A resurrection experiment finds evidence of both reduced genetic diversity and adaptive evolution in the agricultural weed Ipomoea purpurea
Adam Kuester, Shu-Mei Chang, Regina Baucom
Despite the negative economic and ecological impact of weeds, relatively little is known about the evolutionary mechanisms that influence their ability to persist and thrive in agricultural fields. Here, we use a resurrection ecology approach and compare the neutral and adaptive genetic variation of temporally sampled seed progenies of Ipomoea purpurea, an agricultural weed that is resistant to glyphosate, the most widely used herbicide in current-day agriculture. We found striking reductions in allelic diversity between cohorts sampled nine years apart, suggesting that populations of this species sampled from agricultural fields experience genetic bottleneck and/or founder events through time. We further found that populations of this species exhibit modest increases in herbicide resistance over time and evidence that this increase was due to adaptation and not genetic drift. Our results show that even in light of reduced genetic variation, populations of this noxious weed are capable of adapting to strong selection imparted by herbicide application. We likely uncovered only modest increases in resistance between sampling cohorts due to a strong and previously identified fitness cost of resistance in this species, along with the potential that non-resistant migrants germinate from the seed bank.
Genome divergence and gene flow between Drosophila simulans and D. mauritiana
Sarah B. Kingan, Anthony J. Geneva, Jeffrey P. Vedanayagam, Daniel Garrigan
The fruit fly Drosophila simulans and its sister species D. mauritiana are a model system for studying the genetic basis of reproductive isolation, primarily because interspecific crosses produce sterile hybrid males and their phylogenetic proximity to D. melanogaster. We present an analysis of whole-genome patterns of polymorphism and divergence that shows, on average, the genomes of the two species differ at slightly more than 1% of nucleotide positions and an estimated 40% of autosomal and 60% of X linked loci are reciprocally monophyletic. However, the analysis also identifies 21 major genomic regions, comprising ~1% of the genome, in which one species is segregating for haplotypes that are more similar to haplotypes from the other species than expected, given the levels of sequence divergence in that genomic region. This disjoint distribution of interspecific coalescence times is consistent with recent introgression between the cosmopolitan D. simulans and the island endemic D. mauritiana. We find that the putatively introgressed regions are more likely to have significantly higher rates of crossing-over and are enriched for genes with significantly slower rates of protein evolution. We also uncover instances in which genes experiencing lineage-specific positive selection closely interact with genes experiencing introgression. Finally, we find that a large introgressing region on the X chromosome has experienced a strong selective sweep in D. mauritiana and also has high levels of homozygosity in D. simulans. A detailed analysis reveals that the introgressing X chromosome haplotypes are closely associated with the presence of the MDox locus, which is the progenitor of the Winters sex-ratio meiotic drive genes. These results highlight how genetic systems that evolve rapidly in allopatry, including selfish meiotic drive elements, remain robust in natural hybrid genotypes and do not systematically promote reproductive isolation.
Life history effects on the molecular clock of autosomes and sex chromosomes
Guy Amster, Guy Sella
One of the foundational results of molecular evolution is that the rate at which neutral substitutions accumulate on a lineage equals the rate at which mutations arise. Traits that affect rates of mutation therefore also affect the phylogenetic “molecular clock”. We consider the effects of sex-specific generation times and mutation rates in species with two sexes. In particular, we focus on the effects that the age of onset of male puberty and rates of spermatogenesis have likely had in extant hominines (i.e., human, chimpanzee and gorilla), considering a model that approximates features of the mutational process in most mammals and birds and some other vertebrates. As we show, this model helps explain and reconcile a number of seemingly puzzling observations. In hominines, it can explain the puzzlingly low X-to-autosome ratios of substitution rates and how the ratios and rates of autosomal substitutions differ among lineages. Importantly, it suggests how to translate pedigree-based estimates of human mutation rates into split times among apes, given sex-specific life histories. In so doing, it helps bridge the gap between estimates of split times of apes based on fossil and molecular evidence. Finally, considering these effects can help to reconcile recent evidence that changes in generation times should have small effects on mutation rates in humans with classic studies suggesting that they have had major effects on rates of evolution in the mammalian phylogeny.
Successful asexual lineages of the Irish potato Famine pathogen are triploidYing Li, Qian Zhou, Kun Qian, Theo van der Lee, Sanwen Huang
The oomycete Phytophthora infestans was the causal agent of the Irish Great Famine and is a recurring threat to global food security. The pathogen can reproduce both sexually and asexually and has a potential to adapt both abiotic and biotic environment. Although in many regions the A1 and A2 mating types coexist, the far majority of isolates belong to few clonal, asexual lineages. As other oomycetes, P. infestans is thought to be diploid during the vegetative phase of its life cycle, but it was observed that trisomy correlated with virulence and mating type locus and that polyploidy can occur in some isolates. It remains unknown about the frequency of polyploidy occurrence in nature and the relationship between ploidy level and sexuality. Here we discovered that the sexuality of P. infestans isolates correlates with ploidy by comparison of microsatellite fingerprinting, genome-wide polymorphism, DNA quantity, and chromosome numbers. The sexual progeny of P. infestans in nature are diploid, whereas the asexual lineages are mostly triploids, including successful clonal lineages US-1 and 13_A2. This study reveals polyploidization as an extra evolutionary risk to this notorious plant destroyer.
S/HIC: Robust identification of soft and hard sweeps using machine learningDaniel R Schrider, Andrew D Kern
Detecting the targets of adaptive natural selection from whole genome sequencing data is a central problem for population genetics. However, to date most methods have shown sub-optimal performance under realistic demographic scenarios. Moreover, over the past decade there has been a renewed interest in determining the importance of selection from standing variation in adaptation of natural populations, yet very few methods for inferring this model of adaptation at the genome scale have been introduced. Here we introduce a new method, S/HIC, which uses supervised machine learning to precisely infer the location of both hard and soft selective sweeps. We show that S/HIC has unrivaled accuracy for detecting sweeps under demographic histories that are relevant to human populations, and distinguishing sweeps from linked as well as neutrally evolving regions. Moreover we show that S/HIC is uniquely robust among its competitors to model misspecification. Thus even if the true demographic model of a population differs catastrophically from that specified by the user, S/HIC still retains impressive discriminatory power. Finally we apply S/HIC to the case of resequencing data from human chromosome 18 in a European population sample and demonstrate that we can reliably recover selective sweeps that have been identified earlier using less specific and sensitive methods.
Genomic study of the Ket: a Paleo-Eskimo-related ethnic group with significant ancient North Eurasian ancestryPavel Flegontov, Piya Changmai, Anastassiya Zidkova, Maria D. Logacheva, Olga Flegontova, Mikhail S. Gelfand, Evgeny S. Gerasimov, Ekaterina V. Khrameeva, Olga P. Konovalova, Tatiana Neretina, Yuri V. Nikolsky, George Starostin, Vita V. Stepanova, Igor V. Travinsky, Martin Tříska, Petr Tříska, Tatiana V. Tatarinova
The Kets, an ethnic group in the Yenisei River basin, Russia, are considered the last nomadic hunter-gatherers of Siberia, and Ket language has no transparent affiliation with any language family. We investigated connections between the Kets and Siberian and North American populations, with emphasis on the Mal’ta and Paleo-Eskimo ancient genomes, using original data from 46 unrelated samples of Kets and 42 samples of their neighboring ethnic groups (Uralic-speaking Nganasans, Enets, and Selkups). We genotyped over 130,000 autosomal SNPs, determined mitochondrial and Y-chromosomal haplogroups, and performed high-coverage genome sequencing of two Ket individuals. We established that the Kets belong to the cluster of Siberian populations related to Paleo-Eskimos. Unlike other members of this cluster (Nganasans, Ulchi, Yukaghirs, and Evens), Kets and closely related Selkups have a high degree of Mal’ta ancestry. Implications of these findings for the linguistic hypothesis uniting Ket and Na-Dene languages into a language macrofamily are discussed.
Genomic analysis of allele-specific expression in the mouse liverAshutosh K Pandey, Robert W Williams
Genetic differences in gene expression contribute significantly to phenotypic diversity and differences in disease susceptibility. In fact, the great majority of causal variants highlighted by genome-wide association are in non-coding regions that modulate expression. In order to quantify the extent of allelic differences in expression, we analyzed liver transcriptomes of isogenic F1 hybrid mice. Allele-specific expression (ASE) effects are pervasive and are detected in over 50% of assayed genes. Genes with strong ASE do not differ from those with no ASE with respect to their length or promoter complexity. However, they have a higher density of sequence variants, higher functional redundancy, and lower evolutionary conservation compared to genes with no ASE. Fifty percent of genes with no ASE are categorized as house-keeping genes. In contrast, the high ASE set may be critical in phenotype canalization. There is significant overlap between genes that exhibit ASE and those that exhibit strong cis expression quantitative trait loci (cis eQTLs) identified using large genetic expression data sets. Eighty percent of genes with cis eQTLs also have strong ASE effects. Conversely, 40% of genes with ASE effects are associated with strong cis eQTLs. Cis-acting variation detected at the protein level is also detected at the transcript level, but the converse is not true. ASE is a highly sensitive and direct method to quantify cis-acting variation in gene expression and complements and extends classic cis eQTL analysis. ASE differences can be combined with coding variants to produce a key resource of functional variants for precision medicine and genome-to-phenome mapping.