A Multivariate Approach to Determine the Dimensionality of Human Facial Asymmetry.

Many studies have suggested that developmental instability (DI) could lead to asymmetric development, otherwise known as fluctuating asymmetry (FA). Several attempts to unravel the biological meaning of FA have been made, yet the main step in estimating FA is to remove the effects of directional asymmetry (DA), which is defined as the average bilateral asymmetry at the population level. Here, we demonstrate in a multivariate context that the conventional method of DA correction does not adequately compensate for the effects of DA in other dimensions of asymmetry. This appears to be due to the presence of between-individual variation along the DA dimension. Consequently, we propose to decompose asymmetry into its different orthogonal dimensions, where we introduce a new measure of asymmetry, namely fluctuating directional asymmetry (F-DA). This measure describes individual variation in the dimension of DA, and can be used to adequately correct the asymmetry measurements for the presence of DA. We provide evidence that this measure can be useful in disentangling the different dimensions of asymmetry, and further studies on this measure can provide valuable insight into the underlying biological processes leading to these different asymmetry dimensions.

Evaluating Complex Mixtures in the Zebrafish Embryo by Reconstituting Field Water Samples: A Metal Pollution Case Study.

Accurately assessing the toxicity of complex, environmentally relevant mixtures remains an important challenge in ecotoxicology. The goal was to identify biological effects after exposure to environmental water samples and to determine whether the observed effects could be explained by the waterborne metal mixture found in the samples. Zebrafish embryos were exposed to water samples of five different sites originating from two Flemish (Mol and Olen, Belgium) metal contaminated streams: "Scheppelijke Nete" (SN) and "Kneutersloop" (K), and a ditch (D), which is the contamination source of SN. Trace metal concentrations, and Na, K, Mg and Ca concentrations were measured using ICP-MS and were used to reconstitute site-specific water samples. We assessed whether the effects that were observed after exposure to environmental samples could be explained by metal mixture toxicity under standardized laboratory conditions. Exposure to "D" or "reconstituted D" water caused 100% mortality. SN and reconstituted SN water caused similar effects on hatching, swim bladder inflation, growth and swimming activity. A canonical discriminant analysis confirmed a high similarity between both exposure scenarios, indicating that the observed toxicity was indeed primarily caused by metals. The applied workflow could be a valuable approach to evaluate mixture toxicity that limits time and costs while maintaining biological relevance.

A critical re-evaluation of the association between 2D:4D ratios and fluctuating asymmetry in humans.

BACKGROUND: Both asymmetry and the second and fourth digit ratio (2D:4D) relate to various aspects of human life history. Higher asymmetry with extreme 2D:4D ratios have suggested a link between both processes, indicating that early development determines levels of developmental instability. However, most observed associations may be biased because combinations of the digit lengths are used as both dependent and independent variables. AIM: The present study explored the usefulness of asymmetry in digits 2 and 4 to study this association. SUBJECTS AND METHODS: A combination of literature review, a small simulation study, and a study of 2D:4D ratios and asymmetry in a total of 100 young males and females is presented. RESULTS: Using asymmetry in digits 2 and 4 results in bias and increased type I error rates, leading to stronger associations between asymmetry and 2D:4D ratios. In spite of low sample sizes, significant associations between asymmetry measured in hands and face and 2D:4D ratios were detected for asymmetries in digits 2 and 4 only. CONCLUSIONS: Using asymmetry in digits 2 and/or 4 causes bias. Excluding such results, there is currently little evidence of any association between asymmetry and digit ratios. Future studies should carefully select traits to investigate these correlations further.