Positive Directional Selection Research Articles

Precopulatory sexual selection in the seed bug Lygaeus equestris: a comparison of choice and no-choice paradigms

Mate choice has long been appreciated as a key component of sexual selection. However, how we measure mate choice, both in the field and in the laboratory, remains problematic. Mating preferences may be tested using either no-choice or choice tests, but explicit comparisons between these two experimental paradigms remain limited. It has been suggested that preferences may be stronger in choice tests as they allow simultaneous comparison, and some studies have indeed found stronger mating preferences in choice tests compared to no-choice tests. Here we explicitly tested the effect of experimental choice paradigm on the measurement of sexual selection on male and female morphology in the promiscuous seed bug Lygaeus equestris (Heteroptera, Lygaeidae). We performed mating trials in which we varied the amount of choice presented to each sex, giving four choice treatments: no-choice, male choice, female choice and mutual choice. Overall we found evidence for significant positive directional selection on female body length and stabilizing selection on an overall measure of male body size. However, we found no significant effect of choice paradigm on the patterns of sexual selection for males or females. We suggest this may be because of the method of mate assessment in L. equestris, which appears to be primarily via contact cues, which may limit simultaneous comparison between options. • We investigated sexual selection on male and female morphology in a lygaeid bug. • We varied the level of choice available to males and females during mating trials. • We found significant sexual selection on male and female morphology. • However we found no effect of choice paradigm on the strength of selection. • The mechanism of mate choice in this species may limit comparison between options.

Two mitochondrial genes under episodic positive selection in subterranean octodontoid rodents

Tuco-tucos (Ctenomys) and related coruros (Spalacopus) are South American subterranean rodents. An energetically demanding lifestyle within the hypoxic, underground atmosphere may change the selective regime on oxidative phosphorylation. We examined whether weak and/or episodic positive directional selection affected the evolution of two mitochondrial genes (COX2, CytB), in a background of purifying selection in these lineages. We estimated rates of synonymous (dS) and non-synonymous (dN) substitutions and found: 1) significantly higher dN/dS ratio in subterranean groups relative to non-subterranean related species, and 2) two codons in each gene under episodic selection: 94 and 277 of COX2 and 269 and 307 of CytB.

Phenotypic selection on the staminal lever mechanism in Salvia digitaloides (Labiaceae)

The staminal lever mechanism of pollination in Salvia has been considered as a key innovation, which was proposed to contribute to adaptive radiation of the genus. Considering its sophisticated interaction with pollinators, we hypothesized that relevant floral traits in the lever mechanism might be subject to intense selection in natural populations. In the present study, Salvia digitaloides was employed to investigate phenotypic selection on the floral traits. The results indicated that there were large variations in the floral structural traits and spatial position of the stigma. No significant selection was detected on the plausible key trait, i.e. the stamen lever length. Strong stabilizing selection acted consistently on the size of corolla entrance; that is, the intermediate corolla entrance was at a selective advantage. Both corolla length and style exsertion were subject to opposite directional selection in both years; and corolla entrance height was subject to disruptive selection, which was attributed to divergent adaptation of the trait to different body-sized pollinators. Furthermore, correlational selection acted on a few pairs of traits which described corolla entrance size and stigma position. Flower production was consistently subject to significantly positive directional selection in both years. In conclusion, it was most likely that floral traits involved in the staminal lever mechanism evolved in a functionally integrated manner for the enhancement of pollination efficiency in the natural population of S. digitaloides.

A general linear model-based approach for inferring selection to climate

BackgroundMany efforts have been made to detect signatures of positive selection in the human genome, especially those associated with expansion from Africa and subsequent colonization of all other continents. However, most approaches have not directly probed the relationship between the environment and patterns of variation among humans. We have designed a method to identify regions of the genome under selection based on Mantel tests conducted within a general linear model framework, which we call MAntel-GLM to Infer Clinal Selection (MAGICS). MAGICS explicitly incorporates population-specific and genome-wide patterns of background variation as well as information from environmental values to provide an improved picture of selection and its underlying causes in human populations.ResultsOur results significantly overlap with those obtained by other published methodologies, but MAGICS has several advantages. These include improvements that: limit false positives by reducing the number of independent tests conducted and by correcting for geographic distance, which we found to be a major contributor to selection signals; yield absolute rather than relative estimates of significance; identify specific geographic regions linked most strongly to particular signals of selection; and detect recent balancing as well as directional selection.ConclusionsWe find evidence of selection associated with climate (P < 10-5) in 354 genes, and among these observe a highly significant enrichment for directional positive selection. Two of our strongest 'hits’, however, ADRA2A and ADRA2C, implicated in vasoconstriction in response to cold and pain stimuli, show evidence of balancing selection. Our results clearly demonstrate evidence of climate-related signals of directional and balancing selection.

Indirect costs counteract the effects of pollinator-mediated phenotypic selection on corolla size in the Mediterranean shrub Halimium atriplicifolium

Larger corollas receive more pollinator visits but involve higher production and maintenance costs, especially under hot and dry conditions. This can result in indirect costs on reproductive output, which may counteract the effects of pollinator-mediated phenotypic selection on corolla size. In this study, I explored the relationship between corolla size and indirect costs and whether these costs counteract the effects of pollinator-mediated phenotypic selection on this trait in the Mediterranean shrub Halimium atriplicifolium. I hypothesized that (i) corolla production entails direct costs in dry mass, N and P, (ii) corollas entail significant indirect costs in terms of fruit and seed production, (iii) indirect costs increase with corolla size, (iv) this species may suffer pollen limitation to a certain degree and (v) indirect costs counteract the effects of pollinator-mediated selection on corolla size. I compared fruit set and seed production of petal-removed flowers (R flowers) and unmanipulated control flowers (C flowers) and evaluated the influence of individual mean corolla size on relative fruit and seed gain of R compared to C flowers. I also estimated phenotypic selection on corolla size mediated by indirect costs and the combined effect of costs and pollinators (i.e. total selection). Corollas allocated sizeable amount of resources in terms of dry mass relative to the other floral structures. Fruit set and seed per fruit were significantly higher in R flowers, while individual mean corolla size showed a positive relationship with relative fruit gain. Phenotypic selection analysis revealed cost-mediated negative directional selection and absence of positive directional total selection on corolla size through fruit set. This translated into stabilizing total selection. These results suggest that Mediterranean environments can impose constraints on corolla size, counteracting advantages of larger corollas from the pollination point of view with increased indirect costs of such flowers.

Phenotypic Selection Favors Missing Trait Combinations in Coexisting Annual Plants

Trade-offs among traits are important for maintaining biodiversity, but the role of natural selection in their construction is not often known. It is possible that trade-offs reflect fundamental constraints, negative correlational selection, or directional selection operating on costly, redundant traits. In a Sonoran Desert community of winter annual plants, we have identified a trade-off between relative growth rate and water-use efficiency among species, such that species with high relative growth rate have low water-use efficiency and vice versa. We measured selection on water-use efficiency, relative growth rate, and underlying traits within populations of four species at two study sites with different average climates. Phenotypic trait correlations within species did not match the among-species trade-off. In fact, for two species with high water-use efficiency, individuals with high relative growth rate also had high water-use efficiency. All populations experienced positive directional selection for water-use efficiency and relative growth rate. Selection tended to be stronger on water-use efficiency at the warmer and drier site, and selection on relative growth rate tended to be stronger at the cooler and wetter site. Our results indicate that directional natural selection favors a phenotype not observed among species in the community, suggesting that the among-species trade-off could be due to pervasive genetic constraints, perhaps acting in concert with processes of community assembly.

Genetic diversity and signatures of selection of drug resistance in Plasmodium populations from both human and mosquito hosts in continental Equatorial Guinea

BackgroundIn Plasmodium, the high level of genetic diversity and the interactions established by co-infecting parasite populations within the same host may be a source of selection on pathogen virulence and drug resistance. As different patterns have already been described in humans and mosquitoes, parasite diversity and population structure should be studied in both hosts to properly assess their effects on infection and transmission dynamics. This study aimed to characterize the circulating populations of Plasmodium spp and Plasmodium falciparum from a combined set of human blood and mosquito samples gathered in mainland Equatorial Guinea. Further, the origin and evolution of anti-malarial resistance in this area, where malaria remains a major public health problem were traced.MethodsPlasmodium species infecting humans and mosquitoes were identified by nested-PCR of chelex-extracted DNA from dried blood spot samples and mosquitoes. Analysis of Pfmsp2 gene, anti-malarial-resistance associated genes, Pfdhps, Pfdhfr, Pfcrt and Pfmdr1, neutral microsatellites (STR) loci and Pfdhfr and Pfdhps flanking STR was undertaken to evaluate P. falciparum diversity.ResultsPrevalence of infection remains high in mainland Equatorial Guinea. No differences in parasite formula or significant genetic differentiation were seen in the parasite populations in both human and mosquito samples. Point mutations in all genes associated with anti-malarial resistance were highly prevalent. A high prevalence was observed for the Pfdhfr triple mutant in particular, associated with pyrimethamine resistance.Analysis of Pfdhps and Pfdhfr flanking STR revealed a decrease in the genetic diversity. This finding along with multiple independent introductions of Pfdhps mutant haplotypes suggest a soft selective sweep and an increased differentiation at Pfdhfr flanking microsatellites hints a model of positive directional selection for this gene.ConclusionsChloroquine is no longer recommended for malaria treatment in Equatorial Guinea but sulphadoxine-pyrimethamine (SP) remains in use in combination with artesunate and is the only drug recommended in preventive chemotherapy in pregnancy. The high prevalence of point mutations in Pfdhfr and Pfdhps points to the danger of an eventual reduction in the efficacy of SP combined therapy in P. falciparum populations in Equatorial Guinea and to the essential continuous monitoring of these two genes.

Fluctuating viability selection on morphology of cliff swallows is driven by climate

The extent to which fluctuating selection can maintain evolutionary stasis in most populations remains an unresolved question in evolutionary biology. Climate has been hypothesized to drive reversals in the direction of selection among different time periods and may also be responsible for intense episodic selection caused by rare weather events. We measured viability selection associated with morphological traits in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska, USA, over a 14-year period following a rare climatic event. We used mark-recapture to estimate the annual apparent survival of over 26000 individuals whose wing, tail, tarsus and bill had been measured. The fitness functions associated with tarsus length and bill dimensions fluctuated depending on annual climate conditions on the birds' breeding grounds. The oscillating yearly patterns may have slowed and occasionally reversed directional change in trait trajectories, although there was a trend over time for all traits except tarsus to increase in size. The net positive directional selection on some traits, despite periodic climate-associated fluctuations, suggests that cliff swallow morphology in the population is likely to keep changing and supports recent work contending that selection in general does not fluctuate enough to be an effective driver of stasis.

Phenotypic selection on leaf functional traits of two congeneric species in a temperate rainforest is consistent with their shade tolerance

Several studies across species have linked leaf functional traits with shade tolerance. Because evolution by natural selection occurs within populations, in order to explain those interspecific patterns it is crucial to examine variation of traits associated with shade tolerance and plant fitness at an intraspecific scale. In a southern temperate rainforest, two climbing plant species coexist but differ in shade tolerance. Whereas Luzuriaga radicans is most abundant in the shaded understory, L. polyphylla typically occurs in intermediate light environments. We carried out an intraspecific approach to test the hypothesis of differential selection patterns in relation to shade tolerance in these congeneric species. The probability of showing reproductive structures increased with specific leaf area (SLA) in L. polyphylla, and decreased with dark respiration in L. radicans. When reproductive output of fertile individuals was the fitness variable, we detected positive directional selection on SLA in L. polyphylla, and negative directional selection on dark respiration and positive directional selection on leaf size in L. radicans. Total light radiation differed between the microsites where the Luzuriaga species were sampled in the old-growth forest understory. Accordingly, L. radicans had a lower minimum light requirement and showed fertile individuals in darker microsites. L. radicans showed lower dark respiration, higher chlorophyll content, and greater leaf size and SLA than L. polyphylla. Results suggest that in more shade-tolerant species, established in the darker microsites, selection would favor functional traits minimizing carbon losses, while in less shade-tolerant species, plants displaying leaf traits enhancing light capture would be selected.

Overlapping genetic codes for overlapping frameshifted genes in Testudines, and Lepidochelys olivacea as special case

Mitochondrial genes code for additional proteins after +2 frameshifts by reassigning stops to code for amino acids, which defines overlapping genetic codes for overlapping genes. Turtles recode stops UAR→Trp and AGR→Lys (AGR→Gly in the marine Olive Ridley turtle, Lepidochelys olivacea). In Lepidochelys the +2 frameshifted mitochondrial Cytb gene lacks stops, open reading frames from other genes code for unknown proteins, and for regular mitochondrial proteins after frameshifts according to the overlapping genetic code. Lepidochelys’ inversion between proteins coded by regular and overlapping genetic codes substantiates the existence of overlap coding. ND4 differs among Lepidochelys mitochondrial genomes: it is regular in DQ486893; in NC_011516, the open reading frame codes for another protein, the regular ND4 protein is coded by the frameshifted sequence reassigning stops as in other turtles. These systematic patterns are incompatible with Genbank/sequencing errors and DNA decay. Random mixing of synonymous codons, conserving main frame coding properties, shows optimization of natural sequences for overlap coding; Ka/Ks analyses show high positive (directional) selection on overlapping genes. Tests based on circular genetic codes confirm programmed frameshifts in ND3 and ND4l genes, and predicted frameshift sites for overlap coding in Lepidochelys. Chelonian mitochondria adapt for overlapping gene expression: cloverleaf formation by antisense tRNAs with predicted anticodons matching stops coevolves with overlap coding; antisense tRNAs with predicted expanded anticodons (frameshift suppressor tRNAs) associate with frameshift-coding in ND3 and ND4l, a potential regulation of frameshifted overlap coding. Anaeroby perhaps switched between regular and overlap coding genes in Lepidochelys.

Herbivores Modify Selection on Plant Functional Traits in a Temperate Rainforest Understory

There is limited evidence regarding the adaptive value of plant functional traits in contrasting light environments. It has been suggested that changes in these traits in response to light availability can increase herbivore susceptibility. We tested the adaptive value of plant functional traits linked with carbon gain in contrasting light environments and also evaluated whether herbivores can modify selection on these traits in each light environment. In a temperate rainforest, we examined phenotypic selection on functional traits in seedlings of the pioneer tree Aristotelia chilensis growing in sun (canopy gap) and shade (forest understory) and subjected to either natural herbivory or herbivore exclusion. We found differential selection on functional traits depending on light environment. In sun, there was positive directional selection on photosynthetic rate and relative growth rate (RGR), indicating that selection favors competitive ability in a high-resource environment. Seedlings with high specific leaf area (SLA) and intermediate RGR were selected in shade, suggesting that light capture and conservative resource use are favored in the understory. Herbivores reduced the strength of positive directional selection acting on SLA in shade. We provide the first demonstration that natural herbivory rates can change the strength of selection on plant ecophysiological traits, that is, attributes whose main function is resource uptake. Research addressing the evolution of shade tolerance should incorporate the selective role of herbivores.

Selective processes and adaptive evolution of the cytochrome b gene in salamanders of the genus Salamandrella

Sequence analysis of the cytochrome b gene fragment in the salamanders of the genus Salamandrella, Siberian salamander and Schrenk salamander was performed with the purpose to elucidate the effect of natural selection on the evolution of mitochondrial DNA (mtDNA) in these species. It was demonstrated that despite of notable influence of negative selection (expressed as very low dN/dS values), speciation and intraspecific divergence in salamanders was accompanied by the appearance of radical amino acid substitutions, caused by the influence of positive (directional) selection. To examine the evolutionary pattern of synonymous mtDNA sites, distribution of conservative and non-conservative substitutions was analyzed. The rates of conservative and non-conservative substitutions were nearly equal, pointing to neutrality of mutation process at synonymous mtDNA sites of salamanders. Analysis of conservative and non-conservative synonymous substitution distributions in different parts of phylogenetic trees showed that the differences between the synonymous groups compared were statistically significant only in one phylogenetic group of Siberian salamander (haplogroup C) (P = 0.02). In the group of single substitutions, located at terminal phylogenetic branches of Siberian salamanders from group C, increased rate of conservative substitutions was observed. Based on these findings, it was suggested that selective processes could have an influence on the formation of the synonymous substitution profile in the Siberian salamander mtDNA fragment examined.

Phycoerythrin evolution and diversification of spectral phenotype in marine Synechococcus and related picocyanobacteria

In marine Synechococcus there is evidence for the adaptive evolution of spectrally distinct forms of the major light harvesting pigment phycoerythrin (PE). Recent research has suggested that these spectral forms of PE have a different evolutionary history than the core genome. However, a lack of explicit statistical testing of alternative hypotheses or for selection on these genes has made it difficult to evaluate the evolutionary relationships between spectral forms of PE or the role horizontal gene transfer (HGT) may have had in the adaptive phenotypic evolution of the pigment system in marine Synechococcus. In this work, PE phylogenies of picocyanobacteria with known spectral phenotypes, including newly co-isolated strains of marine Synechococcus from the Gulf of Mexico, were constructed to explore the diversification of spectral phenotype and PE evolution in this group more completely. For the first time, statistical evaluation of competing evolutionary hypotheses and tests for positive selection on the PE locus in picocyanobacteria were performed. Genes for PEs associated with specific PE spectral phenotypes formed strongly supported monophyletic clades within the PE tree with positive directional selection driving evolution towards higher phycourobilin (PUB) content. The presence of the PUB-lacking phenotype in PE-containing marine picocyanobacteria from cyanobacterial lineages identified as Cyanobium is best explained by HGT into this group from marine Synechococcus. Taken together, these data provide strong examples of adaptive evolution of a single phenotypic trait in bacteria via mutation, positive directional selection and horizontal gene transfer.

Selection pressures have driven population differentiation of domesticated and wild common carp (Cyprinus carpio L.)

Selection pressures are the principle evolutionary forces for the genetic differentiation of populations. Recent changes in selection pressures on mitochondrial DNA and microsatellite have been described in a wide variety of organisms. The common carp (Cyprinus carpio) has experienced strong selection pressure, in particular artificial selection, during its domestication. However, the contribution and extent of artificial selection in driving genome-wide population differentiation remain unclear. We investigated the genetic differentiation of 4 domesticated strains (Xingguo red common carp, Glass red common carp, Purse red common carp, and Jian common carp, which have been generated by artificial selection since 1970s) and 2 wild populations (Shishou section in Hubei and Yangzhou section in Jiangsu of the Yangtze River) of common carp in China by sequencing the mitochondrial DNA D-loop and by genotyping 10 microsatellite loci. It was found that the domesticated strains exhibited linkage disequilibrium within the population and less genetic variability, higher inbreeding coefficients (F(IS) = 0.101 vs 0.038), and higher genetic differentiation (F(ST) = 0.087 vs 0.001) than the wild populations, which indicates strong selection pressures in the process of domestication. Of the 10 loci, 5 appeared to be under positive directional selection in the domesticated strains, and all 10 loci in wild populations were potentially under balancing selection. We conclude that strong selection pressures, artificial selection in particular, have caused genetic differentiation between populations of domesticated and wild common carp.

Rhizome of life, catastrophes, sequence exchanges, gene creations, and giant viruses: how microbial genomics challenges Darwin

Darwin's theory about the evolution of species has been the object of considerable dispute. In this review, we have described seven key principles in Darwin's book The Origin of Species and tried to present how genomics challenge each of these concepts and improve our knowledge about evolution. Darwin believed that species evolution consists on a positive directional selection ensuring the “survival of the fittest.” The most developed state of the species is characterized by increasing complexity. Darwin proposed the theory of “descent with modification” according to which all species evolve from a single common ancestor through a gradual process of small modification of their vertical inheritance. Finally, the process of evolution can be depicted in the form of a tree. However, microbial genomics showed that evolution is better described as the “biological changes over time.” The mode of change is not unidirectional and does not necessarily favors advantageous mutations to increase fitness it is rather subject to random selection as a result of catastrophic stochastic processes. Complexity is not necessarily the completion of development: several complex organisms have gone extinct and many microbes including bacteria with intracellular lifestyle have streamlined highly effective genomes. Genomes evolve through large events of gene deletions, duplications, insertions, and genomes rearrangements rather than a gradual adaptative process. Genomes are dynamic and chimeric entities with gene repertoires that result from vertical and horizontal acquisitions as well as de novo gene creation. The chimeric character of microbial genomes excludes the possibility of finding a single common ancestor for all the genes recorded currently. Genomes are collections of genes with different evolutionary histories that cannot be represented by a single tree of life (TOL). A forest, a network or a rhizome of life may be more accurate to represent evolutionary relationships among species.

Phylogeny of the Tachycineta genus of New World swallows: Insights from complete mitochondrial genomes

The Tachycineta genus of swallows is comprised of nine species that range from Alaska to southern Chile. We sequenced the entire mitochondrial genome of each member of Tachycineta and generated a completely resolved phylogenetic hypothesis for the corresponding mitochondrial gene tree. Our analyses confirm the presence of two sub-clades within Tachycineta that are associated with geography: a North American/Caribbean clade and a South/Central American clade. We found considerable variation among regions of the mitochondrial genome in both substitution rates and the level of information that each region supplied for phylogenetic reconstruction. We found no evidence of positive directional selection within mitochondrial coding regions, but we identified numerous sites under purifying selection. This finding suggests that, despite differences in life history traits and distributions, mitochondrial genes in Tachycineta are predominantly under purifying selection for conserved function.

Ontogenetic contingency of tolerance mechanisms in response to apical damage.

Plants are able to tolerate tissue loss through vigorous branching which is often triggered by release from apical dominance and activation of lateral meristems. However, damage-induced branching might not be a mere physiological outcome of released apical dominance, but an adaptive response to environmental signals, such as damage timing and intensity. Here, branching responses to both factors were examined in the annual plant Medicago truncatula. Branching patterns and allocation to reproductive traits were examined in response to variable clipping intensities and timings in M. truncatula plants from two populations that vary in the onset of reproduction. Phenotypic selection analysis was used to evaluate the strength and direction of selection on branching under the damage treatments. Plants of both populations exhibited an ontogenetic shift in tolerance mechanisms: while early damage induced greater meristem activation, late damage elicited investment in late-determined traits, including mean pod and seed biomass, and supported greater germination rates. Severe damage mostly elicited simultaneous development of multiple-order lateral branches, but this response was limited to early damage. Selection analyses revealed positive directional selection on branching in plants under early- compared with late- or no-damage treatments. The results demonstrate that damage-induced meristem activation is an adaptive response that could be modified according to the plant's developmental stage, severity of tissue loss and their interaction, stressing the importance of considering these effects when studying plastic responses to apical damage.

Testing Mechanisms of Bergmann’s Rule: Phenotypic Decline but No Genetic Change in Body Size in Three Passerine Bird Populations

Bergmann's rule predicts a decrease in body size with increasing temperature and has much empirical support. Surprisingly, we know very little about whether "Bergmann size clines" are due to a genetic response or are a consequence of phenotypic plasticity. Here, we use data on body size (mass and tarsus length) from three long-term (1979-2008) study populations of great tits (Parus major) that experienced a temperature increase to examine mechanisms behind Bergmann's rule. We show that adult body mass decreased over the study period in all populations and that tarsus length increased in one population. Both body mass and tarsus length were heritable and under weak positive directional selection, predicting an increase, rather than a decrease, in body mass. There was no support for microevolutionary change, and thus the observed declines in body mass were likely a result of phenotypic plasticity. Interestingly, this plasticity was not in direct response to temperature changes but seemed to be due to changes in prey dynamics. Our results caution against interpreting recent phenotypic body size declines as adaptive evolutionary responses to temperature changes and highlight the importance of considering alternative environmental factors when testing size clines.

Viability selection on body size in a non-marine ostracod

One of the most important research topics in evolutionary ecology is body size evolution. Actually, several hypotheses have been proposed to explain the many observed patterns—also known as “rules”—of body size variation in across latitude, temperature, and time. The temperature–size rule (TSR), describes an inverse relationship between body size and temperature. We took advantage of the “natural laboratory” that the crustacean populations at the Chilean altiplano offers, to study the TSR in ostracods. We studied three populations of Limnocythere atacamae that are physically separated by several kilometers, and differ mainly by their permanent thermal regime. We found larger individuals in the hotspring compared to the cold ponds. Also, in the hotspring we found a significant quadratic selection coefficient, suggesting stabilizing selection in this population. The fitness profiles showed stabilizing selection in the hotspring, and positive directional selection in the ponds. Our results suggest the existence of an optimal body size above the population means. This optimal size is apparently attained in the hotspring population. Then, natural selection appears to be promoting a shift in the mean phenotype that, for some reason, is not attained in the cold environments. Genetic slippage and population bottleneck would explain this absence of response to selection.

Reproductive protein evolution in two cryptic species of marine chordate

BackgroundReproductive character displacement (RCD) is a common and taxonomically widespread pattern. In marine broadcast spawning organisms, behavioral and mechanical isolation are absent and prezygotic barriers between species often operate only during the fertilization process. Such barriers are usually a consequence of differences in the way in which sperm and egg proteins interact, so RCD can be manifest as faster evolution of these proteins between species in sympatry than allopatry. Rapid evolution of these proteins often appears to be a consequence of positive (directional) selection. Here, we identify a set of candidate gamete recognition proteins (GRPs) in the ascidian Ciona intestinalis and showed that these GRPs evolve more rapidly than control proteins (those not involved in gamete recognition). Choosing a subset of these gamete recognition proteins that show evidence of positive selection (CIPRO37.40.1, CIPRO60.5.1, CIPRO100.7.1), we then directly test the RCD hypothesis by comparing divergence (omega) and polymorphism (McDonald-Kreitman, Tajima's D, Fu and Li's D and F, Fay and Wu's H) statistics in sympatric and allopatric populations of two distinct forms of C. intestinalis (Types A and B) between which there are strong post-zygotic barriers.ResultsCandidate gamete recognition proteins from two lineages of C. intestinalis (Type A and B) are evolving more rapidly than control proteins, consistent with patterns seen in insects and mammals. However, ω (dN/dS) is not significantly different between the sympatric and allopatric populations, and none of the polymorphism statistics show significant differences between sympatric and allopatric populations.ConclusionsEnhanced prezygotic isolation in sympatry has become a well-known feature of gamete recognition proteins in marine broadcast spawners. But in most cases the evolutionary process or processes responsible for this pattern have not been identified. Although gamete recognition proteins in C. intestinalis do appear to evolve more rapidly, on average, than proteins with other functions, rates of evolution are not different in allopatric and sympatric populations of the two reproductively isolated forms. That sympatry is probably human-mediated, and therefore recent, may explain the absence of RCD.