Selection with gene-cytoplasm interactions. I. Maintenance of cytoplasm polymorphisms

General conditions for the protectedness of gene-cytoplasm polymorphisms are considered for a biallelic model with two cytoplasm types and under the assumption that nuclear polymorphisms cannot be maintained in the presence of only one cytoplasm type. Analytical results involving male fertilities, female fertilities, viabilities and selfing rates are obtained, and numerical results show spiral and cyclic behavior of population trajectories. It is shown that a maternally inherited cytoplasmic polymorphism cannot be maintained in the absence of a nuclear polymorphism, and that a gene-cytoplasm polymorphism can only be maintained if the population shows sexual asymmetry, i.e. , if the ratio of male to female fertility varies among genotypes. Thus, the classical viability selection model does not allow gene-cytoplasm polymorphisms.     

Identification of sterility-inducing cytoplasms in rye using the plasmotype–genotype interaction test and newly developed SCAR markers

A series of 25 rye (Secale cereale L.) inbred lines was tested with respect to three mitochondrial sequence-characterized amplified region (SCAR) polymorphisms. The analysis revealed a close association between the marker-determined mitotypes and plasmotypes (cytoplasm types known from breeding data) represented by the inbreds. The mitochondrial markers also confirmed normal (N-) cytoplasmic character of three wild rye species: Secale montanum, S. vavilovii and S. kuprijanovii. For 186 plants from open-pollinated cultivars of Turkish and South American origin, cytoplasm identification was performed with the use of crossing with double non-restoring tester. In 77 plants the normal (N) cytoplasm was detected, and for 63 of these the PCR analysis was performed producing results which were consistent with the genetic data based on testcrossing and phenotype assessment. The mitochondrial markers also confirmed a presence of sterility-inducing cytoplasm in the remaining 109 plants. Moreover, the markers allowed for differentiation between Pampa (P-) and Vavilovii (V-) cytoplasmic individuals. For 60 plants the latter results were verified using crosses with a line maintaining P-cytoplasmic sterility and acting as a restorer of the V-cytoplasm. For two of these plants contradicting results were produced with the applied methods of cytoplasm identification and the basis of this discrepancy is discussed. Regardless of the identification method, widespread occurrence of a sterility-inducing cytoplasm was revealed, especially in South American populations.     

Comparative ecology of sexual and asexual gecko species (Lepidodactylus) in French Polynesia

Summary The asexual geckoLepidodactylus lugubris, its sexual congenerL. sp. (Takapoto) and hybrids between the two species inhabit the atoll of Takapoto, providing a natural experiment for studying co-existence and interactions between asexual and sexual populations. The range of the sexual species is confined to one section of the lagoon beach and the trees and buildings which abut it, whereas the asexual is distributed across the whole atoll and occupies many habitats. Behavioural experiments revealed no asymmetry in levels of aggression between the two species, suggesting that the confinement ofL. sp. (Takapoto) to the lagoon beach is not due to agonistic interactions. Ecological differences among the constituent clones of the asexual species exist but cannot completely account for the broader habitat use of the asexual. Within a single habitat, one clone ofL. lugubris consumes a wider range of prey items than its sexual relative. Other studies have found that the asexuals are extremely heterozygous relative to the sexuals; we hypothesize that their broad ecological tolerance may be attributable to heterosis. The co-existence of the sexual and parthenogenetic lizards on this small island seems to be stable and may be facilitated by the specialization of the sexual taxon to beach habitats.     

Interphase Nuclei of Many Mammalian Cell Types Contain Deep, Dynamic, Tubular Membrane-bound Invaginations of the Nuclear Envelope

The nuclear envelope consists of a doublemembraned extension of the rough endoplasmic reticulum. In this report we describe long, dynamic tubular channels, derived from the nuclear envelope, that extend deep into the nucleoplasm. These channels show cell-type specific morphologies ranging from single short stubs to multiple, complex, branched structures. Some channels transect the nucleus entirely, opening at two separate points on the nuclear surface, while others terminate at or close to nucleoli. These channels are distinct from other topological features of the nuclear envelope, such as lobes or folds.

The channel wall consists of two membranes continuous with the nuclear envelope, studded with features indistinguishable from nuclear pore complexes, and decorated on the nucleoplasmic surface with lamins. The enclosed core is continuous with the cytoplasm, and the lumenal space between the membranes contains soluble ER-resident proteins (protein disulphide isomerase and glucose-6-phosphatase).

Nuclear channels are also found in live cells labeled with the lipophilic dye DiOC₆. Time-lapse imaging of DiOC₆-labeled cells shows that the channels undergo changes in morphology and spatial distribution within the interphase nucleus on a timescale of minutes.

The presence of a cytoplasmic core and nuclear pore complexes in the channel walls suggests a possible role for these structures in nucleo–cytoplasmic transport. The clear association of a subset of these structures with nucleoli would also be consistent with such a transport role.

     

Intergenomic epistasis for fitness: within-population interactions between cytoplasmic and nuclear genes in Drosophila melanogaster

The symbiotic relationship between the mitochondrial and nuclear genomes coordinates metabolic energy production and is fundamental to life among eukaryotes. Consequently, there is potential for strong selection to shape interactions between these two genomes. Substantial research attention has focused on the possibility that within-population sequence polymorphism in mitochondrial DNA (mtDNA) is maintained by mitonuclear fitness interactions. Early theory predicted that selection will often eliminate mitochondrial polymorphisms. However, recent models demonstrate that intergenomic interactions can promote the maintenance of polymorphism, especially if the nuclear genes involved are linked to the X chromosome. Most empirical studies to date that have assessed cytonuclear fitness interactions have studied variation across populations and it is still unclear how general and strong such interactions are within populations. We experimentally tested for cytonuclear interactions within a laboratory population of Drosophila melanogaster using 25 randomly sampled cytoplasmic genomes, expressed in three different haploid nuclear genetic backgrounds, while eliminating confounding effects of intracellular bacteria (e.g., Wolbachia). We found sizable cytonuclear fitness interactions within this population and present limited evidence suggesting that these effects were sex specific. Moreover, the relative fitness of cytonuclear genotypes was environment specific. Sequencing of mtDNA (2752 bp) revealed polymorphism within the population, suggesting that the observed cytoplasmic genetic effects may be mitochondrial in origin.     

Selection with Gene-Cytoplasm Interactions. II. Maintenance of Gynodioecy

Gynodioecy is apparently frequently inherited through gene-cytoplasm interactions. General conditions for the protectedness of gene-cytoplasm polymorphisms for a biallelic model with two cytoplasm types were obtained previously, and these are applied to seven special cases of gene-cytoplasm interactions controlling gynodioecy and involving dominance. It is assumed that nuclear polymorphisms cannot be maintained in one cytoplasm type only. It is held that pure cytoplasmic inheritance of gynodioecy without nuclear interactions is unlikely, and it is shown that gynodioecy with gene-cytoplasm interactions is easier to establish than purely nuclear gynodioecy, for monogenic biallelic dominant or recessive inheritance. For three special cases, a resource-allocation model with simple assumptions always leads to conditions for protectedness of gynodioecy.     

Selection caused by self-fertilization I. Four measures of self-fertilization and their effects on fitness

The standard models of selfing in seed plants consider only the ovules, which are assumed to have a constant selfing rate. It has recently become clear, however, that hermaphrodite or monoecious populations frequently show sexual asymmetry (nonconstant pollen:ovule fertilities among individuals). Such asymmetry usually results in pollen selfing rates which differ from those for the ovules and are frequency-dependent even for constant ovule selfing rates. A recent study of selfing rates for all gametes of an individual is extended here to include four selfing rates (for ovules, pollen, all gametes, and zygotes), and simple mathematical relationships linking the four rates are obtained. Unlike earlier models of selfing, it is not assumed that the ovule selfing rate is constant, but instead that this rate, like all the others, is determined by the mobility of the pollen, which in turn is determined by the floral biology and ecology. It is found that all four selfing rates are usually frequency-dependent. The selfing rate for all gametes (the combined selfing rate) is usually intermediate between those for the ovules and pollen, and the zygotic rate is usually the smallest of the four. The exceptions to the above statements occur for relatively extreme situations, such as complete selfing for pollen or ovules, no selfing, or sexual symmetry. Three modes of selfing are considered: prior (PS), competing (CS), and delayed (DS) self-fertilization. It is shown that if there are at least two types with different selfing rates in the population, then the ranking of their selfing rates may depend upon the frequencies of the types (for the combined and the zygotic rates), may be frequency-independent (ovule rate), or may be dependent or independent, according to the mode of selfing (pollen rate). The effects of the various influences on the amount of selfing are by no means negligible. Thus a numerical study shows pollen selfing rates for one type which vary from 0.09 to 0.96, according to its frequency. Another numerical result shows a change in combined selfing rate from 0.13 to 0.86, depending solely on the mode of selfing. Results for Scots Pine show that an ovule selfing rate of 0.5 was accompanied by a combined rate of 0.143.The population selfing rate is not the same as the mean of individual selfing rates, and can only be obtained if female fitnesses as well as ovule selfing rates are known for each type.Previous models of selfing have failed to distinguish between the effects of increased selfing and increased pollen fertility, with the result that increased selfing always resulted in greater fitness. In the present models the two effects are distinguishable, and it is found that increased selfing may result in increased or decreased fitness, depending also on population density and on a form of pollen density. Thus the old dogma that in the absence of viability and fertility selection increased selfing always results in increased fitness is finally refuted, and the importance of the influence of ecological parameters on selfing and fitness is emphasized, since population density and pollen density influence the selfing rates.     

Temperature-specific outcomes of cytoplasmic-nuclear interactions on egg-to-adult development time in seed beetles

The integration of the mitochondrial and nuclear genomes coordinates cellular energy production and is fundamental to life among eukaryotes. Therefore, there is potential for strong selection to shape the interactions between the two genomes. Several studies have now demonstrated that epistatic interactions between cytoplasmic and nuclear genes for fitness can occur both at a "within" and "across" population level. Genotype-by-environment interactions are common for traits that are encoded by nuclear genes, but the effects of environmental heterogeneity on traits that are partly encoded by cytoplasmic genes have received little attention despite the fact that there are reasons to believe that phenotypic effects of cytoplasmic genetic variation may often be environment specific. Consequently, the importance of environmental heterogeneity to the outcomes of cyto-nuclear fitness interactions and to the maintenance of mitochondrial polymorphism is unclear. Here, we assess the influence of temperature on cyto-nuclear effects on egg-to-adult development time in seed beetles (Callosobruchus maculatus). We employed an "across-population" design, sourcing beetles from five distinct populations and using backcrossing to create orthogonal combinations of distinct introgression lines, fixed for their cytoplasmic and nuclear lineages. We then assayed development times at two different temperatures and found sizeable cyto-nuclear effects in general, as well as temperature- and block-specific cyto-nuclear effects. These results demonstrate that environmental factors such as temperature do exert selection on cytoplasmic genes by favoring specific cyto-nuclear genetic combinations, and are consistent with the suggestion that complex genotype-by-environment interactions may promote the maintenance of polymorphism in mitochondrial genes.     

Analysis of partial assortative mating and sexual selection models for a polygamous species involving a trait based on levels of heterozygosity

The consequences of preferential mating in the presence of partial assortative and sexual selection mechanisms are ascertained for a two-allele one-locus trait involving two phenotype classes C₁ = {all homozygotes} and C₂ = {heterozygotes}. Relevant biological cases may include Burley (1977, Proc. Nat. Acad. Sci. USA74, 3476–3479), Wilbur et al. (1978, Evolution32, 264–270), and Singh and Zouros (1978, Evolution32, 342–353). When the preference rate for the heterozygote exceeds that for homozygotes, it is established that the unique stable state is the central Hardy-Weinberg equilibrium. The rate of approach is faster with sexual selection than for the corresponding model of assortative mating. When the preference rates favor the homozygotes then in this symmetric model of sexual selection two asymmetric Hardy-Weinberg polymorphisms can evolve, and which succeeds depends on initial conditions. The models are also analyzed with natural selection acting on phenotypes superimposed on assortative mating. In this case we can have up to three coexisting stable states involving both fixation alternatives and a central polymorphism. The corresponding model with sexual selection maintains either the central equilibrium as in assortative mating or two asymmetric polymorphic equilibria.     

Promyelocytic leukemia bodies tether to early endosomes during mitosis

During mitosis the nuclear envelope breaks down, leading to potential interactions between cytoplasmic and nuclear components. PML bodies are nuclear structures with tumor suppressor and antiviral functions. Early endosomes, on the other hand, are cytoplasmic vesicles involved in transport and growth factor signaling. Here we demonstrate that PML bodies form stable interactions with early endosomes immediately following entry into mitosis. The 2 compartments remain stably associated throughout mitosis and dissociate in the cytoplasm of newly divided daughter cells. We also show that a minor subset of PML bodies becomes anchored to the mitotic spindle poles during cell division. The study demonstrates a stable mitosis-specific interaction between a cytoplasmic and a nuclear compartment.     

Frequency-dependent selection in hermaphrodites: the rule rather than the exception

It is shown for simple assumptions that frequency-dependent selection in seed plants may have at least three causes, namely: (a) sexual asymmetry; (b) differential genotypic selfing rates; and (c) gene-cytoplasm fertility interactions. Sexual asymmetry is defined as the situation where the ratio of pollen: ovule fertility is not the same for all genotypes or phenotypes in a population. A review of the literature suggests that all three causes of frequency-dependent selection probably occur in nature, and that at least the first cause, sexual asymmetry, is probably widespread in hermaphrodites. It is shown that resource-allocation theory leads to the expectation that sexual asymmetry and, therefore, frequency-dependent selection should be almost universal in hermaphrodite populations which are not largely selfed. A method for obtaining frequency-dependent fitness values is presented and applied to a population of Scots pine ( Pinus sylvestris ) showing strong sexual asymmetry.     

Selection in plant populations of effectively infinite size II. Protectedness of a biallelic polymorphism

The biologically important problem of protectedness of genetic polymorphisms in monoecious plant populations exhibiting genotypically determined variation in rates of self-fertilization and sexually asymmetrical fertilities has hitherto escaped exact, analytical treatment for the reason that appropriate mathematical techniques relying on allelic frequencies do not seem to exist. For the particular case of one locus and two alleles it was possible to develop such a technique which provides conditions of high precision for protectedness of an allele. A comparison of the results with those already known from models that appear to be specializations of the present model showed that some of the earlier conclusions can be generalized, while others have to be handled with great care or should even be rejected. Above all, this concerns the role of self-fertilization, which is frequently considered to counteract the establishment of genetic polymorphisms. However, it turned out that increasing the heterozygote selfing rate also increases protectedness for both alleles in all situations. Moreover, even if the amount of self-fertilization is the same for all genotypes, asymmetry in the production of ovules and pollen, which is more the rule than an exception, may imply protectedness only for comparatively large selfing rates. The probably most outstanding finding is that, depending on the ovule and pollen fertilities, protectedness may be realized only within small ranges of selfing rates, and these ranges may vary from arbitrarily low to arbitrarily high rates. On the other hand, if the ovule fertilities show strong overdominance for the heterozygote—more precisely, if the heterozygote produces more than twice as many ovules as either of the homozygotes—both alleles are protected irrespective of the pollen fertilities and rates of self-fertilization; this generalizes earlier results obtained for more specific models.     

Interactions between cytoplasmic and nuclear genomes confer sex‐specific effects on lifespan in Drosophila melanogaster

Genetic variation outside of the cell nucleus can affect the phenotype. The cytoplasm is home to the mitochondria, and in arthropods often hosts intracellular bacteria such as Wolbachia. Although numerous studies have implicated epistatic interactions between cytoplasmic and nuclear genetic variation as mediators of phenotypic expression, two questions remain. Firstly, it remains unclear whether outcomes of cyto‐nuclear interactions will manifest differently across the sexes, as might be predicted given that cytoplasmic genomes are screened by natural selection only through females as a consequence of their maternal inheritance. Secondly, the relative contribution of mitochondrial genetic variation to other cytoplasmic sources of variation, such as Wolbachia infection, in shaping phenotypic outcomes of cyto‐nuclear interactions remains unknown. Here, we address these questions, creating a fully crossed set of replicated cyto‐nuclear populations derived from three geographically distinct populations of Drosophila melanogaster, measuring the lifespan of males and females from each population. We observed that cyto‐nuclear interactions shape lifespan and that the outcomes of these interactions differ across the sexes. Yet, we found no evidence that placing the cytoplasms from one population alongside the nuclear background of others (generating putative cyto‐nuclear mismatches) leads to decreased lifespan in either sex. Although it was difficult to partition mitochondrial from Wolbachia effects, our results suggest at least some of the cytoplasmic genotypic contribution to lifespan was directly mediated by an effect of sequence variation in the mtDNA. Future work should explore the degree to which cyto‐nuclear interactions result in sex differences in the expression of other components of organismal life history.     

MITOCHONDRIAL GENOTYPES HAVE NO DETECTABLE EFFECTS ON MERISTIC TRAITS IN CUTTHROAT TROUT HYBRID SWARMS

Efforts to detect effects of cytoplasmic genes are confounded by the problem of partitioning nuclear and cytoplasmic effects. In this study we test for effects of mtDNA haplotype on development in hybrid populations of cutthroat trout (Oncorhynchus clarki) with randomly associated nuclear and mtDNA genotypes. We have previously described several intraspecific hybrid swarms formed by interbreeding of westslope cutthroat trout (O. c. lewisi) and Yellowstone cutthroat trout (O. c. bouvieri). Genetic distance between these subspecies is high (Nei's D = 0.30; mtDNA P = 0.02), and diagnostic alleles at multiple nuclear loci and two distinct mtDNA haplotypes are present in the hybrids. Historical associations between alleles at nuclear loci and between cytotypes and nuclear alleles have largely decayed. We test for differences in meristic characters between fish with alternate mtDNA haplotypes. Counts and fluctuating bilateral asymmetry for these characters have been previously shown to be sensitive indicators of genetic differences that affect development. No differences were found between mtDNA types in meristic counts or fluctuating asymmetry. Therefore, the alternate mtDNA haplotypes had no detectable effect on development as measured by meristic counts in these hybrid populations. However, diagnostic alleles at one nuclear allozyme locus (CK-CI) were associated with several fin ray counts.     

New facts about callose events in the young ovules of some sexual and apomictic species of the Asteraceae family

Callose (β-1,3-glucan) is one of the cell wall polymers that plays an important role in many biological processes in plants, including reproductive development. In angiosperms, timely deposition and degradation of callose during sporogenesis accompanies the transition of cells from somatic to generative identity. However, knowledge on the regulation of callose biosynthesis at specific sites of the megasporocyte wall remains limited and the data on its distribution are not conclusive. Establishing the callose deposition pattern in a large number of species can contribute to full understanding of its function in reproductive development. Previous studies focused on callose events in sexual species and only a few concerned apomicts. The main goal of our research was to establish and compare the pattern of callose deposition during early sexual and diplosporous processes in the ovules of some Hieracium, Pilosella and Taraxacum (Asteraceae) species; aniline blue staining technique was used for this purpose. Our findings indicate that callose deposition accompanies both meiotic and diplosporous development of the megaspore mother cell. This suggests that it has similar regulatory functions in intercellular communication regardless of the mode of reproduction. Interestingly, callose deposition followed a different pattern in the studied sexual and diplosporous species compared to most angiosperms as it usually began at the micropylar pole of the megasporocyte. Here, it was only in sexually reproducing H. transylvanicum that callose first appeared at the chalazal pole of the megasporocyte. The present paper additionally discusses the occurrence of aposporous initial cells with callose-rich walls in the ovules of diploid species.

     

MITOCHONDRIAL INHERITANCE PATTERNS ACROSS A COTTONWOOD HYBRID ZONE: CYTONUCLEAR DISEQUILIBRIA AND HYBRID ZONE DYNAMICS

In this study we examine the cytoplasmic inheritance patterns of an interspecific hybridizing population of Fremont and narrowleaf cottonwoods, using mitochondrial DNA. Three mitochondrial probes showing polymorphisms were used to distinguish between trees of known nuclear inheritance. Every tree screened had only one cytoplasmic genotype, either Fremont or narrowleaf. Thus, these results demonstrate that mitochondria are uniparentally inherited in these trees. Previous studies of the nuclear inheritance of this interspecific hybridizing population of cottonwood trees indicated an asymmetry in the frequency of parental genes. Using mitochondrial markers we tested one hypothesis potentially responsible for this asymmetric distribution (i.e., trees of mixed genotypes will be sterile or will not survive if their cytoplasm is derived from one or the other parent). Our results, however, show that both Fremont and narrowleaf mitochondrial markers are found in trees with mixed nuclear genotypes. Thus, nuclear-cytoplasmic incompatibilities do not appear to account for the asymmetric distribution of nuclear genotypes within the hybrid swarm. An alternative explanation for the observed asymmetric distribution of nuclear genotypes is advanced. Although nuclear-cytoplasmic incompatibilities do not appear to explain the asymmetric distribution of nuclear alleles within the hybrid zone, nonrandom associations between nuclear and cytoplasmic genotypes do exist. For example, all F₁ hybrids had Fremont mitochondrial genotypes. Furthermore, backcrosses between F₁ hybrid and narrowleaf trees have a higher than expected proportion of heterozygous loci and a higher than expected proportion of Fremont mitochondria. We propose that seeds, seedlings, or trees with high proportions of heterozygous loci are at a disadvantage unless they also have the Fremont mitochondrial genotype. While it is generally difficult to infer dynamic processes from static patterns, studies such as ours enable one to gain new insights to the dynamics of plant hybrid zones. A hybridization pattern of decreasingly complex backcrosses as one proceeds from higher to lower elevation within the hybrid swarm, a residue of Fremont cytoplasmic DNA within the pure narrowleaf population, and the unidirectional nature of these crosses suggest that the narrowleaf population may be spreading down the canyon and the Fremont population receding. The eventual fate of the hybrid zone, in relation to these processes, is discussed.     

Natural Selection with Nuclear and Cytoplasmic Transmission. I. a Deterministic Model

A deterministic model allowing variation at a nuclear genetic locus in a population segregating two cytoplasmic types is formulated. Additive, multiplicative and symmetric viability matrices are analyzed for existence and stability of equilibria. The protectedness of polymorphisms in both nuclear genes and cytoplasmic types is also investigated in the general model. In no case is a complete polymorphism protected with this deterministic model. Results are discussed in light of the extensive variation in mtDNA that has recently been reported.     

Some multiallele partial assortative mating systems for a polygamous species

The consequences of preferential mating in the presence of partial assortative and sexual selection mechanisms are ascertained for a two-allele one-locus trait involving two phenotype classes C₁ = {all homozygotes} and C₂ = {heterozygotes}. Relevant biological cases may include Burley (1977, Proc. Nat. Acad. Sci. USA 74, 3476–3479), Wilbur et al. (1978, Evolution 32, 264–270), and Singh and Zouros (1978, Evolution 32, 342–353). When the preference rate for the heterozygote exceeds that for homozygotes, it is established that the unique stable state is the central Hardy-Weinberg equilibrium. The rate of approach is faster with sexual selection than for the corresponding model of assortative mating. When the preference rates favor the homozygotes then in this symmetric model of sexual selection two asymmetric Hardy-Weinberg polymorphisms can evolve, and which succeeds depends on initial conditions. The models are also analyzed with natural selection acting on phenotypes superimposed on assortative mating. In this case we can have up to three coexisting stable states involving both fixation alternatives and a central polymorphism. The corresponding model with sexual selection maintains either the central equilibrium as in assortative mating or two asymmetric polymorphic equilibria.     

Sexual conflict and the maintenance of genetic variation in natural populations

Understanding the factors that maintain genetic variation in natural populations is a foundational goal of evolutionary biology. To this end, population geneticists have developed a variety of models that can produce stable polymorphisms. In one of the earliest models, Owen ( 1953 ) demonstrated that differences in selection pressures acting on males and females could maintain multiple alleles of a gene at a stable equilibrium. If the selection pressures act in opposite directions in males and females, we refer to this as (inter‐) sexual conflict or sexual antagonism (Arnqvist & Rowe, 2005 ). Testing if sexual conflict maintains genetic variation in natural populations is a tremendous challenge—it requires both identifying loci that harbor sexually antagonistic alleles and determining whether those alleles are maintained as stable polymorphisms (Mank, 2017 ). Doing so genome‐wide is even harder because it is not tractable to identify sexually antagonistic alleles and test for stable polymorphisms at all loci. Dutoit et al. ( 2018 ) confront this challenge in a paper published in this issue of Molecular Ecology. Using gene expression and population genomic data from the collared flycatcher, Dutoit et al. ( 2018 ) identify associations and correlations between genomic signatures of balanced polymorphisms and sexual conflict.     

Attack of the clones: reproductive interference between sexuals and asexuals in the Crepis agamic complex

Negative reproductive interactions are likely to be strongest between close relatives and may be important in limiting local coexistence. In plants, interspecific pollen flow is common between co‐occurring close relatives and may serve as the key mechanism of reproductive interference. Agamic complexes, systems in which some populations reproduce through asexual seeds (apomixis), while others reproduce sexually, provide an opportunity to examine effects of reproductive interference in limiting coexistence. Apomictic populations experience little or no reproductive interference, because apomictic ovules cannot receive pollen from nearby sexuals. Oppositely, apomicts produce some viable pollen and can exert reproductive interference on sexuals by siring hybrids. In the Crepis agamic complex, sexuals co‐occur less often with other members of the complex, but apomicts appear to freely co‐occur with one another. We identified a mixed population and conducted a crossing experiment between sexual diploid C. atribarba and apomictic polyploid C. barbigera using pollen from sexual diploids and apomictic polyploids. Seed set was high for all treatments, and as predicted, diploid–diploid crosses produced all diploid offspring. Diploid–polyploid crosses, however, produced mainly polyploidy offspring, suggesting that non‐diploid hybrids can be formed when the two taxa meet. Furthermore, a small proportion of seeds produced in open‐pollinated flowers was also polyploid, indicating that polyploid hybrids are produced under natural conditions. Our results provide evidence for asymmetric reproductive interference, with pollen from polyploid apomicts contributing to reduce the recruitment of sexual diploids in subsequent generations. Existing models suggest that these mixed sexual–asexual populations are likely to be transient, eventually leading to eradication of sexual individuals from the population.