Competition between random genetic drift and natural selection play a central role in evolution: Whereas nonbeneficial mutations often prevail in small populations by chance, mutations that sweep through large populations typically confer a selective advantage. Here, however, we observe chance effects during range expansions that dramatically alter the gene pool even in large microbial populations. Initially well mixed populations of two fluorescently labeled strains of Escherichia coli develop well defined, sector-like regions with fractal boundaries in expanding colonies. The formation of these regions is driven by random fluctuations that originate in a thin band of pioneers at the expanding frontier. A comparison of bacterial and yeast colonies (Saccharomyces cerevisiae) suggests that this large-scale genetic sectoring is a generic phenomenon that may provide a detectable footprint of past range expansions.genetic distance ͉ founder effect ͉ microorganisms ͉ range expansion A principal tenet of modern evolutionary biology is that Darwinian selection and random genetic drift compete in driving evolutionary change (1). It is widely accepted that genetic drift can have significant effects on small populations (2) that may even lead to speciation (3). In large populations, however, random sampling effects are generally considered weak compared with selection (4-6) (law of large numbers). A major departure from this paradigmatic behavior occurs when large populations undergo range expansions. The descendents of individuals first settling in a new territory are most likely to dominate the gene pool as the expansion progresses (7-9). Random sampling effects among these pioneers results in genetic drift that can have profound consequences on the diversity of the expanding population. Indeed, spatially varying levels of genetic diversity and colonization patterns appear to be correlated in many species (10-13). For example, the often observed south-north gradient in neutral genetic diversity [''southern richness to northern purity'' (14)] on the northern hemisphere is thought to reflect past range expansions induced by glacial cycles (12). Although these trends indicate that genetic drift during range expansions has shaped the gene pool of many species, the underlying spatial mechanism remains obscure: Diversity gradients are often difficult to interpret and potentially interfere with the signal of spreading beneficial mutations (15,16). In fact, a major challenge of present-day population genetics is to decide whether natural selection or a past demographic process is responsible for the prevalence of common mutations (17).Here, we use simple microbial systems to study the nature of random genetic drift in range expansions of large populations. We observe chance effects that segregate the gene pool into well defined, sector-like regions of reduced genetic diversity. The genetic segregation on the population level is the consequence of number fluctuations on a much smaller scale, within a thin region of reproducing p...

Loss of sexual reproduction is considered an evolutionary dead end for metazoans, but bdelloid rotifers challenge this view as they appear to have persisted asexually for millions of years 1 . Neither male sex organs nor meiosis have ever been observed in these microscopic animals: oocytes are formed through mitotic divisions, with no reduction of chromosome number and no indication of chromosome pairing 2 . However, current evidence does not exclude that they may engage in sex on rare, cryptic occasions. Here we report the genome of a bdelloid rotifer, Adineta vaga (Davis, 1873) 3 , and show that its structure is incompatible with conventional meiosis. At gene scale, the genome of A. vaga is tetraploid and comprises both anciently duplicated segments and less divergent allelic regions. However, in contrast to sexual species, the allelic regions are rearranged and sometimes even found on the same chromosome. Such structure does not allow meiotic pairing; instead, we find abundant evidence of gene conversion, which may limit the accumulation of deleterious mutations in the absence of meiosis. Gene families involved in resistance to oxidation, carbohydrate metabolism and defence against transposons are significantly expanded, which may explain why transposable elements cover only 3% of the assembled sequence. Furthermore, 8% of the genes are likely to be of non-metazoan origin and were probably acquired horizontally. This apparent convergence between bdelloids and prokaryotes sheds new light on the evolutionary significance of sex.With more than 460 described species 4 , bdelloid rotifers ( Fig. 1) represent the highest metazoan taxonomic rank in which males, hermaphrodites and meiosis are unknown. Such persistence and diversification of an ameiotic clade of animals are in contradiction with the supposed long-term disadvantages of asexuality, making bdelloids an 'evolutionary scandal' 5 . Another unusual feature of bdelloid rotifers is their extreme resistance to desiccation at any stage of their life cycle 6 , enabling these microscopic animals to dwell in ephemeral freshwater habitats such as mosses, lichens and forest litter; this ability is presumably the source of their extreme resistance to ionizing radiation 7 .We assembled the genome of a clonal A. vaga lineage into separate haplotypes with a N 50 of 260 kilobases (kb) (that is, half of the assembly was composed of fragments longer than 260 kb). Assembly size was 218 megabases (Mb) but 26 Mb of the sequence had twice the average sequencing coverage, suggesting that some nearly identical regions were not resolved during assembly ( Supplementary Fig. 3); hence, the total genome size is likely to be 244 Mb, which corresponds to the estimate obtained independently using fluorometry (Supplementary Note C2). Annotation of the complete assembly (including all haplotypes) yielded 49,300 genes. Intragenomic sequence comparisons revealed numerous homologous blocks with conserved gene order (colinear regions). For each such block we computed the per-site synonymous d...

Spatially resolved genetic data is increasingly used to reconstruct the migrational history of species. To assist such inference, we study, by means of simulations and analytical methods, the dynamics of neutral gene frequencies in a population undergoing a continual range expansion in one dimension. During such a colonization period, lineages can fix at the wave front by means of a "surfing" mechanism [Edmonds C.A., Lillie A.S. & Cavalli-Sforza L.L. (2004) Proc Natl Acad Sci USA 101: 975-979]. We quantify this phenomenon in terms of (i) the spatial distribution of lineages that reach fixation and, closely related, (ii) the continual loss of genetic diversity (heterozygosity) at the wave front, characterizing the approach to fixation. Our simulations show that an effective population size can be assigned to the wave that controls the (observable) gradient in heterozygosity left behind the colonization process. This effective population size is markedly higher in pushed waves than in pulled waves, and increases only sub-linearly with deme size. To explain these and other findings, we develop a versatile analytical approach, based on the physics of reaction-diffusion systems, that yields simple predictions for any deterministic population dynamics.Population expansions in space are common events in the evolutionary history of many species [1,2,3,4,5,6,7] and have a profound effect on their genealogy. It is widely appreciated that any range expansion leads to a reduction of genetic diversity ("Founder Effect") because the gene pool for the new habitat is provided only by a small number of individuals, which happen to arrive in the unexplored territory first. In many species, the genetic footprints of these pioneers are still recognizable today and provide information about the migrational history of the species. For instance, a frequently observed south-north gradient in genetic diversity ("southern richness to northern purity" [8]) on the northern hemisphere is thought to reflect the range expansions induced by the glacial cycles. In the case of humans, the genetic diversity decreases essentially linearly with increasing geographic distance from Africa [2,3], which is indicative of the human migration out of Africa. It is hoped [9], that the observed patterns of neutral genetic diversity can be used to infer details of the corresponding colonization pathways.Such an inference requires an understanding of how a colonization process generates a gradient in genetic diversity, and which parameters chiefly control the magnitude of this gradient. Traditional models of population genetics [10], which mainly focus on populations of constant size and distribution, apply to periods before and after a range expansion has occurred, when the population is at demographic equilibrium. However, the spatio-temporal dynamics in the transition period, on which we focus in this article, is less amenable to the standard analytical tools of population genetics, and has been so far studied mostly by means of simulations [11,12,13,14,15...