A highly pleiotropic amino acid polymorphism in the Drosophila insulin receptor contributes to life‐history adaptation

Finding the specific nucleotides that underlie adaptive variation is a major goal in evolutionary biology, but polygenic traits pose a challenge because the complex genotype–phenotype relationship can obscure the effects of individual alleles. However, natural selection working in large wild populations can shift allele frequencies and indicate functional regions of the genome. Previously, we showed that the two most common alleles of a complex amino acid insertion–deletion polymorphism in the Drosophila insulin receptor show independent, parallel clines in frequency across the North American and Australian continents. Here, we report that the cline is stable over at least a five‐year period and that the polymorphism also demonstrates temporal shifts in allele frequency concurrent with seasonal change. We tested the alleles for effects on levels of insulin signaling, fecundity, development time, body size, stress tolerance, and life span. We find that the alleles are associated with predictable differences in these traits, consistent with patterns of Drosophila life‐history variation across geography that likely reflect adaptation to the heterogeneous climatic environment. These results implicate insulin signaling as a major mediator of life‐history adaptation in Drosophila, and suggest that life‐history trade‐offs can be explained by extensive pleiotropy at a single locus.     

Modulation of methuselah expression targeted to Drosophila insulin‐producing cells extends life and enhances oxidative stress resistance

Ubiquitously reduced signaling via Methuselah (MTH), a G‐protein‐coupled receptor (GPCR) required for neurosecretion, has previously been reported to extend life and enhance stress resistance in flies. Whether these effects are due to reduced MTH signalling in specific tissues remains unknown. We determined that reduced expression of mth targeted to the insulin‐producing cells (IPCs) of the fly brain was sufficient to extend life and enhance oxidative stress resistance. Paradoxically, we discovered that overexpression of mth targeted to the same cells has similar phenotypic effects to reduced expression due to MTH's interaction with β‐arrestin, which uncouples GPCRs from their G‐proteins. We confirmed the functional relationship between MTH and β‐arrestin by finding that IPC‐targeted overexpression of β‐arrestin alone mimics the longevity phenotype of reduced MTH signaling. As reduced MTH signaling also inhibits insulin secretion from the IPCs, the most parsimonious mechanistic explanation of its longevity and stress‐resistance enhancement might be through reduced insulin/IGF signaling (IIS). However, examination of phenotypic features of long‐lived IPC‐mth modulated flies as well as several downstream IIS targets implicates enhanced activity of the JNK stress‐resistance pathway more directly than insulin signaling in the longevity and stress‐resistance phenotypes.     

Identification of a candidate adaptive polymorphism for Drosophila life history by parallel independent clines on two continents

Life history traits are critical components of fitness and frequently reflect adaptive responses to environmental pressures. However, few genes that contribute to natural life history variation have been identified. Insulin signalling mediates the determination of life history traits in many organisms, and single gene manipulation in Drosophila melanogaster suggests that individual genes in the pathway have the potential to produce major effects on these quantitative traits. We evaluated allelic variation at two insulin signalling genes, the Insulin‐like Receptor (InR) and its substrate, chico, in natural populations of D. melanogaster. We found different patterns of variation: InR shows evidence of positive selection and clines in allele frequency across latitude; chico exhibits neutral patterns of evolution. The clinal patterns at InR are replicated between North America and Australia, showing striking similarity in the distribution of specific alleles and the rate at which allele frequencies change across latitude. Moreover, we identified a polymorphism at InR that appears to be functionally significant and consistent with hypothetical patterns of selection across geography. This polymorphism provides new characterization of genic regions of functionality within InR, and is likely a component in a suite of genes and traits that respond adaptively to climatic variation.     

Longevity in response to lowered insulin signaling requires glycine N‐methyltransferase‐dependent spermidine production

Reduced insulin/IGF signaling (IIS) extends lifespan in multiple organisms. Different processes in different tissues mediate this lifespan extension, with a set of interplays that remain unclear. We here show that, in Drosophila, reduced IIS activity modulates methionine metabolism, through tissue‐specific regulation of glycine N‐methyltransferase (Gnmt), and that this regulation is required for full IIS‐mediated longevity. Furthermore, fat body‐specific expression of Gnmt was sufficient to extend lifespan. Targeted metabolomics showed that reducing IIS activity led to a Gnmt‐dependent increase in spermidine levels. We also show that both spermidine treatment and reduced IIS activity are sufficient to extend the lifespan of Drosophila, but only in the presence of Gnmt. This extension of lifespan was associated with increased levels of autophagy. Finally, we found that increased expression of Gnmt occurs in the liver of liver‐specific IRS1 KO mice and is thus an evolutionarily conserved response to reduced IIS. The discovery of Gnmt and spermidine as tissue‐specific modulators of IIS‐mediated longevity may aid in developing future therapeutic treatments to ameliorate aging and prevent disease.     

DILP‐producing median neurosecretory cells in the Drosophila brain mediate the response of lifespan to nutrition

Dietary restriction extends lifespan in diverse organisms, but the gene regulatory mechanisms and tissues mediating the increased survival are still unclear. Studies in worms and flies have revealed a number of candidate mechanisms, including the target of rapamycin and insulin/IGF‐like signalling (IIS) pathways and suggested a specific role for the nervous system in mediating the response. A pair of sensory neurons in Caenorhabditis elegans has been found to specifically mediate DR lifespan extension, but a neuronal focus in the Drosophila nervous system has not yet been identified. We have previously shown that reducing IIS via the partial ablation of median neurosecretory cells in the Drosophila adult brain, which produce three of the seven fly insulin‐like peptides, extends lifespan. Here, we show that these cells are required to mediate the response of lifespan to full feeding in a yeast dilution DR regime and that they appear to do so by mechanisms that involve both altered IIS and other endocrine effects. We also present evidence of an interaction between these mNSCs, nutrition and sleep, further emphasising the functional homology between the DILP‐producing neurosecretory cells in the Drosophila brain and the hypothalamus of mammals in their roles as integration sites of many inputs for the control of lifespan and behaviour.     

The genetic component of human longevity: New insights from the analysis of pathway‐based SNP‐SNP interactions

In human longevity studies, single nucleotide polymorphism (SNP) analysis identified a large number of genetic variants with small effects, yet not easily replicable in different populations. New insights may come from the combined analysis of different SNPs, especially when grouped by metabolic pathway. We applied this approach to study the joint effect on longevity of SNPs belonging to three candidate pathways, the insulin/insulin‐like growth factor signalling (IIS), DNA repair and pro/antioxidant. We analysed data from 1,058 tagging SNPs in 140 genes, collected in 1825 subjects (1,089 unrelated nonagenarians from the Danish 1905 Birth Cohort Study and 736 Danish controls aged 46–55 years) for evaluating synergic interactions by SNPsyn. Synergies were further tested by the multidimensional reduction (MDR) approach, both intra‐ and interpathways. The best combinations (FDR<0.0001) resulted those encompassing IGF1R‐rs12437963 and PTPN1‐rs6067484, TP53‐rs2078486 and ERCC2‐rs50871, TXNRD1‐rs17202060 and TP53‐rs2078486, the latter two supporting a central role of TP53 in mediating the concerted activation of the DNA repair and pro‐antioxidant pathways in human longevity. Results were consistently replicated with both approaches, as well as a significant effect on longevity was found for the GHSR gene, which also interacts with partners belonging to both IIS and DNA repair pathways (PAPPA, PTPN1, PARK7, MRE11A). The combination GHSR‐MREA11, positively associated with longevity by MDR, was further found influencing longitudinal survival in nonagenarian females (p = .026). Results here presented highlight the validity of SNP‐SNP interactions analyses for investigating the genetics of human longevity, confirming previously identified markers but also pointing to novel genes as central nodes of additional networks involved in human longevity.     

Modulation of caveolae by insulin/IGF‐1 signaling regulates aging of Caenorhabditis elegans

Reducing insulin/IGF‐1 signaling (IIS) extends lifespan, promotes protein homeostasis (proteostasis), and elevates stress resistance of worms, flies, and mammals. How these functions are orchestrated across the organism is only partially understood. Here, we report that in the nematode Caenorhabditis elegans, the IIS positively regulates the expression of caveolin‐1 (cav‐1), a gene which is primarily expressed in neurons of the adult worm and underlies the formation of caveolae, a subtype of lipid microdomains that serve as platforms for signaling complexes. Accordingly, IIS reduction lowers cav‐1 expression and lessens the quantity of neuronal caveolae. Reduced cav‐1 expression extends lifespan and mitigates toxic protein aggregation by modulating the expression of aging‐regulating and signaling‐promoting genes. Our findings define caveolae as aging‐governing signaling centers and underscore the potential for cav‐1 as a novel therapeutic target for the promotion of healthy aging.     

Competitiveness and life‐history characteristics of Daphnia with respect to susceptibility to a bacterial pathogen

Costs of resistance, i.e. trade‐offs between resistance to parasites or pathogens and other fitness components, may prevent the fixation of resistant genotypes and therefore explain the maintenance of genetic polymorphism for resistance in the wild. Using two approaches, the cost of resistance to a sterilizing bacterial pathogen were tested for in the crustacean Daphnia magna. First, groups of susceptible and resistant hosts from each of four natural populations were compared in terms of their life‐history characteristics. Secondly, we examined the competitiveness of nine clones from one population for which more detailed information on genetic variation for resistance was known. In no case did the results show that competitiveness or life history characteristics of resistant Daphnia systematically differed from susceptible ones. These results suggest that costs of resistance are unlikely to explain the maintenance of genetic variation in D. magna populations. We discuss methods for measuring fitness and speculate on which genetic models of host‐parasite co‐evolution may apply to the Daphnia‐microparasite system.     

Movement decline across lifespan of Caenorhabditis elegans mutants in the insulin/insulin‐like signaling pathway

Research in aging biology has identified several pathways that are molecularly conserved across species that extend lifespan when mutated. The insulin/insulin‐like signaling (IIS) pathway is one of the most widely studied of these. It has been assumed that extending lifespan also extends healthspan (the period of life with minimal functional loss). However, data supporting this assumption conflict and recent evidence suggest that life extension may, in and of itself, extend the frail period. In this study, we use Caenorhabditis elegans to further probe the link between lifespan and healthspan. Using movement decline as a measure of health, we assessed healthspan across the entire lifespan in nine IIS pathway mutants. In one series of experiments, we studied healthspan in mass cultures, and in another series, we studied individuals longitudinally. We found that long‐lived mutants display prolonged mid‐life movement and do not prolong the frailty period. Lastly, we observed that early‐adulthood movement was not predictive of late‐life movement or survival, within identical phenotypes. Overall, these observations show that extending lifespan does not prolong the period of frailty. Both genotype and a stochastic component modulate aging, and movement late in life is more variable than early‐life movement.     

Polymorphism at the Clock gene predicts phenology of long‐distance migration in birds

Dissecting phenotypic variance in life history traits into its genetic and environmental components is at the focus of evolutionary studies and of pivotal importance to identify the mechanisms and predict the consequences of human‐driven environmental change. The timing of recurrent life history events (phenology) is under strong selection, but the study of the genes that control potential environmental canalization in phenological traits is at its infancy. Candidate genes for circadian behaviour entrained by photoperiod have been screened as potential controllers of phenological variation of breeding and moult in birds, with inconsistent results. Despite photoperiodic control of migration is well established, no study has reported on migration phenology in relation to polymorphism at candidate genes in birds. We analysed variation in spring migration dates within four trans‐Saharan migratory species (Luscinia megarhynchos; Ficedula hypoleuca; Anthus trivialis; Saxicola rubetra) at a Mediterranean island in relation to Clock and Adcyap1 polymorphism. Individuals with larger number of glutamine residues in the poly‐Q region of Clock gene migrated significantly later in one or, respectively, two species depending on sex and whether the within‐individual mean length or the length of the longer Clock allele was considered. The results hinted at dominance of the longer Clock allele. No significant evidence for migration date to covary with Adcyap1 polymorphism emerged. This is the first evidence that migration phenology is associated with Clock in birds. This finding is important for evolutionary studies of migration and sheds light on the mechanisms that drive bird phenological changes and population trends in response to climate change.     

Remarkable life history polymorphism may be evolving under divergent selection in the silverleaf sunflower

Substantial intraspecific variation in life history is rare and potentially a signal of incipient ecological speciation, if variation is driven by geographically heterogenous natural selection. We present the first report of extensive life history polymorphism in Helianthus argophyllus, the silverleaf sunflower, and examine evidence for its evolution by divergent selection. In 18 populations sampled from across the species range and grown in a common garden, most quantitative traits covaried such that individuals could be assigned to two distinct life history syndromes: tall and late flowering with small initial flowerheads, or short and early flowering with larger initial flowerheads. Helianthus argophyllus exhibits regional genetic structure, but this population structure does not closely correspond with patterns of phenotypic variation. The early‐flowering syndrome is primarily observed in populations from coastal barrier islands, while populations from the nearby mainland coast, although geographically and genetically close, are primarily late flowering. Additionally, several traits are more differentiated among regions than expected based on neutral genetic divergence (Q_ST > F_ST), including the first principal component score corresponding with life history syndrome. This discordance between patterns of phenotypic and genetic variation suggests that divergent selection is driving genetic differences in life history across the species range. If so, the silverleaf sunflower may be in early stages of ecological speciation.     

Characterization of polymorphic microsatellites in the castration parasite plant‐ant Allomerus octoarticulatus cf. demerarae (Formicidae: Myrmicinae)

Fifteen microsatellite loci were isolated from the Peruvian tropical plant‐ant Allomerus octoarticulatus cf. demerarae (Hymenoptera: Myrmicinae) and their polymorphism was characterized. High levels of within‐population variation were observed at most loci, with number of alleles ranging from one to 21, and heterozygosity from 0 to 1 per population sample. Cross‐species amplification of these loci was also tested in one other species of the ant genus Allomerus (Allomerus decemarticulatus), displaying similar life history.