trees indicating individual synchrony, which may be mediated by two mechanisms. First, early active adults are captured more frequently on early trees and late adults on late trees. As adult activity period is positively correlated with date of egg hatch, early clutches will tend to be laid on earlier trees while late clutches will be laid more frequently on late trees. Secondly, a presumed active choice mechanism may additionally increases individual host synchronization. Yet, the latter requires further study. 5. Our data support that synchronization of larval hatching with host budburst is adaptive as it increases adult size and thus expected fitness.

Random amplified polymorphic DNA (RAPD) and parsimony methods

Abstract Random amplified polymorphic DNA (RAPD) data possess a number of undesirable features for parsimony analysis. These features include their non-codominant inheritance, their anonymous nature, their different (a)symmetrical transformation probabilities, and their possible GC priming bias. As a consequence, no single parsimony method seems appropriate for RAPD data. Moreover, the presence/absence coding of RAPDs is equivalent to the invalid independent allele model for allozymes. These issues are discussed and the way in which parsimony analysis of RAPDs can be misleading is illustrated.

Plague and the Human Flea, Tanzania

Pulex irritans fleas were more common in villages with high plague incidence.

Cross-species amplification of microsatellite primers in passerine birds

Developing species specific microsatellite primers can be avoidedby using existing markers which amplify across species. However,for passerines, such ‘cross-species’ markers are mostly lackingand few guidelines exist for selecting them from the wide rangeof existing markers. Here cross-species amplification tests of 40microsatellite primers in 13 passerine species show an increasein probability of amplification and polymorphism with decreasingphylogenetic distance. Primers which successfully amplified inmany species had a higher chance to be polymorphic. However,since the amplification success, across a broad range of species,of particular primersets remains difficult to predict it iscrucial to identify such markers empirically. Here we describesuch widely applicable bird (passerines) microsatellite markers.

Morphological and genetic variation in the speckled wood butterfly (Pararge aegeria L.) among differently fragmented landscapes

We compared morphological and genetic variation between populations of the speckled wood butterfly (Parage aegeria L.) from four landscapes differing in degree of fragmentation: a continuous woodland, a set of woodland fragments, another, more isolated set of woodlots and a highly fragmented area with very small woodland fragments and hedgerows scattered in an intensively used agricultural landscape. Male butterflies were collected, weighed and their wing features (size and colour) measured by means of an image analysis system. We used Polyacrylamide Gel Electrophoresis (PAGE) to investigate genetic variation. The clearest morphological differences were found between the large, continuous woodland and the three other areas. Males of this woodland had a paler basal and distal forewing colour and larger yellow patches on the dorsal side of the forewing. Since these features are relevant to theroregulation and anti-predation, this result suggests differences with respect to microclimate and predation between the study areas. Relative thorax size also increased with degree of fragmentation. Since the thorax contains the flight muscles, this result may relate to differences in patterns of mobility or different costs for the same mobility pattern. The genetic diversity of the butterflies of the large, continuous woodland was significantly higher than in the other study areas. There were also indications for genetic differentiation. Hence, this study may point at adaptive morphological variation in the speckled wood butterfly among differently fragmented landscapes.

Environmental stress increases skeletal fluctuating asymmetry in the moor frog Rana arvalis

Whether fluctuating asymmetry (FA) provides a useful metric indicator of the degree of environmental stress experienced by populations is still a contentious issue. We investigated whether the degree of FA in skeletal elements is useful in elucidating the degree of environmental stress experienced by frog populations, and further, tested the proposition that a trait’s sensitivity to stress—as reflected in the degree of FA—is related to the degree of directional selection experienced by the given trait. We compared the degree of FA in four bilateral skeletal elements of male and female moor frogs (Rana arvalis) originating from low (acidified) and neutral pH populations. While the degree of uncorrected FA was unrelated to the degree of acidity, the growth rate and age of the individuals, the size-corrected FA was significantly higher in low than in neutral pH populations and decreased with individual ages and growth rates. In addition, both measures of FA were significantly higher in males and in particular in traits presumably under high sexual selection as indicated by the degree of sexual size dimorphism. All in all, the results indicate that individuals from acidified localities are smaller, younger and exhibit a significantly higher degree of FA than individuals from neutral pH populations. These results constitute the first assessment of FA in amphibians and suggest that the degree of FA in skeletal traits can be a useful indicator of the degree of environmental stress experienced by amphibian populations.

Heritability and artificial selection on ambulatory dispersal distance in Tetranychus urticae: effects of density and maternal effects.

Dispersal distance is understudied although the evolution of dispersal distance affects the distribution of genetic diversity through space. Using the two-spotted spider mite, Tetranychus urticae, we tested the conditions under which dispersal distance could evolve. To this aim, we performed artificial selection based on dispersal distance by choosing 40 individuals (out of 150) that settled furthest from the home patch (high dispersal, HDIS) and 40 individuals that remained close to the home patch (low dispersal, LDIS) with three replicates per treatment. We did not observe a response to selection nor a difference between treatments in life-history traits (fecundity, survival, longevity, and sex-ratio) after ten generations of selection. However, we show that heritability for dispersal distance depends on density. Heritability for dispersal distance was low and non-significant when using the same density as the artificial selection experiments while heritability becomes significant at a lower density. Furthermore, we show that maternal effects may have influenced the dispersal behaviour of the mites. Our results suggest primarily that selection did not work because high density and maternal effects induced phenotypic plasticity for dispersal distance. Density and maternal effects may affect the evolution of dispersal distance and should be incorporated into future theoretical and empirical studies.

Analysis of cervical ribs in a series of human fetuses

In humans, an increasing body of evidence has linked the frequency of cervical ribs to stillbirths, other malformations and early childhood cancers. However, the frequency of cervical ribs in a putatively healthy fetal population is not sufficiently known to assess the actual medical risks of these prenatal findings. We therefore analyzed the presence of skeletal anomalies in a series of 199 electively aborted fetuses, which were whole‐mount stained with alizarin red specific for skeletal tissues. Results show that approximately 40% of the fetuses had cervical ribs, even though external congenital abnormalities such as craniofacial and limb defects were absent. A literature overview indicates that the observed frequency of cervical ribs is comparable to results previously obtained for deceased fetuses with no or minor congenital anomalies, and higher than expected for healthy fetuses. This unexpected result can probably in part be explained by a higher detection rate of small cervical ribs when using alizarin red staining instead of radiographs. Additionally, studies in the literature suggest that the size of a cervical rib may indicate the severity of abnormalities, but this possibility requires further research. Anomalies of the axial skeleton are known to be caused by a disturbance of early development, which alters Hox gene expression, but in this study the origin of the stress could not be verified as maternal medical data were not available. The co‐occurrence of rudimentary or absent 12th ribs in 23.6% of the cases with cervical ribs indicates that in approximately 8% of the fetuses a homeotic shift occurred over a larger part of the vertebral column. This suggests that the expression of multiple Hox genes may have been affected in these fetuses. Together, the high incidence of cervical ribs and also their co‐occurrence with rudimentary or absent 12th ribs suggests that there may have been a disturbance of early development such that the studied fetuses are probably not informative about the general population. Future studies determining the frequency of cervical ribs in a more healthy fetal population are therefore needed to evaluate their potential as an indicator of medical risks.

The effect of air pollution and other environmental stressors on leaf fluctuating asymmetry and specific leaf area of Salix alba L.

We aimed at evaluating the effect of low-level air pollution on leaf area fluctuating asymmetry (FAA) and specific leaf area (SLA) of Salix alba L., taking into account other environmental factors. Cuttings were grown in standardized conditions in the near vicinity of air quality measuring stations in Belgium. Variability of SLA and FAA between measuring stations explained 83% and 7.26%, respectively, of the total variability. FAA was not influenced by air pollution or environmental factors such as shading, herbivory, air temperature and humidity. SLA was increased by an increase in shadow, while NO(x) and O(3) concentrations had only a marginal influence. The influence of SO(2) concentration was negligible. Although our data analysis suggests a relationship between SLA and NO(x)/O(3) concentration, the absence of a straightforward relationship between FAA and SLA and air pollution still questions the usefulness of these bio-indicators for monitoring air pollution.