[
International Worm Meeting,
2015]
Seemingly similar phenotypic traits can vary considerably in the extent of genetic variation in the trait. If two traits in the same set of organisms have different levels of genetic variation, there are two potential underlying causes: mutation and/or selection. Traits may differ in the mutational target they present, or in the rate at which those loci mutate. Traits may also differ in the average effect that mutations have on the trait. Alternatively, selection may differ between traits.We investigated these issues by comparing the input of genetic variation by mutation to the standing genetic variation found in a worldwide collection of C. elegans for a suite of 19 traits related to the first mitotic cell division. The results are striking: the mutational variance (VM) explains 90% of the variation in the standing genetic variance (VG). One of the traits - embryo length - has been previously shown to be under stabilizing selection. The strong positive relationship between VM and VG remains after accounting for variation in embryo length. Mutations were allowed to accumulate in two separate experiments in two different genetic backgrounds (N2 and PB306). The correlation between VM in the two experiments was ~0.9, which indicates that for a given set of traits, the mutational process produces highly repeatable outcomes.The ratio of VG/VM can be interpreted as the number of generations of mutation required to produce that amount of variation in the population. For 18 of the 19 traits (embryo length notwithstanding) the ratio of VG/VM is on the order of a few hundred generations. That result is consistent with molecular evidence suggesting that C. elegans has recently undergone a species-wide selective sweep that purged most of the standing genetic variation.We conclude that variation among traits in the amount of genetic variation in the population is a predictable function of the mutational input, and that the role of direct and/or linked natural selection is secondary. This result has implications for understanding the genetic architecture of complex traits in any organism, including humans.
[
Evolutionary Biology of Caenorhabditis and Other Nematodes,
2014]
Cellular mechanisms are essential to translate molecular information into phenotypes. Although genomic, molecular, development and phenotypic evolution are largely explored; there is currently little understanding on the evolutionary dynamics of cell biological processes. We chose to analyze the mitotic spindle in an evolutionary context. The mitotic spindle is a microtubule-based structure that generates mechanical forces to segregate chromosomes apart and find its position within the cell during anaphase. In the one-cell C. elegans embryo, the spindle undergoes very reproducible movements leading to its elongation and asymmetric positioning towards the posterior side of the cell. Consequently, the first embryonic division is asymmetric in size. We recorded the first asymmetric embryonic division of 42 different species of the Rhabditidae family (corresponding to 111 different strains). We quantified spindle dynamics in details by measuring ~25 different cellular parameters, such as spindle size, cell size, centrosome motion, etc. We found very different spindle movements depending on the species observed, despite a conserved final asymmetric position. We also found some clear intra- species variations. Therefore, we reveal cryptic evolutionary changes in the regulation of mechanical forces behind a conserved cellular process. We found that transverse oscillations of the spindle, as found in C. elegans, were observed only in species from the Caenorhabditis genus, except C. sp. 1. Otherwise, the majority of parameters varied randomly on the phylogeny and did not show any directionality of changes. We will present our final results that allow us to 1) describe the diversity of cryptic changes of an fundamental cellular structure and 2) identify the mechanisms that are either flexible or highly constrained. At the same time, such comparative approach allows us to uncover essential properties of the spindle mechanics shared between species 1. Ultimately, we want to identify the compensatory mechanisms allowing the system to maintain its final output despite the accumulation of cryptic changes.1. Riche & al. "Evolutionary comparisons reveal a positional switch for spindle pole oscillations in Caenorhabditis embryos". Journal of Cell Biology. 2013: 201, 653-662.
[
International Worm Meeting,
2013]
An obvious feature of living organisms is that their development is robust to variation in the environmental circumstances in which the organism finds itself - within limits. That is, the phenotype is more or less "canalized" (canalized "robust"). The "more or less" is of considerable interest to evolutionary biologists, for a variety of reasons, i.e., under what environmental or genetic circumstances does development become more or less canalized? Environmental canalization can be quantified as the phenotypic variation among genetically identical individuals raised in a uniform environment - the "(micro)environmental variance" in the lingo of quantitative genetics, V(E). Here we provide quantitative estimates of the effects of spontaneous mutations on V(E) for a variety of phenotypic traits subject to different selective regimes. Three general quantitative trends emerge. First, in almost all cases, mutation accumulation tends to de-canalize the phenotype (i.e., V(E) increases), typically at a rate similar to the rate of change of the trait itself. Second, there is a strong positive association between the rate of increase of V(E) for a trait and the mutational variance, V(M) for the trait itself. Third, and most intriguingly, mutations affecting V(E) for a trait appear to be usually under stronger selection than mutations affecting the trait itself.