Recognition Loci Research Articles

Influence of Trp-Cage on the Function and Stability of GLP-1R Agonist Exenatide Derivatives.

Exenatide (Ex4), a GLP-1 incretin mimetic polypeptide, is an effective therapeutic agent against diabetes and obesity. We highlight the indirect role of Ex4's structure-stabilizing Trp-cage (Tc) motif in governing GLP-1 receptor (GLP-1R) signal transduction. We use various Ex4 derivatives to explore how Tc compactness influences thermal stability, aggregation, enhancement of insulin secretion, and GLP-1R binding. We found that Ex4 variants decorated with fortified Tc motifs exhibit increased resistance to unfolding and aggregation but show an inverse relationship between the bioactivity and stability. Molecular dynamics simulations coupled with a rigid-body segmentation protocol to analyze dynamic interconnectedness revealed that the constrained Tc motifs remain intact within the receptor-ligand complexes but interfere with one of the major stabilizing contacts and recognition loci on the extracellular side of GLP-1R, dislodging the N-terminal activating region of the hormone mimetics, and restrict the free movement of TM6, the main signal transduction device of GLP-1R.

QTL mapping reveals mating type gene LeHD1 regulating mycelial growth in shiitake mushroom, Lentinula edodes

Mycelial growth plays a vital role in substrate colonization, and understanding the genetic mechanisms of mycelial growth is of great significance for the fundamental and applied interest in mushrooms. The existence of conserved pleiotropic genes for mating type and mycelial growth in Agaricales is worthy to be penetratingly explored. Herein in Lentinula edodes, we refined an 1176 kb QTL-hotspot region regulating mycelial growth rate by quantitative trait loci (QTL) mapping. The mating gene LeHD1 encoding homeodomain type 1 transcription factor was found in this QTL-hotspot, and was prioritized as a candidate gene. We then cloned and characterized LeHD1 in L. edodes. The coding sequence of the LeHD1 is 2650 bp in length and encodes 808 amino acids. To further explore the function of LeHD1 in mycelial growth, vectors for LeHD1-overexpression (OE) and LeHD1-RNAi were constructed and transformed into shiitake strain WX1. Overexpression of LeHD1 increased mycelial growth rate by 8.6% to 18.5%. Contrary to this, interference of LeHD1 caused an 8.5% to 26.8% decrease in mycelial growth rate. Taken together, these results demonstrate that LeHD1 promotes mycelial growth in L. edodes. To our knowledge, LeHD1 was the first mating homeodomain gene identified to regulate mycelia growth in mushrooms providing additional insights into the association between mycelial growth and sexual recognition loci.

Search for MHC/TCR-Like Systems in Living Organisms.

Highly polymorphic loci evolved many times over the history of species. These polymorphic loci are involved in three types of functions: kind recognition, self-incompatibility, and the jawed vertebrate adaptive immune system (AIS). In the first part of this perspective, we reanalyzed and described some cases of polymorphic loci reported in the literature. There is a convergent evolution within each functional category and between functional categories, suggesting that the emergence of these self/non-self recognition loci has occurred multiple times throughout the evolutionary history. Most of the highly polymorphic loci are coding for proteins that have a homophilic interaction or heterophilic interaction between linked loci, leading to self or non-self-recognition. The highly polymorphic MHCs, which are involved in the AIS have a different functional mechanism, as they interact through presented self or non-self-peptides with T cell receptors, whose diversity is generated by somatic recombination. Here we propose a mechanism called “the capacity of recognition competition mechanism” that might contribute to the evolution of MHC polymorphism. We propose that the published cases corresponding to these three biological categories represent a small part of what can be found throughout the tree of life, and that similar mechanisms will be found many times, including the one where polymorphic loci interact with somatically generated loci.

Communicate and Fuse: How Filamentous Fungi Establish and Maintain an Interconnected Mycelial Network.

Cell-to-cell communication and cell fusion are fundamental biological processes across the tree of life. Survival is often dependent upon being able to identify nearby individuals and respond appropriately. Communication between genetically different individuals allows for the identification of potential mating partners, symbionts, prey, or predators. In contrast, communication between genetically similar (or identical) individuals is important for mediating the development of multicellular organisms or for coordinating density-dependent behaviors (i.e., quorum sensing). This review describes the molecular and genetic mechanisms that mediate cell-to-cell communication and cell fusion between cells of Ascomycete filamentous fungi, with a focus on Neurospora crassa. Filamentous fungi exist as a multicellular, multinuclear network of hyphae, and communication-mediated cell fusion is an important aspect of colony development at each stage of the life cycle. Asexual spore germination occurs in a density-dependent manner. Germinated spores (germlings) avoid cells that are genetically different at specific loci, while chemotropically engaging with cells that share identity at these recognition loci. Germlings with genetic identity at recognition loci undergo cell fusion when in close proximity, a fitness attribute that contributes to more rapid colony establishment. Communication and cell fusion also occur between hyphae in a colony, which are important for reinforcing colony architecture and supporting the development of complex structures such as aerial hyphae and sexual reproductive structures. Over 70 genes have been identified in filamentous fungi (primarily N. crassa) that are involved in kind recognition, chemotropic interactions, and cell fusion. While the hypothetical signal(s) and receptor(s) remain to be described, a dynamic molecular signaling network that regulates cell-cell interactions has been revealed, including two conserved MAP-Kinase cascades, a conserved STRIPAK complex, transcription factors, a NOX complex involved in the generation of reactive oxygen species, cell-integrity sensors, actin, components of the secretory pathway, and several other proteins. Together these pathways facilitate the integration of extracellular signals, direct polarized growth, and initiate a transcriptional program that reinforces signaling and prepares cells for downstream processes, such as membrane merger, cell fusion and adaptation to heterokaryon formation.

Balancing selection at nonself recognition loci in the chestnut blight fungus, Cryphonectria parasitica, demonstrated by trans-species polymorphisms, positive selection, and even allele frequencies.

Balancing selection has been inferred in diverse organisms for nonself recognition genes, including those involved in immunity, mating compatibility, and vegetative incompatibility. Although selective forces maintaining polymorphisms are known for genes involved in immunity and mating, mechanisms of balancing selection for vegetative incompatibility genes in fungi are being debated. We hypothesized that allorecognition and its consequent inhibition of virus transmission contribute to the maintenance of polymorphisms in vegetative incompatibility loci (vic) in the chestnut blight fungus, Cryphonectria parasitica. Balancing selection was demonstrated at two loci, vic2 and vic6, by trans-species polymorphisms in C. parasitica, C. radicalis, and C. japonica and signatures of positive selection in gene sequences. In addition, more than half (31 of 54) of allele frequency estimates at six vic loci in nine field populations of C. parasitica from Asia and the eastern US were not significantly different from 0.5, as expected at equilibrium for two alleles per locus under balancing selection. At three vic loci, deviations from 0.5 were predicted based on the effects of heteroallelism on virus transmission. Twenty-five of 27 allele frequency estimates were greater than or equal to 0.5 for the allele that confers significantly stronger inhibition of virus transmission at three loci with asymmetric transmission. These results are consistent with the allorecognition hypothesis that vegetative incompatibility genes are under selection because of their role in reducing infection by viruses.

Duplicated pollen-pistil recognition loci control intraspecific unilateral incompatibility in Brassica rapa.

In plants, cell-cell recognition is a crucial step in the selection of optimal pairs of gametes to achieve successful propagation of progeny. Flowering plants have evolved various genetic mechanisms, mediated by cell-cell recognition, to enable their pistils to reject self-pollen, thus preventing inbreeding and the consequent reduced fitness of progeny (self-incompatibility, SI), and to reject foreign pollen from other species, thus maintaining species identity (interspecific incompatibility)1. In the genus Brassica, the SI system is regulated by an S-haplotype-specific interaction between a stigma-expressed female receptor (S receptor kinase, SRK) and a tapetum cell-expressed male ligand (S locus protein 11, SP11), encoded by their respective polymorphic genes at the S locus2-6. However, the molecular mechanism for recognition of foreign pollen, leading to reproductive incompatibility, has not yet been identified. Here, we show that recognition between a novel pair of proteins, a pistil receptor SUI1 (STIGMATIC UNILATERAL INCOMPATIBILITY 1) and a pollen ligand PUI1 (POLLEN UNILATERAL INCOMPATIBILITY 1), triggers unilateral reproductive incompatibility between plants of two geographically distant self-incompatible Brassica rapa lines, even though crosses would be predicted to be compatible based on the S haplotypes of pollen and stigma. Interestingly, SUI1 and PUI1 are similar to the SI genes, SRK and SP11, respectively, and are maintained as cryptic incompatibility genes in these two populations. The duplication of the SRK and SP11 followed by reciprocal loss in different populations would provide a molecular mechanism of the emergence of a reproductive barrier in allopatry.

Quantifying the coevolutionary potential of multistep immune defenses.

Coevolutionary models often assume host infection by parasites depends on a single bout of molecular recognition. As detailed immunological studies accumulate, however, it becomes increasingly apparent that the outcome of host-parasite interactions more generally depends on complex multiple step infection processes. For example, in plant and animal innate immunity, recognition steps are followed by downstream effector steps that kill recognized parasites, with the outcome depending on an escalatory molecular arms race. Here, we explore the consequences of such multistep infection processes for coevolution using a genetically explicit model. Model analyses reveal that polymorphism is much greater at recognition loci than effector loci, that host-genotype by parasite-genotype (Gh × Gp) interactions are larger for the recognition step, and that the recognition step contributes more to local adaptation than the effector step. These results suggest that (1) local adaptation is more likely when fitness measures are related to recognition versus downstream effectors, (2) effector loci, while mechanistically important, are less likely to harbor the Gh × Gp variation that fuels coevolution, and (3) recognition loci are better candidates for genomic hotspots of coevolution.

Assortative Mating Drives Linkage Disequilibrium between Sperm and Egg Recognition Protein Loci in the Sea Urchin Strongylocentrotus purpuratus

Sperm and eggs have interacting proteins on their surfaces that influence their compatibility during fertilization. These proteins are often polymorphic within species, producing variation in gamete affinities. We first demonstrate the fitness consequences of various sperm bindin protein (Bindin) variants in the sea urchin Strongylocentrotus purpuratus, and assortative mating between males and females based on their sperm Bindin genotype. This empirical finding of assortative mating based on sperm Bindin genotype could arise by linkage disequilibrium (LD) between interacting sperm and egg recognition loci. We then examine sequence variation in eight exons of the sea urchin egg receptor for sperm Bindin (EBR1). We find little evidence of LD among the eight exons of EBR1, yet strong evidence for LD between sperm Bindin and EBR1 overall, and varying degrees of LD between sperm Bindin among the eight exons. We reject the alternate hypotheses of LD driven by shared evolutionary histories, population structure, or close physical linkage between these interacting loci on the genome. The most parsimonious explanation for this pattern of LD is that it represents selection driven by assortative mating based on interactions among these sperm and egg loci. These findings indicate the importance of ongoing sexual selection in the maintenance of protein polymorphisms and LD, and more generally highlight how LD can be used as an indication of current mate choice, as opposed to historic selection.

Joint evolution of kin recognition and cooperation in spatially structured rhizobium populations.

In the face of costs, cooperative interactions maintained over evolutionary time present a central question in biology. What forces maintain this cooperation? Two potential ways to explain this problem are spatially structured environments (kin selection) and kin-recognition (directed benefits). In a two-locus population genetic model, we investigated the relative roles of spatial structure and kin recognition in the maintenance of cooperation among rhizobia within the rhizobia-legume mutualism. In the case where the cooperative and kin recognition loci are independently inherited, spatial structure alone maintains cooperation, while kin recognition decreases the equilibrium frequency of cooperators. In the case of co-inheritance, spatial structure remains a stronger force, but kin recognition can transiently increase the frequency of cooperators. Our results suggest that spatial structure can be a dominant force in maintaining cooperation in rhizobium populations, providing a mechanism for maintaining the mutualistic nodulation trait. Further, our model generates unique and testable predictions that could be evaluated empirically within the legume-rhizobium mutualism.

Highly expressed genes in gonads of the bat star Patiria miniata: gene ontology, expression differences, and gamete recognition loci

AbstractEvolutionary analysis of mating systems in broadcast‐spawning marine animals and plants has focused on sperm‐ and egg‐surface proteins and glycopeptides that mediate reproductive interactions at different stages of gamete recognition. Improved understanding of the ecology and evolution of such interactions depends on extending our knowledge to multiple genes expressed in both sperm and eggs of diverse taxonomic groups with different modes of fertilization. Here, we use readily accessible next‐generation sequencing methods and desktop bioinformatics to characterize the repertoire of highly expressed genes in testes and ovaries of the asterinid sea star Patiria miniata, including gene ontology annotations for male‐ and female‐expressed molecules, and descriptions of two genes that encode egg‐surface molecules involved in fertilization that have not previously been studied in sea stars. The results are used to contrast expression differences between the testis and ovary, and to develop hypotheses of gamete‐specific expression. We also explore differences in ovary gene expression among multiple females from northern and southern populations that show nucleotide differentiation at many non‐expressed loci and at a gamete recognition locus.

Geographic variation in allele frequency of the gamete recognition protein M7 lysin throughout a mosaic blue mussel hybrid zone

Divergence at gamete recognition loci is hypothesized to result in speciation in broadcast spawning invertebrates. Many gamete recognition loci evolve quickly and show patterns of positive selection, yet the advantage of divergence is rarely known. M7 lysin is a sperm protein in the Mytilus edulis species complex that shows evidence of adaptive evolution. This locus is polymorphic with two distinct clades within Mytilus galloprovincialis, one of which, the D clade, shows the strongest signal of positive selection. We tested whether the geographic patterns in allele frequency were consistent with the hypothesis that positive selection on D clade alleles (G D ) was due to reinforcement. Populations of M. edulis showed little evidence of introgression of G D alleles, but there was no consistent evidence of reproductive character displacement. The lack of consistent patterns expected of reinforcement suggests that another mechanism is likely responsible for the adaptive divergence of M7 lysin.

Crozier’s paradox revisited: maintenance of genetic recognition systems by disassortative mating

BackgroundOrganisms are predicted to behave more favourably towards relatives, and kin-biased cooperation has been found in all domains of life from bacteria to vertebrates. Cooperation based on genetic recognition cues is paradoxical because it disproportionately benefits individuals with common phenotypes, which should erode the required cue polymorphism. Theoretical models suggest that many recognition loci likely have some secondary function that is subject to diversifying selection, keeping them variable.ResultsHere, we use individual-based simulations to investigate the hypothesis that the dual use of recognition cues to facilitate social behaviour and disassortative mating (e.g. for inbreeding avoidance) can maintain cue diversity over evolutionary time. Our model shows that when organisms mate disassortatively with respect to their recognition cues, cooperation and recognition locus diversity can persist at high values, especially when outcrossed matings produce more surviving offspring. Mating system affects cue diversity via at least four distinct mechanisms, and its effects interact with other parameters such as population structure. Also, the attrition of cue diversity is less rapid when cooperation does not require an exact cue match. Using a literature review, we show that there is abundant empirical evidence that heritable recognition cues are simultaneously used in social and sexual behaviour.ConclusionsOur models show that mate choice is one possible resolution of the paradox of genetic kin recognition, and the literature review suggests that genetic recognition cues simultaneously inform assortative cooperation and disassortative mating in a large range of taxa. However, direct evidence is scant and there is substantial scope for future work.

LOW RATES OF BINDIN CODON EVOLUTION IN LECITHOTROPHIC HELIOCIDARIS SEA URCHINS

Life-history variables including egg size affect the evolutionary response to sexual selection in broadcast-spawning sea urchins and other marine animals. Such responses include high or low rates of codon evolution at gamete recognition loci that encode sperm- and egg-surface peptides. Strong positive selection on such loci affects intraspecific mating success and interspecific reproductive divergence (and may play a role in speciation). Here, we analyze adaptive codon evolution in the sperm acrosomal protein bindin from a brooding sea urchin (Heliocidaris bajulus, with large eggs and nonfeeding or lecithotrophic larval development) and compare our results to previously published data for two closely related congeners. Purifying selection and low relative rates of bindin nonsynonymous substitution in H. bajulus were significantly different from selectively neutral bindin evolution in H. erythrogramma despite similar large egg size in those two species, but were similar to the background rate of nonsynonymous bindin substitution for other closely related sea urchins (including H. tuberculata, all with small egg size and feeding planktonic larval development). Bindin evolution is not driven by egg size variation among Heliocidaris species, but may be more consistent with an alternative mechanism based on the effects of high or low spatial density of conspecific mates.

Evolutionary Consequences of Introgression at M7 Lysin, a Gamete Recognition Locus, Following Secondary Contact Between Blue Mussel Species

Hybridizing populations of blue mussels, Mytilus edulis and Mytilus trossulus, in Cobscook Bay (eastern Maine) have been used by our laboratory to study the evolution of gamete incompatibility and molecular evolution of the vitelline coat lysin proteins expressed in sperm. The M7 lysin locus has been the most studied of the three lysins, but evidence for positive selection necessary to help confirm its role in gamete recognition in western Atlantic hybrid zones is contradictory. We developed an alternative test, based on rates of introgression at M7 lysin. Contrary to expectations, introgression at this locus is much higher (instead of much lower) than is introgression at neutral markers. In this article, we present simulations, constructed using synthetic populations of combinations of admixed genotypes, representing various hybrid offspring categories. Simulations produced variation in introgression across loci, but did not generate the massive introgression at M7 lysin observed in natural populations in Cobscook Bay. We consider these results in the context of selection operating on gamete recognition loci, both within and between species, during the third stage of allopatric speciation in Mytilus.

Evolution and Diversity of a Fungal Self/Nonself Recognition Locus

BackgroundSelf/nonself discrimination is an essential feature for pathogen recognition and graft rejection and is a ubiquitous phenomenon in many organisms. Filamentous fungi, such as Neurospora crassa, provide a model for analyses of population genetics/evolution of self/nonself recognition loci due to their haploid nature, small genomes and excellent genetic/genomic resources. In N. crassa, nonself discrimination during vegetative growth is determined by 11 heterokaryon incompatibility (het) loci. Cell fusion between strains that differ in allelic specificity at any of these het loci triggers a rapid programmed cell death response.Methodology/Principal FindingsIn this study, we evaluated the evolution, population genetics and selective mechanisms operating at a nonself recognition complex consisting of two closely linked loci, het-c (NCU03493) and pin-c (NCU03494). The genomic position of pin-c next to het-c is unique to Neurospora/Sordaria species, and originated by gene duplication after divergence from other species within the Sordariaceae. The het-c pin-c alleles in N. crassa are in severe linkage disequilibrium and consist of three haplotypes, het-c1/pin-c1, het-c2/pin-c2 and het-c3/pin-c3, which are equally frequent in population samples and exhibit trans-species polymorphisms. The absence of recombinant haplotypes is correlated with divergence of the het-c/pin-c intergenic sequence. Tests for positive and balancing selection at het-c and pin-c support the conclusion that both of these loci are under non-neutral balancing selection; other regions of both genes appear to be under positive selection. Our data show that the het-c2/pin-c2 haplotype emerged by a recombination event between the het-c1/pin-c1 and het-c3/pin-c3 approximately 3–12 million years ago.Conclusions/SignificanceThese results support models by which loci that confer nonself discrimination form by the association of polymorphic genes with genes containing HET domains. Distinct allele classes can emerge by recombination and positive selection and are subsequently maintained by balancing selection and divergence of intergenic sequence resulting in recombination blocks between haplotypes.

How effective is recognition of siblings on the basis of genotype?

The ability to recognize kin based on genetic markers has been widely proposed as a mechanism to facilitate altruistic behaviour and inbreeding avoidance. Siblings are an important group of relatives to discriminate from unrelated individuals but present a problem, because siblings can share 0, 1 or 2 alleles at any single recognition locus. Here, we present a Bayesian model of kin recognition that defines the potential for genotypic information to convey kinship. Under the direct comparison model, where the signaller's genotype is compared with that of the receiver, the odds ratio that a pair of individuals were siblings was substantially increased if they shared both alleles at a single locus, but only a minority of siblings were recognized; increasing the number of recognition loci used could not increase both the odds ratio and the proportion of siblings recognized. A maternal comparison model, where the signaller's genotype is compared with that of the receiver's mother, performed poorly when only a single recognition locus was considered, but became increasingly effective with more recognition loci. Nevertheless, incorporating partial-matching information across multiple, independent loci are likely to be difficult. Further empirical work needs to establish the mechanistic basis of genetic kin recognition used by different taxa.

Genetic linkage map for Amylostereum areolatum reveals an association between vegetative growth and sexual and self-recognition

Amylostereum areolatum is a filamentous fungus that grows through tip extension, branching and hyphal fusion. In the homokaryotic phase, the hyphae of different individuals are capable of fusing followed by heterokaryon formation, only if they have dissimilar allelic specificities at their mating-type ( mat) loci. In turn, hyphal fusion between heterokaryons persists only when they share the same alleles at all of their heterokaryon incompatibility ( het) loci. In this study we present the first genetic linkage map for A. areolatum, onto which the mat and het loci, as well as quantitative trait loci (QTLs) for mycelial growth rate are mapped. The recognition loci ( mat-A and het-A) are positioned near QTLs associated with mycelial growth, suggesting that the genetic determinants influencing recognition and growth rate in A. areolatum are closely associated. This was confirmed when isolates associated with specific mat and het loci displayed significantly different mycelial growth rates. Although the link between growth and sexual recognition has previously been observed in other fungi, this is the first time that an association between growth and self-recognition has been shown.

A ‘dirty’ business: testing the limitations of terminal restriction fragment length polymorphism (TRFLP) analysis of soil fungi

Terminal restriction fragment length polymorphism (TRFLP) is an increasingly popular method in molecular ecology. However, several key limitations of this method have not been fully examined especially when used to study fungi. We investigated the impact of spore contamination, intracollection ribosomal DNA internal transcribed spacer (ITS) region variation, and conserved restriction enzyme recognition loci on the results produced by TRFLP to characterize soil fungal communities. We find that (i) the potential for nontarget structures such as spores to contribute DNA to target sample extractions is high; (ii) multiple fragments (i.e. 'extra peaks') per PCR primer-restriction enzyme combination can be detected that are caused by restriction enzyme inefficiency and intracollection ribosomal DNA ITS variation; and (iii) restriction enzyme digestion in conserved vs. variable gene regions leads to different characterizations of community diversity. Based on these results, we suggest that studies employing TRFLP need to include information from known, identified fungi from sites within which studies take place and not to rely only on TRFLP profiles as a short cut to fungal community description.

Binding of norbinaltorphimine (norBNI) congeners to wild-type and mutant mu and kappa opioid receptors: molecular recognition loci for the pharmacophore and address components of kappa antagonists.

Molecular modifications of both the kappa opioid antagonist norbinaltorphimine (norBNI, 1) and the kappa receptor have provided evidence that the selectivity of this ligand is conferred through ionic interaction if its N17' protonated amine group (an "address") with a nonconserved acidic residue (Glu297) on the kappa receptor. In the present study, we have examined the effect of structural modifications on the affinity of norBNI analogues for wild-type and mutant kappa and mu opioid receptors expressed in COS-7 cells. Compounds 2, 3, and 7, which have an antagonist pharmacophore and basic N17' group in common with norBNI, retained high affinity for the wild-type kappa but exhibited greatly reduced affinity for mutant kappa receptors (E297K and E297A). Modification of the phenolic or N-substituent groups of the antagonist pharmacophore (4 and 5) or removal of basicity at the address N17' center (6) led to greatly reduced affinity for the wild-type and mutant receptors. The reduced affinity upon modification of the kappa receptor is consistent with the ionic interaction of the protonated N17' group of kappa antagonists (1-3, 7) with the carboxylate group of E297 at the top of TM6. This was supported by the greatly enhanced affinity of compounds 1-3 for the mutant mu receptor (K303E), as compared to the wild-type mu receptor, given that residue K303 occupies a position equivalent to that of E297 in the kappa receptor. In view of the high degree of homology of the seven TM domains of the kappa and mu opioid receptors, it is suggested that the antagonist pharmacophore is bound within this highly conserved region of the kappa or mutant mu receptor and that an anionic residue at the top of TM6 (E297 or K303E, respectively) provides additional binding affinity.

All males are not created equal: fertility differences depend on gamete recognition polymorphisms in sea urchins.

Behaviors, morphologies, and genetic loci directly involved in reproduction have been increasingly shown to be polymorphic within populations. Explaining how such variants are maintained by selection is crucial to understanding the genetic basis of fertility differences, but direct tests of how alleles at reproductive loci affect fertility are rare. In the sea urchin genus Echinometra, the protein bindin mediates sperm attachment to eggs, evolves quickly, and is polymorphic within species. Eggs exposed to experimental sperm mixtures show strong discrimination on the basis of the males' bindin genotype. Different females produce eggs that nonrandomly select sperm from different males, showing that variable egg-sperm interactions determine fertility. Eggs select sperm with a bindin genotype similar to their own, suggesting strong linkage between female choice and male trait loci. These experiments demonstrate that alleles at a single locus can have a strong effect on fertilization and that reproductive loci may retain functional polymorphisms through epistatic interactions between male and female traits. They also suggest that positive selection at gamete recognition loci like bindin involves strong selection within species on mate choice interactions.