To adhere or not to adhere

The modulation of cell adhesion is fundamental to the morphogenesis that accompanies proper embryonic development. Cadherins are a large family of calcium-dependent cell adhesion molecules whose spatial and temporal expression is critical to the formation of the neural crest, a unique, multipotent cell type that contributes to the patterning of the vertebrate body plan. Neural crest cells arise from the embryonic ectoderm through inductive interactions and reside in the dorsal aspect of the neural tube. These cells under go an epithelial-to-mesenchymal transition and migrate to precise destinations in the embryo, where they go on to differentiate into such diverse structures as melanocytes, elements of the peripheral nervous system, and the craniofacial skeleton. Distinct cadherins are expressed during the induction, migration and differentiation of the neural crest. With the advent of genomic sequencing, assembly and annotation for various model organisms, it has become possible to elucidate the molecular mechanisms underlying cadherin expression, and how these cadherins function, during neural crest development. This review explores the known roles of cadherins and details, where relevant, how different cadherins are regulated during the formation of the neural crest.     

To age or not to age

According to the antagonistic pleiotropy theory of ageing, natural selection has favoured genes conferring short-term benefits to the organism at the cost of deterioration in later life. The 'disposable soma' theory expresses this as a life-history strategy in which somatic maintenance is below the level required to prevent ageing, thus enabling higher immediate fertility. It has been argued that a non-ageing strategy will always be bettered by a low but non-zero rate of ageing, because the costs of such ageing will be felt only in the distant future when they are of negligible importance. Here, we examine this argument critically. We find that a non-ageing strategy will be locally optimal if, in the presence of ageing, the onset of deterioration is sufficiently rapid or early. Conversely, ageing will be optimal if deterioration is sufficiently slow or late. As the temporal profile of ageing changes from one of steady deterioration to one involving a sudden loss of vitality after a period of little or no decline, the conditions for a non-ageing strategy to be locally optimal become progressively more stringent. But for all forms of profile considered, conditions can be found for which a strategy involving no ageing is locally optimal.     

To tree or not to tree

The practice of tracking geographical divergence along a phylogenetic tree has added an evolutionary perspective to biogeographic analysis within single species. In spite of the popularity of phylogeography, there is an emerging problem. Recurrent mutation and recombination both create homoplasy, multiple evolutionary occurrences of the same character that are identical in state but not identical by descent. Homoplasic molecular data are phylogenetically ambiguous. Converting homoplasic molecular data into a tree represents an extrapolation, and there can be myriad candidate trees among which to choose. Derivative biogeographic analyses of 'the tree' are analyses of that extrapolation, and the results depend on the tree chosen. I explore the informational aspects of converting a multicharacter data set into a phylogenetic tree, and then explore what happens when that tree is used for population analysis. Three conclusions follow: (i) some trees are better than others; good trees are true to the data, whereas bad trees are not; (ii) for biogeographic analysis, we should use only good trees, which yield the same biogeographic inference as the phenetic data, but little more; and (iii) the reliable biogeographic inference is inherent in the phenetic data, not the trees.     

To permute or not to permute

Permutation test is a popular technique for testing a hypothesis of no effect, when the distribution of the test statistic is unknown. To test the equality of two means, a permutation test might use a test statistic which is the difference of the two sample means in the univariate case. In the multivariate case, it might use a test statistic which is the maximum of the univariate test statistics. A permutation test then estimates the null distribution of the test statistic by permuting the observations between the two samples. We will show that, for such tests, if the two distributions are not identical (as for example when they have unequal variances, correlations or skewness), then a permutation test for equality of means based on difference of sample means can have an inflated Type I error rate even when the means are equal. Our results illustrate permutation testing should be confined to testing for non-identical distributions. CONTACT: calian@raunvis.hi.is.     

To model or not to model

Summary Self-improving control systems may belong to either of two categories, according to whether or not they embody an explicit model of the part of their environment with which they interact. The two forms of operation are discussed and compared, and it is shown that the two may be mathematically equivalent. The treatment also gives theoretical justification for a particular mode of operation for nonmodel-forming controllers.     

To die or not to die: Shigella has an answer

Invasion of epithelial cells by Shigella is a critical step in the pathogenesis of bacillary dysentery. In this issue of Cell Host & Microbe, Bergounioux et?al. (2012) uncover a complex interplay of proinvasion, prosurvival, and prodeath signals centered on the activation of calpain protease by the Shigella VirA protein.