Effects of ambient and preceding temperatures and metabolic genes on flight metabolism in the Glanville fritillary butterfly

Flight is essential for foraging, mate searching and dispersal in many insects, but flight metabolism in ectotherms is strongly constrained by temperature. Thermal conditions vary greatly in natural populations and may hence restrict fitness-related activities. Working on the Glanville fritillary butterfly (Melitaea cinxia), we studied the effects of temperature experienced during the first 2 days of adult life on flight metabolism, genetic associations between flight metabolic rate and variation in candidate metabolic genes, and genotype–temperature interactions. The maximal flight performance was reduced by 17% by 2 days of low ambient temperature (15 °C) prior to the flight trial, mimicking conditions that butterflies commonly encounter in nature. A SNP in phosphoglucose isomerase (Pgi) had a significant association on flight metabolic rate in males and a SNP in triosephosphate isomerase (Tpi) was significantly associated with flight metabolic rate in females. In the Pgi SNP, AC heterozygotes had higher flight metabolic rate than AA homozygotes following low preceding temperature, but the trend was reversed following high preceding temperature, consistent with previous results on genotype–temperature interaction for this SNP. We suggest that these results on 2-day old butterflies reflect thermal effect on the maturation of flight muscles. These results highlight the consequences of variation in thermal conditions on the time scale of days, and they contribute to a better understanding of the complex dynamics of flight metabolism and flight-related activities under conditions that are relevant for natural populations living under variable thermal conditions.     

Effect of multiple-locus heterozybosity and salinity on clearance rate in a brackish-water clam,Rangia cuneata (Sowerby)

Previous studies of several species of marine bivalves and gastropods have reported a positive correlation between growth or size and level of multiple-locus heterozygosity. There is some evidence that the growth advantage of relatively heterozygous individuals is due to a lower rate of standard or routine metabolism, compared with more homozygous individuals, although heterozygosity-dependent differences in feeding rate may also be involved. The present study examined the relationship between clearance rate in three salinity treatments (5,15, and 25%.) and multiple-locus heterozygosity at nine polymorphic allozyme loci in the clam Rangia cuneata (Sowerby). Clearance rates were determined by disappearance of an algal suspension from a flowing-water system. Allozyme genotypes were determined using starch-gel electrophoresis. The polymorphic loci examined were those coding for a nonspecific esterase (Est), mannosephosphate isomerase (Mpi), leucine aminopeptidase (Lap), 6-phosphogluconate dehydrogenase (6-Pgd), phosphoglucose isomerase (Pgi), isocitrate dehydrogenase (Idh), malate dehydrogenase (Mdh), adenylate kinase (Adk), and phosphoglucomutase (Pgm). Weight-corrected clearance rates increased significantly (P < 0.05) with increasing multiple-locus heterozygosity and decreased significantly (P < 0.05) with increasing salinity. These data support the idea that heterozygosity-growth correlations may be due in part to differences in clearance rate. However, further study is needed to understand the exact physiological processes which relate heterozygosity and growth.     

Genetical control and linkage relationships of Isozyme markers in sugar beet (B. vulgaris L.). 2. NADP- and NAD-specific malate dehydrogenases, 6-P-gluconate dehydrogenase,shikimate dehydrogenase,diaphorase and aconitase

Summary The NADP-specific malate dehydrogenase isozymes were controlled by multiple gene systems. Three genes coding for dimeric enzymes segregated in a dependent fashion (NADP-Mdh ₁, NADP-Mdh ₂, NADP-Mdh ₃). A fourth gene (NADP-Mdh ₄), also coded for dimers, but was not polymorphic in B. vulgaris. A fifth gene (NADP-Me ₁) coded for enzymes active as monomers. Two genes were found to control the main zone of NAD-specific malate dehydrogenase: one coded for dimers (Mdh ₁), while a second (Mdh ₂) was not polymorphic in the assessions studied. 6-P-Gluconate dehydrogenase was not polymorphic in B. vulgaris; the two types detected on SGE₁ electrophoresis were due to developmental expression of the different systems. No genetical segregations could be detected in progeny of crosses of the distinct phenotypes. A shikimate dehydrogenase gene (Skdh ₁) that coded for monomers was identified. The diaphorase system was rather complex, but one gene (Dia ₁) coding for monomeric enzymes could be identified. Aconitase was found to be controlled by two independent genes (Aco ₁, Aco ₂), both polymorphic and coding for proteins active as monomers. Tight linkage was found between the genes NADP-Mdh ₁, NADP-Mdh ₂ and NADP-Mdh ₃. Linkage was also found between a pollen fertility restorer (Z) and the Mdh ₁ gene. The identification of linkage with Aco ₁ needs further investigation. R segregated independently from Mdh ₁, Aco ₁ and Dia ₁. Independent segregations were scored for isozyme genes Pgm ₂, Icd ₁, Ak ₁, Gpi ₁, Aco ₁ and Dia ₁.Abbreviations Tris-HCl Tris (hydroxymethyl) aminomethane-HCl - NADP nicotinamide adenine dinucleotide phosphate - NBT nitro-blue tetrazolium chloride monohydrate - PMS phenazine methosulphate     

The metabolic enzyme phosphoglucose isomerase (Pgi) affects the outcome of intra‐specific competition in a polyembryonic wasp

1. Polyembryonic parasitoid wasps in the family Encyrtidae (Hymenoptera) have evolved a caste system consisting of morphologically and functionally distinct larvae called soldiers and reproductives. 2. Two selective pressures are thought to underlie the evolution of the soldier caste: defence against competitors and resolution of the sex ratio conflict. Previous studies also indicate that soldier development time strongly affects the outcome of intra‐specific competition in the polyembryonic encyrtid Copidosoma floridanum Ashmead. This study builds on prior findings by showing that alleles of the metabolic enzyme phosphoglucose isomerase (Pgi) differentially affect soldier development time and the outcome of competition. 3. Soldier larvae with the Pgi alleles 100 or 120 emerged on average 65 h post‐parasitism, whereas soldier larvae with a third allele, 54, emerged at 67 h. In turn, C. floridanum broods homozygous for the 100 and 120 alleles outcompete broods homozygous for the 54 allele. 4. Pgi allelic diversity may be maintained through a life‐history trade‐off affecting female brood sizes with homozygous broods bearing the developmentally disadvantageous 54 allele producing more adult females than broods bearing alternate common alleles.     

Genetic hitchhiking associated with life history divergence and colonization of North America in the European corn borer moth

A primary goal for evolutionary biology is to reveal the genetic basis for adaptive evolution and reproductive isolation. Using Z and E pheromone strains the European corn borer (ECB) moth, I address this problem through multilocus analyses of DNA polymorphism. I find that the locus Triose phosphate isomerase (Tpi) is a statistically significant outlier in coalescent simulations of demographic histories of population divergence, including strict allopatric isolation, restricted migration, secondary contact, and population growth or decline. This result corroborates a previous QTL study that identified the Tpi chromosomal region as a repository for gene(s) contributing to divergence in life history. Patterns of nucleotide polymorphism at Tpi suggest a recent selective sweep and genetic hitchhiking associated with colonization of North America from Europe ~200 generations ago. These results indicate that gene genealogies initially diverge during speciation because of selective sweeps, but differential introgression may play a role in the maintenance of differentiation for sympatric populations.     

Subcellular localization of isozymes of NAD-dependent malate dehydrogenase in sugar beet <Emphasis Type="Italic">Beta vulgaris</Emphasis> L.

Subcellular localization of isozymes of NAD-dependent malate dehydrogenase (MDH) in sugar beet was studied. Isozymes ss and ll controlled by loci Mdh2 and Mdh3, respectively, were shown to locate in mitochondria, whereas isozyme pp controlled by locus Mdh1, in microbodies. All examined samples lack hybrid MDH isozymes, which could testify to the interaction between products of nonallelic Mdh genes. This can be explained by the localization of nonallelic isozymes in various compartments of the cell and organelles.     

Modelling single nucleotide effects in phosphoglucose isomerase on dispersal in the Glanville fritillary butterfly: coupling of ecological and evolutionary dynamics

Dispersal comprises a complex life-history syndrome that influences the demographic dynamics of especially those species that live in fragmented landscapes, the structure of which may in turn be expected to impose selection on dispersal. We have constructed an individual-based evolutionary sexual model of dispersal for species occurring as metapopulations in habitat patch networks. The model assumes correlated random walk dispersal with edge-mediated behaviour (habitat selection) and spatially correlated stochastic local dynamics. The model is parametrized with extensive data for the Glanville fritillary butterfly. Based on empirical results for a single nucleotide polymorphism (SNP) in the phosphoglucose isomerase (Pgi) gene, we assume that dispersal rate in the landscape matrix, fecundity and survival are affected by a locus with two alleles, A and C, individuals with the C allele being more mobile. The model was successfully tested with two independent empirical datasets on spatial variation in Pgi allele frequency. First, at the level of local populations, the frequency of the C allele is the highest in newly established isolated populations and the lowest in old isolated populations. Second, at the level of sub-networks with dissimilar numbers and connectivities of patches, the frequency of C increases with decreasing network size and hence with decreasing average metapopulation size. The frequency of C is the highest in landscapes where local extinction risk is high and where there are abundant opportunities to establish new populations. Our results indicate that the strength of the coupling of the ecological and evolutionary dynamics depends on the spatial scale and is asymmetric, demographic dynamics having a greater immediate impact on genetic dynamics than vice versa.     

Enhanced Synonymous Site Divergence in Positively Selected VertebrateAntimicrobial Peptide Genes

Nonrandom patterns associated with adaptively evolving genes can shed light on how selection and mutation produce rapid changes in sequences. I examine such patterns in two independent families of antimicrobial peptide genes: those in frogs, which are known to have evolved under positive selection, and those in flatfishes, which I show have also evolved under positive selection. I address two recently proposed hypotheses about the molecular evolution of antimicrobial peptide genes. The first is that the mature peptide region is replicated by an error-prone polymerase that increases the mutation rate and the transversion/transition ratio compared to the signal sequence of the same genes. The second is that mature peptides evolve in a coordinated fashion with their propieces, such that a change in net charge in one molecular region prompts an opposite change in charge in the other region. I test these hypotheses using alternative methods that minimize alignment errors, correct for phylogenetic nonindependence, reduce sequence saturation, and account for differing selection pressures on different regions of the gene. In both gene families I show that divergence at both synonymous and nonsynonymous sites within the mature peptide region is enhanced. However, in neither gene family is there evidence of an increased mutational transversion/transition ratio or coordinated evolution. My observations are consistent with either an elevated mutation rate in an adaptively evolving gene region or widespread selection on “silent” sites. These hypotheses challenge the assumption that mutations are random and can be measured by the synonymous substitution rate. [Reviewing Editor: Dr. Willie J. Swanson]     

Fitness differences associated with Pgi SNP genotypes in the Glanville fritillary butterfly (Melitaea cinxia)

Allozyme variation at the phosphoglucose isomerase (PGI) locus in the Glanville fritillary butterfly (Melitaea cinxia) is associated with variation in flight metabolic rate, dispersal rate, fecundity and local population growth rate. To map allozyme to DNA variation and to survey putative functional variation in genomic DNA, we cloned the coding sequence of Pgi and identified nonsynonymous variable sites that determine the most common allozyme alleles. We show that these single‐nucleotide polymorphisms (SNPs) exhibit significant excess of heterozygotes in field‐collected population samples as well as in laboratory crosses. This is in contrast to previous results for the same species in which other allozymes and SNPs were in Hardy–Weinberg equilibrium or exhibited an excess of homozygotes. Our results suggest that viability selection favours Pgi heterozygotes. Although this is consistent with direct overdominance at Pgi, we cannot exclude the possibility that heterozygote advantage is caused by the presence of one or more deleterious alleles at linked loci.     

Detection of regional allozyme divergence in the rocky intertidal goby Chaenogobius annularis

Genetic differentiation in the intertidal goby Chaenogobius annularis was studied using allozyme markers. Samples were collected from six localities along the coasts of Japanese islands and the Korean Peninsula. Six out of 13 loci showed allelic variation in at least one population. Although the predominant alleles of 12 loci were the same among all populations, Mdh-1 showed clear differences among the populations located along the coasts of the Pacific Ocean and the Sea of Japan. These two possible geographic groups, the Pacific Ocean and the Sea of Japan groups, were characterized by diagnostic alleles of Mdh-1, namely Mdh-1 ¹⁰⁰ and Mdh-1 ⁷⁰.     

Genetic control of malate dehydrogenase isozymes in maize

At least six nuclear loci are responsible for the genetic control of malate dehydrogenase (L-malate: NAD oxidoreductase; EC 1.1.1.37; MDH) in coleoptiles of maize. Three independently segregating loci (Mdh1, Mdh2, Mdh3) govern the production of MDH isozymes resistant to inactivation by ascorbic acid and found largely or solely in the mitochondria. A rare recessive allele found at a fourth nuclear locus (mmm) causes increased electrophoretic mobility of the MDH isozymes governed by the Mdh1, Mdh2 and Mdh3 loci.—Two loci (Mdh4, Mdh5) govern MDH isozymes that are selectively inactivated by homogenization in an ascorbic acid solution and that appear to be nonmitochondrial (soluble). Mdh4 and Mdh5 segregate independently of each other and independently of Mdh1, Mdh2 and Mdh3. However, there is close linkage between the migration modifier and Mdh4.——Multiple alleles have been found for all of the Mdh loci except the migration modifier, and electrophoretically "null" or near "null" alleles (as expressed in standardized sections of maize coleoptile) have been found for all loci except Mdh4. Duplicate inheritance commonly occurs for Mdh1 and Mdh2 and also for Mdh4 and Mdh5.——Inter- and intragenic heterodimers are formed between sub-units specified by the three loci governing the mitochondrial MDH isozymes. The same is true of the alleles and nonalleles at the two loci governing the soluble variants. No such heterodimers are formed by interactions between mitochondrial and soluble MDH isozymes.     

Biochemical properties and level of expression of alcohol dehydrogenases in the allotetraploid plant Tragopogon miscellus and its diploid progenitors

This study demonstrates that homoeologous genes in two diploid plant species that specify different amounts of an enzyme maintain the same relative level of expression in an allotetraploid derivative. The three predominant alcohol dehydrogenase (ADH) isozymes (DD, DP, PP) in seeds of the recently evolved allotetraploid plant Tragopogon miscellus (Compositae) are dimers specified by Adh3-D and Adh3-P genes derived from its diploid progenitors T. dubius and T. pratensis. Seeds of T. pratensis contain twice as much ADH activity as those of T. dubius, while T. miscellus is intermediate. The three isozymes were similar in a number of catalytic properties; the densitometric ratio of the isozymes purified from T. miscellus was 1 DD:4DP:4PP for both ADH activity and protein; and dissociation-reassociation of the DP enzyme gave a 1:2:1 ratio of the three isozymes. Therefore, the enzymes were similar in specific activity, but twice as many P as D subunits were present in active enzymes in T. miscellus, precisely the difference in activity between the parents. In T. miscellus, the specific activity of ADH and its activity per mg tissue are intermediate to those of the diploids, because relative expression of the Adh gene in each genome is not influenced by the presence of the other genome.     

A comparison of kinetic parameters obtained with three major non-interconvertible isozymes of rat pyruvate kinase

The kinetic properties of partially purified kidney cortex, liver and muscle isozymes of rat pyruvate kinase (EC 2.7.1.40) were compared. The liver and kidney cortex enzymes were isolated in forms which were homotropically activated by phosphoenolpyruvate and heterotropically activated by fructose-1,6-diphosphate. In the absence of added modulators, the liver enzyme was less active, but both isozymes were fruther inactivated by l-alanine, l-phenylalanine or ATP. The liver enzyme was relatively more sensitive to ATP, but less sensitive to l-phenylalanine. The muscle enzyme, on the other hand, was isolated in a more active form which was insensitive to ATP or l-alanine inhibition and of intermediate sensitivity to l-phenylalanine inhibition. In the presence of l-phenylalanine, muscle enzyme also underwent homotropic and heterotropic activation. Not any of the isozymes were inhibited by NADH.All three isozymes were activated by K⁺ or NH₄⁺. NH₄⁺ was the more effective activator for the kidney cortex or liver enzymes, in the former case because of a greater affinity, the latter because of a higher catalytic efficiency. Of the divalent cations tested only Mg²⁺ and Mn²⁺ activated. All three isozymes had lower maximal rates when activated by Mn²⁺, but this ion also consistently acted as a typical K-type activator.Evidence also was obtained which suggested that the change from one conformational form to another might take minutes and therefore, measured kinetic parameters could reflect conformational as well as catalytic phenomena. This observation, plus suggested independent subunit interactions, were considered to be evidence favoring a sequential rather than a concerted mechanism of conformational transition.     

Lack of Variation at Phosphoglucose Isomerase (Pgi) in Bumblebees: Implications for Conservation Genetics Studies

Assessing genetic variation underlying ecologically important traits is increasingly of interest and importance in population and conservation genetics. For some groups generally useful markers exist for examining the relative role of selection and drift in shaping genetic diversity e.g. the major histocompatibility complex in vertebrates and self-incompatibility loci in plants. For invertebrates there is no such generally useful locus. However, phosphoglucose isomerase (Pgi) has been proposed as a useful functional marker in the conservation genetics of invertebrates. Where thermal microclimate varies, balanced polymorphisms may be maintained due to trade-offs between thermally stable and kinetically advantageous allelic forms. We here report very low levels of Pgi variation in bumblebees rendering this locus to be of little use as an adaptive marker in a conservation genetics context in this group. Potential explanations for this lack of variation are considered.     

Biochemical analysis of reactive oxygen species production and antioxidative responses in unripe avocado (Persea americana Mill var Hass) fruits in response to wounding

We analyzed the production of reactive oxygen species (ROS) and of detoxifying enzymes and enzymes of the ascorbate (ASC) acid cycle in avocado fruit (Pesea Americana Mill cv Hass) in response to wounding. The levels of superoxide anion (O₂ ^?), hydroxyl radicals (OH^.) and hydrogen peroxide (H₂O₂) increased at 15 min and 2 and 15 h post-wounding. Peroxidase (POD) activity had increased to high levels 24 h after wounding; in contrast, catalase and superoxide dismutase (SOD) levels hat decreased significantly at 24 h post-treatment. Basic POD was the major POD form induced, and the levels of at least three apoplastic POD isozymes –increased following wounding. Using specific inhibitors, we characterized one MnSOD and two CuZnSOD isozymes. CuZnSOD activities decreased notably 12 h after treatment. The activities of dehydroascorbate reductase and glutathione reductase increased dramatically following the wounding treatment, possibly as a means to compensate for the redox changes due to ROS production.     

COMPONENTS OF REPRODUCTIVE ISOLATION BETWEEN NORTH AMERICAN PHEROMONE STRAINS OF THE EUROPEAN CORN BORER

Of 12 potential reproductive isolating barriers between closely related Z‐ and E‐pheromone strains of the European corn borer moth (Ostrinia nubilalis), seven significantly reduced gene flow but none were complete, suggesting that speciation in this lineage is a gradual process in which multiple barriers of intermediate strength accumulate. Estimation of the cumulative effect of all barriers resulted in nearly complete isolation (>99%), but geographic variation in seasonal isolation allowed as much as ～10% gene flow. With the strongest barriers arising from mate‐selection behavior or ecologically relevant traits, sexual and natural selection are the most likely evolutionary processes driving population divergence. A recent multilocus genealogical study corroborates the roles of selection and gene flow ( Dopman et al. 2005 ), because introgression is supported at all loci besides Tpi, a sex‐linked gene. Tpi reveals strains as exclusive groups, possesses signatures of selection, and is tightly linked to a QTL that contributes to seasonal isolation. With more than 98% of total cumulative isolation consisting of prezygotic barriers, Z and E strains of ECB join a growing list of taxa in which species boundaries are primarily maintained by the prevention of hybridization, possibly because premating barriers evolve during early stages of population divergence.     

Slow Co-Evolution of the MAGO and Y14 Protein Families Is Required for the Maintenance of Their Obligate Heterodimerization Mode

The exon junction complex (EJC) plays important roles in RNA metabolisms and the development of eukaryotic organisms. MAGO (short form of MAGO NASHI) and Y14 (also Tsunagi or RBM8) are the EJC core components. Their biological roles have been well investigated in various species, but the evolutionary patterns of the two gene families and their protein-protein interactions are poorly known. Genome-wide survey suggested that the MAGO and Y14 two gene families originated in eukaryotic organisms with the maintenance of a low copy. We found that the two protein families evolved slowly; however, the MAGO family under stringent purifying selection evolved more slowly than the Y14 family that was under relative relaxed purifying selection. MAGO and Y14 were obliged to form heterodimer in a eukaryotic organism, and this obligate mode was plesiomorphic. Lack of binding of MAGO to Y14 as functional barrier was observed only among distantly species, suggesting that a slow co-evolution of the two protein families. Inter-protein co-evolutionary signal was further quantified in analyses of the Tol-MirroTree and co-evolution analysis using protein sequences. About 20% of the 41 significantly correlated mutation groups (involving 97 residues) predicted between the two families was clade-specific. Moreover, around half of the predicted co-evolved groups and nearly all clade-specific residues fell into the minimal interaction domains of the two protein families. The mutagenesis effects of the clade-specific residues strengthened that the co-evolution is required for obligate MAGO-Y14 heterodimerization mode. In turn, the obliged heterodimerization in an organism serves as a strong functional constraint for the co-evolution of the MAGO and Y14 families. Such a co-evolution allows maintaining the interaction between the proteins through large evolutionary time scales. Our work shed a light on functional evolution of the EJC genes in eukaryotes, and facilitates to understand the co-evolutionary processes among protein families.     

Identification of amino acid residues responsible for different GTP preferences of human glutamate dehydrogenase isozymes

Human glutamate dehydrogenase isozymes (hGDH1 and hGDH2) differ markedly in their inhibition by GTP. These regulatory preferences must arise from amino acid residues that are not common between hGDH isozymes. We have constructed chimeric enzymes by reciprocally switching the corresponding amino acid segments 390-465 in hGDH isozymes that are located within or near the C-terminal 48-residue antenna helix, which is thought to be part of the regulatory domain of mammalian GDHs. These resulted in triple mutations in amino acid sequences at 415, 443, and 456 sites that are not common between hGDH1 and hGDH2. The chimeric enzymes did not change their enzyme efficiency (k_cat/K_m) and expression level. Functional analyses, however, revealed that the chimeric mutants almost completely acquired the different GTP regulatory preference between hGDH isozymes. These results suggest that the 415, 443, and 456 residues acting in concert are responsible for the GTP inhibitory properties of hGDH isozymes.     

Significance of Population Size on the Fixation of Nonsynonymous Mutations in Genes Under Varying Levels of Selection Pressure

Previous studies observed a higher ratio of divergences at nonsynonymous and synonymous sites (ω = d_N/d_S) in species with a small population size compared to that estimated for those with a large population size. Here we examined the theoretical relationship between ω, effective population size (N_e), and selection coefficient (s). Our analysis revealed that when purifying selection is high, ω of species with small N_e is much higher than that of species with large N_e. However the difference between the two ω reduces with the decline in selection pressure (s → 0). We examined this relationship using primate and rodent genes and found that the ω estimated for highly constrained genes of primates was up to 2.9 times higher than that obtained for their orthologous rodent genes. Conversely, for genes under weak purifying selection the ω of primates was only 17% higher than that of rodents. When tissue specificity was used as a proxy for selection pressure we found that the ω of broadly expressed genes of primates was up to 2.1-fold higher than that of their rodent counterparts and this difference was only 27% for tissue specific genes. Since most of the nonsynonymous mutations in constrained or broadly expressed genes are deleterious, fixation of these mutations is influenced by N_e. This results in a higher ω of these genes in primates compared to those from rodents. Conversely, the majority of nonsynonymous mutations in less-constrained or tissue-specific genes are neutral or nearly neutral and therefore fixation of them is largely independent of N_e, which leads to the similarity of ω in primates and rodents.     

The Evolution of Selfing Is Accompanied by Reduced Efficacy of Selection and Purging of Deleterious Mutations

The transition from outcrossing to selfing is predicted to reduce the genome-wide efficacy of selection because of the lower effective population size (N_e) that accompanies this change in mating system. However, strongly recessive deleterious mutations exposed in the homozygous backgrounds of selfers should be under strong purifying selection. Here, we examine estimates of the distribution of fitness effects (DFE) and changes in the magnitude of effective selection coefficients (N_es) acting on mutations during the transition from outcrossing to selfing. Using forward simulations, we investigated the ability of a DFE inference approach to detect the joint influence of mating system and the dominance of deleterious mutations on selection efficacy. We investigated predictions from our simulations in the annual plant Eichhornia paniculata, in which selfing has evolved from outcrossing on multiple occasions. We used range-wide sampling to generate population genomic datasets and identified nonsynonymous and synonymous polymorphisms segregating in outcrossing and selfing populations. We found that the transition to selfing was accompanied by a change in the DFE, with a larger fraction of effectively neutral sites (N_es < 1), a result consistent with the effects of reduced N_e in selfers. Moreover, an increased proportion of sites in selfers were under strong purifying selection (N_es > 100), and simulations suggest that this is due to the exposure of recessive deleterious mutations. We conclude that the transition to selfing has been accompanied by the genome-wide influences of reduced N_e and strong purifying selection against deleterious recessive mutations, an example of purging at the molecular level.