Self-compatible Plants Research Articles

Floral, reproductive, and pollination biology of Floral, reproductive, and pollination biology of Eugenia myrcianthes Nied.

Studies on species of the family Myrtaceae are mostly related to floristic surveys, reproduction involving large plant communities, and family taxonomy. Thus, the objective of this study was to elucidate aspects of the floral and reproductive biology, as well as floral visitors of ubajai (Eugenia myrcianthes Nied.). Studies were conducted on floral morphology and morphometry, identification of nectaries and structures attractive to floral visitors, characterization of floral visitors, stigma receptivity, and androecium maturation, pollen storage, in vitro viability testing, and characterization of the reproductive system. Ubajai flowers open at approximately 6:00 a.m., and their anthers are the main attractive structure to floral visitors. The main floral visitors and effective pollinators of ubajai are honeybees (Apis mellifera). The addition of 40% sucrose to the culture medium, using fresh pre-anthesis pollen, allows for a 90% germination rate. Ubajai pollen is recalcitrant, thus, it loses viability before 30 days of storage, even when stored in a refrigerator, freezer, liquid nitrogen, or natural environment. Ubajai can be considered a self-compatible plant; however, fertilization of flowers through cross-pollination also occurs, and apomixis does not occur.

Comparing levels of geitonogamous visitation by honey bees and other pollinators

Geitonogamy, the transfer of pollen from one flower to another on the same plant, is often the primary means of self-pollination in flowering plants. For self-compatible plants, self-fertilization may lead to greatly reduced offspring fitness via inbreeding depression. For self-incompatible plants, geitonogamous pollen transfer can result in low seed set, even when stigmatic pollen loads are substantial. For multiple self-compatible, native California plants, we found that honey bees visited more flowers per plant than native insects, and that offspring resulting from pollination by honey bees had reduced fitness relative to those resulting from native insect pollination. Here we investigate whether honey bees generally make more geitonogamous visits than other pollinators using data from a global survey of 41 manuscripts that reported floral visitation data. Compared to the average of all non-honey bee visitors in a plants pollinator assemblage, honey bees visit significantly more flowers per plant, though they do not differ from the non-honey bee visitor with the highest rate of geitonogamous visitation. However, the disparity between rates of geitonogamous visitation by honey bees and non-honey bee visitors is a function of the frequency of honey bees relative to non-honey bee visitors. As honey bees become increasingly numerically dominant, there is a trend for their rates of geitonogamous visitation to increase, accompanied by a significant decline in flowers visited per plant by non-honey bee visitors. While we found that honey bees visited more flowers per plant compared to the average of other visitors, large or eusocial pollinators were as likely as honey bees to be the most geitonogamous visitor.

Honeybees (Apis mellifera) decrease the fitness of plants they pollinate.

Most flowering plants require animal pollination and are visited by multiple pollinator species. Historically, the effects of pollinators on plant fitness have been compared using the number of pollen grains they deposit, and the number of seeds or fruits produced following a visit to a virgin flower. While useful, these methods fail to consider differences in pollen quality and the fitness of zygotes resulting from pollination by different floral visitors. Here we show that, for three common native self-compatible plants in Southern California, super-abundant, non-native honeybees (Apis mellifera L.) visit more flowers on an individual before moving to the next plant compared with the suite of native insect visitors. This probably increases the transfer of self-pollen. Offspring produced after honeybee pollination have similar fitness to those resulting from hand self-pollination and both are far less fit than those produced after pollination by native insects or by cross-pollination. Because honeybees often forage methodically, visiting many flowers on each plant, low offspring fitness may commonly result from honeybee pollination of self-compatible plants. To our knowledge, this is the first study to directly compare the fitness of offspring resulting from honeybee pollination to that of other floral visitors.

Flowering Biology of Alisma plantago-aquatica (Alismataceae).

Field observations of flowering Alisma plantago-aquatica plants were carried out in Moscow region (Russia). The A. plantago-aquatica flower remains anthetic for a single light day, from 9:00 a.m. to 8:00 p.m. White petals showed a contrast bicolored pattern in UV light, and the pattern probably serves as a nectar clue for pollinators. Flowers were visited by insects in daytime from 11:00 a.m. to 3:00 p.m. Coleopterans (Coccinellidae), dipterans (Drosophilidae, Hybotidae, Muscidae, Sepsidae, and Syrphidae), and hymenopterans (Apidae) were observed as flower visitors. Hoverflies (Syrphidae) and bees (Apidae) were the most frequent visitors. Large amounts of A. plantago-aquatica pollen grains were found on their bodies, and a major role in pollination was consequently assumed for the insects. Based on the original findings and literature data on A. plantago-aquatica reproductive biology in Belgium, Slovakia, and the Czech Republic, hoverflies were identified as the most stable and efficient pollinators of A. plantago-aquatica in various parts of the species range. Bees (Apidae) were recognized as A. plantago-aquatica pollinators for the first time in this work. A flower isolation experiment confirmed that A. plantago-aquatica is a self-compatible plant, but requires insects for the most efficient cross-pollination.

Outcrossing rates in an experimentally admixed population of self-compatible and self-incompatible Arabidopsis lyrata

The transition to self-compatibility from self-incompatibility is often associated with high rates of self-fertilization, which can restrict gene flow among populations and cause reproductive isolation of self-compatible (SC) lineages. Secondary contact between SC and self-incompatible (SI) lineages might re-establish gene flow if SC lineages remain capable of outcrossing. By contrast, intrinsic features of SC plants that reinforce high rates of self-fertilization could maintain evolutionary divergence between lineages. Arabidopsis lyrata subsp. lyrata is characterized by multiple origins of self-compatibility and high rates of self-fertilization in SC-dominated populations. It is unclear whether these high rates of selfing by SC plants have intrinsic or extrinsic causes. We estimated outcrossing rates and examined patterns of pollinator movement for 38 SC and 40 SI maternal parents sampled from an admixed array of 1509 plants sourced from six SC and six SI populations grown under uniform density. Although plants from SI populations had higher outcrossing rates (mean tm = 0.78 ± 0.05 SE) than plants from SC populations (mean tm = 0.56 ± 0.06 SE), outcrossing rates among SC plants were substantially higher than previous estimates from natural populations. Patterns of pollinator movement appeared to contribute to lower outcrossing rates for SC plants; we estimated that 40% of floral visits were geitonogamous (between flowers of the same plant). The relatively high rates of outcrossing for SC plants under standardized conditions indicate that selfing rates in natural SC populations of A. lyrata are facultative and driven by extrinsic features of A. lyrata, including patterns of pollinator movement.

Neofunctionalisation of the Sli gene leads to self-compatibility and facilitates precision breeding in potato

Genetic gain in potato is hampered by the heterozygous tetraploid genome of cultivated potato. Converting potato into a diploid inbred-line based F1-hybrid crop provides a promising route towards increased genetic gain. The introduction of a dominant S-locus inhibitor (Sli) gene into diploid potato germplasm allows efficient generation of self-fertilized seeds and thus the development of potato inbred lines. Little is known about the structure and function of the Sli locus. Here we describe the mapping of Sli to a 12.6 kb interval on chromosome 12 using a recombinant screen approach. One of two candidate genes present in this interval shows a unique sequence that is exclusively present in self-compatible lines. We describe an expression vector that converts self-incompatible genotypes into self-compatible and a CRISPR-Cas9 vector that converts SC genotypes into SI. The Sli gene encodes an F-box protein that is specifically expressed in pollen from self-compatible plants. A 533 bp insertion in the promotor of that gene leads to a gain of function mutation, which overcomes self-pollen rejection.

Cross-Species Metabolic Profiling of Floral Specialized Metabolism Facilitates Understanding of Evolutional Aspects of Metabolism Among Brassicaceae Species.

Plants produce a variety of floral specialized (secondary) metabolites with roles in several physiological functions, including light-protection, attraction of pollinators, and protection against herbivores. Pigments and volatiles synthesized in the petal have been focused on and characterized as major chemical factors influencing pollination. Recent advances in plant metabolomics have revealed that the major floral specialized metabolites found in land plant species are hydroxycinnamates, phenolamides, and flavonoids albeit these are present in various quantities and encompass diverse chemical structures in different species. Here, we analyzed numerous floral specialized metabolites in 20 different Brassicaceae genotypes encompassing both different species and in the case of crop species different cultivars including self-compatible (SC) and self-incompatible (SI) species by liquid chromatography-mass spectrometry (LC-MS). Of the 228 metabolites detected in flowers among 20 Brassicaceae species, 15 metabolite peaks including one phenylacyl-flavonoids and five phenolamides were detected and annotated as key metabolites to distinguish SC and SI plant species, respectively. Our results provide a family-wide metabolic framework and delineate signatures for compatible and incompatible genotypes thereby providing insight into evolutionary aspects of floral metabolism in Brassicaceae species.

A new genetic locus for self-compatibility in the outcrossing grass species perennial ryegrass (Lolium perenne).

BackgroundSelf-incompatibility (SI) is a physiological mechanism that many flowering plants employ to prevent self-fertilization and maintain heterozygosity. In the grass family this is known to be controlled by a two locus (S-Z) system; however, the SI system is intrinsically leaky. Modifier genes of both the S and Z loci and a further locus, T, are known to override SI leading to self-fertilization and self-seed production. This has implications for the ecological and evolutionary success as well as the commercial breeding of grasses. Here we report a study where the genetic control of self-compatibility (SC) was determined from the results of self-pollinating an F2 population of perennial ryegrass from two independently derived inbred lines produced by single-seed descent.Methods In vitro self-pollinations of 73 fertile plants were analysed. A genetic association analysis was made with a panel of 1863 single-nucleotide polymorphism (SNP) markers, generated through genotype-by-sequencing methodology. Markers were placed on a recombination map of seven linkage groups (LGs) created using Joinmap v.5. The seed set on self- and open-pollinated inflorescences was determined on 143 plants, including the 73 plants analysed for self-pollination response.Key ResultsSelf-pollinations revealed a bimodal distribution of percentage SC with peaks at 50 and 100 %. A single quantitative trait locus (QTL) was identified with peak association for marker 6S14665z17875_11873 that mapped to LG 6. Peak position was associated with maximum marker segregation distortion. The self-compatible plants were equally fecund after self- and open pollination.ConclusionsThis is the first report in the Poaceae family of an SC locus located on LG 6. This new SC QTL discovery, as well as indicating the complex nature of the pollen–stigma recognition process and its evolutionary significance, provides an additional source of SC for breeding perennial ryegrass.

The Ecology of Autogamy in Wild Blueberry (Vaccinium angustifolium Aiton): Does the Early Clone Get the Bee?

Wild blueberry, Vaccinium angustifolium Aiton, for the most part requires cross-pollination. However, there is a continuum across a gradient from zero to 100% in self-compatibility. We previously found by sampling many fields that 20–25% of clones during bloom have high levels of self-compatibility (≥50%). In 2009–2011, and 2015 we studied the ecology of self-pollination in wild blueberry, specifically its phenology and bee recruitment and subsequent bee density on bloom. We found that highly self-compatible clones were predominantly early blooming genotypes in the wild blueberry population. On average, fruit set and berry weight were highest in self-compatible genotypes. The bumble bee community (queens only early in the spring) was characterized by bees that spent large amounts of time foraging in self-compatible plant patches that comprised only a small proportion of the blueberry field, the highest density in the beginning of bloom when most genotypes in bloom were self-compatible. As bloom proceeded in the spring, more plants were in bloom and thus more land area was occupied by blooming plants. The absolute density of bumble bee queens per m2 declined, as a dilution effect, and this probably resulted in lower fruit set throughout the field.

Pollen flow within and among isolated populations of two rare, self-compatible plant species from inselbergs of Northeast Brazil

Endangered species in isolated habitats (e.g., inselbergs) may escape mate limitation during patch colonization through autonomous self-pollination. After colonization, the higher the number of plants breeding randomly within populations through cross-pollination, the lower is genetic erosion caused by genetic drift and inbreeding. Additionally, pollen flow among patches can increase population genetic variation [natural genetic recovery (NGR)]. Autonomous self- and cross-pollination were investigated within small isolated populations of the rare hummingbird-pollinated Ameroglossum pernambucense and A. manoel-felixii from inselbergs of Northeastern Brazil. We also used A. manoel-felixii and the widespread Encholirium spectabile (mainly bat-pollinated) to compare potential for NGR. In both Ameroglossum species, we performed controlled pollination experiments to test for autonomous self-pollination. Additionally, we compared fruit and seed production (reproductive output) between flowers exposed to and isolated from pollinators to test the potential for cross-pollination. We assessed the extent of cross-pollination within populations, using Wright’s genetic neighborhood model. In A. manoel-felixii and E. spectabile, we tested the potential for NGR among three nearby inselbergs. Pollen movement was tracked using fluorescent dyes. Populations of both Ameroglossum species self-pollinate, but pollinators increased fecundity, revealing potential for cross-pollination. Nonetheless, the area comprising random cross-pollination corresponded to ~ 3% of the area occupied by the Ameroglossum populations, suggesting poor capacity to counteract genetic erosion. In contrast, although both species showed potential for NGR, bat pollination in E. spectabile may facilitate NGR in more remote populations. This study suggests the predominant role of cross-pollination among small, isolated populations as the main force against genetic erosion.

Assessment of Mating System in Helianthus annuus and H. petiolaris (Asteraceae) Populations

Abstract Helianthus annuus subsp. annuus and H. petiolaris are wild North American species that have been naturalized in central Argentina. They have a sporophytic self-incompatibility genetic system that prevent self-fertilization but the occurrence of self-compatible plants in Argentina was observed in both species and could in part explain their highly invasive ability. Their geographical distribution coincides with the major crop area. The domestic sunflower is self-compatible, can hybridize with both species and presents a considerable amount of gene flow. The aim of this study is to understand the self-incompatibility mechanism in both wild Helianthus species. Reciprocal crossing and seed production were used to identify self-compatible genotypes, the number and distribution of self-incompatibility alleles within populations and the type and extent of allelic interactions in the pollen and pistil. The behaviour of S alleles within each population was explained by five functional S alleles and one non-functional allele in each species, differing in their presence and frequency within accessions. In both species, the allelic interactions were of dominance/recessiveness and codominance in pollen, whereas it was only codominance in the pistil. Inbreeding effects in wild materials appeared in the third generation of self-pollination, with lethal effects in most plants. The number of S alleles is low and they behave in a similar way of other Asteraceae species. The self-compatibility was addressed to non-functional S alleles introgressed in wild Helianthus plants through gene flow from self-compatible sunflower.

Fitness of reciprocal F1 hybrids between Rhinanthus minor and Rhinanthus major under controlled conditions and in the field.

The performance of first-generation hybrids determines to a large extent the long-term outcome of hybridization in natural populations. F1 hybrids can facilitate further gene flow between the two parental species, especially in animal-pollinated flowering plants. We studied the performance of reciprocal F1 hybrids between Rhinanthus minor and R.major, two hemiparasitic, annual, self-compatible plant species, from seed germination to seed production under controlled conditions and in the field. We sowed seeds with known ancestry outdoors before winter and followed the complete life cycle until plant death in July the following season. Germination under laboratory conditions was much lower for the F1 hybrid formed on R.major compared with the reciprocal hybrid formed on R.minor, and this confirmed previous results from similar experiments. However, this difference was not found under field conditions, which seems to indicate that the experimental conditions used for germination in the laboratory are not representative for the germination behaviour of the hybrids under more natural conditions. The earlier interpretation that F1 hybrid seeds formed on R.major face intrinsic genetic incompatibilities therefore appears to be incorrect. Both F1 hybrids performed at leastas well as andsometimes better than R.minor, which had a higher fitness than R.major in one of the two years in the greenhouse and in the field transplant experiment. The high fitness of the F1 hybrids confirms findings from naturally mixed populations, where F1 hybrids appear in the first year after the two species meet, which leads to extensive advanced-hybrid formation and introgression in subsequent generations.

Self-Incompatibility inMatricaria chamomillaL. (Asteraceae) Is Linked to Differential Esterase Activity

Premise of research. Self-incompatibility (SI) is a frequent mechanism of plants to avoid self-pollination. Pollen esterases (EC 3.1.1.−) play an important role in pollination by enabling the penetration of pollen tubes through the cutin layer (cutinases). Therefore, their composition could contribute to SI. To test this hypothesis, esterase activity and isozyme profiles between self-compatible and self-incompatible chamomile plants (Matricaria chamomilla L., Asteraceae) were compared.Methodology. Stable self-incompatible plants were selected, and the approximate number of alleles contributing to SI were determined in a full-diallel cross. Esterase activity of self-compatible and self-incompatible chamomile plants was tested on the four substrates that follow: 1-naphthyl acetate, 2-naphthyl acetate, 1-formyl-2-naphthyl acetate, and indoxyl acetate. Esterases were characterized by electrophoretic methods, both concerning protein size and isozyme pattern in two different buffer systems.Pivotal results. Seventeen percent of all plants analyzed were self-incompatible. Eight self-incompatible plants—crossed in a full diallel—showed six different alleles responsible for SI, a number that can only be regarded as a minimum number of alleles due to the low sample size. Self-incompatible plants showed a significantly lower esterase activity for the two substrates, namely, 1-naphthyl acetate and 2-naphthyl acetate. With 1-formyl-2-naphthylacetate, however, esterase activity of self-incompatible plants was higher than that of self-compatible plants. With indoxyl acetate, no significant difference between self-incompatible and self-compatible plants could be found. The different esterase isozyme patterns observed did not correlate to the compatibility mechanism.Conclusions. The composition of pollen esterases is variable and could contribute to SI by differences in their substrate specificity.

The link between selfing and greater dispersibility in a heterocarpic Asteraceae.

Although an evolutionary link between breeding system and dispersibility has been proposed, to date empirical data and theoretical models of plants show contrasting trends. We tested two competing hypotheses for the association between breeding systems and dispersibility in the heterocarpic Hypochaeris salzmanniana (Asteraceae) by using both an experimental approach and surveys over 2 years of five natural populations along an environmental cline with a gradient of pollinator availability. Hypochaeris salzmanniana produced two types of fruits, beaked (BF) and nonbeaked (NBF), which differ in their dispersal ability. The BF were lighter and had a lower dropping velocity and higher dispersal distance than the NBF. Potential for long-distance dispersal, measured as BF ratio per head, had high narrow-sense heritability. Greater dispersibility and selfing ability were linked at all the scales studied. Both selfed BF and NBF fruits had longer plumes and lower plume loading than outcrossed fruits, characteristics that promote farther dispersal. Natural populations with a higher percentage of self-compatible plants showed a higher BF ratio. Moreover, selfing led to a higher BF ratio than outcrossing. The avoidance of inbreeding depression seems to be the most plausible selective pressure for the greater dispersibility traits of selfed seeds. Furthermore, the ability to modulate the BF ratio and thus the potential for long-distance dispersal of offspring based on its selfed or outcrossed origin could be advantageous, and therefore selected, under unpredictable pollination environments that favor higher dispersive selfers, which overcome both pollen limitation and inbreeding avoidance.

Mating systems and avoidance of inbreeding depression as evolutionary drivers of pollen limitation in animal-pollinated self-compatible plants.

Most theory addressing the evolution of pollen limitation in flowering plants focuses on stochasticity in the relative abundance of plant and pollinator populations affecting trade-offs in resource allocation to ovule production or pollinator attraction vs. seed maturation. Mating system evolution is an underappreciated but potentially widespread additional mechanism for the evolutionary emergence of pollen limitation in animal-pollinated self-compatible plants. We model individual plant flowering phenologies influencing both pollinator attraction and geitonogamous self-fertilization caused by pollinator movements among flowers within plants, incorporating demographic but not environmental stochasticity. Plant phenology and the resulting pollen limitation are analysed at evolutionarily stable equilibria (ESS). Pollen limitation is measured by two quantities: the proportion of unpollinated flowers and the reduction in maternal fitness caused by inbreeding depression in selfed seeds. When pollinators visit multiple flowers per plant, pollen limitation is never minimized at an ESS and results from the evolution of flowering phenologies balancing the amount and genetic composition (outbred vs. inbred) of pollen receipt. Results are consistent with previous theory demonstrating that pollen limitation can be an evolved property, not just a constraint; they complement existing models by showing that plant avoidance of inbreeding depression constitutes a genetic mechanism contributing to evolution of pollen limitation, in addition to ecological mechanisms previously studied.

Seed germination and seedling allogamy in Rosmarinus officinalis: the costs of inbreeding.

Self-pollination by geitonogamy is likely in self-compatible plants that simultaneously expose a large number of flowers to pollinators. However, progeny of these plants is often highly allogamous. Although mechanisms to increase cross-pollination have been identified and studied, their relative importance has rarely been addressed simultaneously in plant populations. We used Rosmarinus officinalis to explore factors that influence the probability of self-fertilisation due to geitonogamy or that purge its consequences, focusing on their effects on seed germination and allogamy rate. We experimentally tested the effect of geitonogamy on the proportion of filled seeds and how it influences germination rate. During two field seasons, we studied how life history and flowering traits of individuals influence seed germination and allogamy rates of their progeny in wild populations at the extremes of the altitudinal range. The traits considered were plant size, population density, duration of the flowering season, number of open flowers, flowering synchrony among individuals within populations and proportion of male-sterile flowers. We found that most seeds obtained experimentally from self-pollination were apparently healthy but empty, and that the proportion of filled seeds drove the differences in germination rate between self- and cross-pollination experiments. Plants from wild populations consistently had low germination rate and high rate of allogamy, as determined with microsatellites. Germination rate related positively to the length of the flowering season, flowering synchrony and the ratio of male-sterile flowers, whereas the rate of allogamous seedlings was positively related only to the ratio of male-sterile flowers. Rosemary plants purge most of the inbreeding caused by its pollination system by aborting the seeds. This study showed that the rates of seed germination and allogamy of the seedlings depend on a complex combination of factors that vary in space and time. Male sterility of flowers, length of the flowering season and flowering synchrony of individuals within populations all favour high rates of cross-pollination, therefore increasing germination and allogamy rates. Flowering traits appear to be highly plastic and respond to local and seasonal conditions.

Relatively weak inbreeding depression in selfing but also in outcrossing populations of North American Arabidopsis lyrata.

Hermaphroditic plants can potentially self-fertilize, but most possess adaptations that promote outcrossing. However, evolutionary transitions to higher selfing rates are frequent. Selfing comes with a transmission advantage over outcrossing, but self-progeny may suffer from inbreeding depression, which forms the main barrier to the evolution of higher selfing rates. Here, we assessed inbreeding depression in the North American herb Arabidopsis lyrata, which is normally self-incompatible, with a low frequency of self-compatible plants. However, a few populations have become fixed for self-compatibility and have high selfing rates. Under greenhouse conditions, we estimated mean inbreeding depression per seed (based on cumulative vegetative performance calculated as the product of germination, survival and aboveground biomass) to be 0.34 for six outcrossing populations, and 0.26 for five selfing populations. Exposing plants to drought and inducing defences with jasmonic acid did not magnify these estimates. For outcrossing populations, however, inbreeding depression per seed may underestimate true levels of inbreeding depression, because self-incompatible plants showed strong reductions in seed set after (enforced) selfing. Inbreeding-depression estimates incorporating seed set averaged 0.63 for outcrossing populations (compared to 0.30 for selfing populations). However, this is likely an overestimate because exposing plants to 5% CO2 to circumvent self-incompatibility to produce selfed seed might leave residual effects of self-incompatibility that contribute to reduced seed set. Nevertheless, our estimates of inbreeding depression were clearly lower than previous estimates based on the same performance traits in outcrossing European populations of A.lyrata, which may help explain why selfing could evolve in North American A.lyrata.

Deciphering the Theobroma cacao self-incompatibility system: from genomics to diagnostic markers for self-compatibility.

Cocoa self-compatibility is an important yield factor and has been described as being controlled by a late gameto-sporophytic system expressed only at the level of the embryo sac. It results in gametic non-fusion and involves several loci. In this work, we identified two loci, located on chromosomes 1 and 4 (CH1 and CH4), involved in cocoa self-incompatibility by two different processes. Both loci are responsible for gametic selection, but only one (the CH4 locus) is involved in the main fruit drop. The CH1 locus acts prior to the gamete fusion step and independently of the CH4 locus. Using fine-mapping and genome-wide association studies, we focused analyses on restricted regions and identified candidate genes. Some of them showed a differential expression between incompatible and compatible reactions. Immunolocalization experiments provided evidence of CH1 candidate genes expressed in ovule and style tissues. Highly polymorphic simple sequence repeat (SSR) diagnostic markers were designed in the CH4 region that had been identified by fine-mapping. They are characterized by a strong linkage disequilibrium with incompatibility alleles, thus allowing the development of efficient diagnostic markers predicting self-compatibility and fruit setting according to the presence of specific alleles or genotypes. SSR alleles specific to self-compatible Amelonado and Criollo varieties were also identified, thus allowing screening for self-compatible plants in cocoa populations.

SIPP, a Novel Mitochondrial Phosphate Carrier, Mediates in Self-Incompatibility.

In Solanaceae, the S-specific interaction between the pistil S-RNase and the pollen S-Locus F-box protein controls self-incompatibility (SI). Although this interaction defines the specificity of the pollen rejection response, the identification of three pistil essential modifier genes unlinked to the S-locus (HT-B, 120K, and NaStEP) unveils a higher degree of complexity in the pollen rejection pathway. We showed previously that NaStEP, a stigma protein with homology with Kunitz-type protease inhibitors, is essential to SI in Nicotiana spp. During pollination, NaStEP is taken up by pollen tubes, where potential interactions with pollen tube proteins might underlie its function. Here, we identified NaSIPP, a mitochondrial protein with phosphate transporter activity, as a novel NaStEP-interacting protein. Coexpression of NaStEP and NaSIPP in pollen tubes showed interaction in the mitochondria, although when expressed alone, NaStEP remains mostly cytosolic, implicating NaSIPP-mediated translocation of NaStEP into the organelle. The NaSIPP transcript is detected specifically in mature pollen of Nicotiana spp.; however, in self-compatible plants, this gene has accumulated mutations, so its coding region is unlikely to produce a functional protein. RNA interference suppression of NaSIPP in Nicotiana spp. pollen grains disrupts the SI by preventing pollen tube inhibition. Taken together, our results are consistent with a model whereby the NaStEP and NaSIPP interaction, in incompatible pollen tubes, might destabilize the mitochondria and contribute to arrest pollen tube growth.

Evolution of the selfing syndrome: Anther orientation and herkogamy together determine reproductive assurance in a self-compatible plant.

Capacity for autonomous self-fertilization provides reproductive assurance, has evolved repeatedly in the plant kingdom, and typically involves several changes in flower morphology and development (the selfing syndrome). Yet, the relative importance of different traits and trait combinations for efficient selfing and reproductive success in pollinator-poor environments is poorly known. In a series of experiments, we tested the importance of anther-stigma distance and the less studied trait anther orientation for efficiency of selfing in the perennial herb Arabis alpina. Variation in flower morphology among eight self-compatible European populations was correlated with efficiency of self-pollination and with pollen limitation in a common-garden experiment. To examine whether anther-stigma distance and anther orientation are subject to directional and/or correlational selection, and whether this is because these traits affect pollination success, we planted a segregating F2 population at two native field sites. Selection strongly favored a combination of introrse anthers and reduced anther-stigma distance at a site where pollinator activity was low, and supplemental hand-pollination demonstrated that this was largely because of their effect on securing self-pollination. The results suggest that concurrent shifts in more than one trait can be crucial for the evolution of efficient self-pollination and reproductive assurance in pollinator-poor habitats.