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.     

Selection on intra‐individual variation in stigma–anther distance in the tropical tree Ipomoea wolcottiana (Convolvulaceae)

• It is well known that animals can exert strong selective pressures on plant traits. However, studies on the evolutionary consequences of plant–animal interactions have mainly focused on understanding how these interactions shape trait means, while overlooking its potential direct effect on the variability among structures within a plant (e.g. flowers and fruits). The degree of within‐plant variability can have strong fitness effects but few studies have evaluated its role as a potential target of selection.
• Here we reanalysed data on Ipomoea wolcottiana stigma–anther distance to test alternate mechanisms driving selection on the mean as well as on intra‐individual variance in 2 years. We found strong negative selection acting on intra‐individual variation but not on mean stigma–anther distance, suggesting independent direct selection on the latter.
• Our result suggests that intra‐individual variance has the potential to be an important target of selection in nature, and that ignoring it could lead to the wrong characterisation of the selection regime.
• We highlight the need for future studies to consider patterns of selection on the mean as well as on intra‐individual variance if we want to understand the full extent of plant–animal interactions as an evolutionary force in nature.

     

Sexual antagonism in the pistil varies among populations of a hermaphroditic mixed‐mating plant

Sexual conflicts and their evolutionary outcomes may be influenced by population‐specific features such as mating system and ecological context; however, very few studies have investigated the link between sexual conflict and mating system. The self‐compatible, mixed‐mating hermaphrodite Collinsia heterophylla (Plantaginaceae) is thought to exhibit a sexual conflict over timing of stigma receptivity. This conflict involves (i) delayed stigma receptivity, which intensifies pollen competition, and (ii) early fertilization forced by pollen, which reduces seed set. We investigated the potential for the conflict to occur under field conditions and performed glasshouse crosses within eight populations to assess its consistency across populations. Flowers were visited, and produced seeds after pollination, at all developmental stages, suggesting that the conflict can be of significance under natural conditions. In the glasshouse, early pollination imposed costs in all populations. Overall, the timing of first seed set was most strongly affected by the maternal parent, denoting stronger female than male ability to influence the onset of stigma receptivity. Crosses also revealed a negative relationship between donor‐ and recipient‐related onset of receptivity within individuals, a novel result hinting at trade‐offs in sex allocation or a history of antagonistic selection. Neither timing of stigma receptivity, timing of first seed set, nor pollen competitive ability covaried with population outcrossing rate. In conclusion, these results indicate that sexually antagonistic selection may be present in varying degrees in different populations of C. heterophylla, but this variation does not appear to be directly related to mating system variation.     

Pollen and stigma size changes during the transition from tristyly to distyly in Oxalis alpina (Oxalidaceae)

• Pollen and stigma size have the potential to influence male fitness of hermaphroditic plants, particularly in species presenting floral polymorphisms characterised by marked differences in these traits among floral morphs. In this study, we take advantage of the evolutionary transition from tristyly to distyly experienced by Oxalis alpina (Oxalidaceae), and examined whether modifications in the ancillary traits (pollen and stigma size) respond to allometric changes in other floral traits. Also, we tested whether these modifications are in accordance with what would be expected under the hypothesis that novel competitive scenarios (as in distylous‐derived reproductive system) exert morph‐ and whorl‐specific selective pressures to match the available stigmas.
• We measure pollen and stigma size in five populations of O. alpina representing the tristyly–distyly transition.
• A general reduction in pollen and stigma size occurred along the tristyly–distyly transition, and pollen size from the two anther levels within each morph converged to a similar size that was characterised by whorl‐specific changes (increases or decreases) in pollen size of different anthers in each floral type.
• Overall, results from this study show that the evolution of distyly in this species is characterised not only by changes in sexual organ position and flower size, but also by morph‐specific changes in pollen and stigma size. This evidence supports the importance of selection on pollen and stigma size, which increase fitness of remaining morphs following the evolution of distyly, and raises questions to explore on the functional value of pollen size in heterostylous systems under pollen competition.

     

Nectar robbers influence the trait–fitness relationship of Primula secundiflora

• The trait–fitness relationship influences the strength and direction of floral evolution. To fully understand and predict the evolutionary trajectories of floral traits, it is critical to disentangle the direct and indirect effects of floral traits on plant fitness in natural populations.
• We experimentally quantified phenotypic selection on floral traits through female fitness and estimated the casual effects of nectar robbing with different nectar robbing intensities on trait–fitness relationships in both the L‐ (long‐style and short‐anther phenotype) and S‐morph (short‐style and long‐anther phenotype) flowers among Primula secundiflora populations.
• A larger number of flowers and wider corolla tubes had both direct and indirect positive effects on female fitness in the P. secundiflora populations. The indirect effects of these two traits on female fitness were mediated by nectar robbers. The indirect effect of the number of flowers on female fitness increased with increasing nectar robbing intensity. In most populations, the direct and/or indirect effects of floral traits on female fitness were stronger in the S‐morph flowers than in the L‐morph flowers. In addition, nectar robbers had a direct positive effect on female fitness, but this effect varied between the L‐ and S‐morph flowers.
• These results show the potential role of nectar robbers in influencing the trait–fitness relationships in this primrose species.

     

A resurrection experiment finds evidence of both reduced genetic diversity and potential adaptive evolution in the agricultural weed Ipomoea purpurea

Despite the negative economic and ecological impact of weeds, relatively little is known about the evolutionary mechanisms that influence their persistence in agricultural fields. Here, we use a resurrection approach to examine the potential for genotypic and phenotypic evolution in Ipomoea purpurea, an agricultural weed that is resistant to glyphosate, the most widely used herbicide in current‐day agriculture. We found striking reductions in allelic diversity between cohorts sampled nine years apart (2003 vs. 2012), suggesting that populations of this species sampled from agricultural fields have experienced genetic bottleneck events that have led to lower neutral genetic diversity. Heterozygosity excess tests indicate that these bottlenecks may have occurred prior to 2003. A greenhouse assay of individuals sampled from the field as seed found that populations of this species, on average, exhibited modest increases in herbicide resistance over time. However, populations differed significantly between sampling years for resistance: some populations maintained high resistance between the sampling years whereas others exhibited increased or decreased resistance. Our results show that populations of this noxious weed, capable of adapting to strong selection imparted by herbicide application, may lose genetic variation as a result of this or other environmental factors. We probably uncovered only modest increases in resistance on average between sampling cohorts due to a strong and previously identified fitness cost of resistance in this species, along with the potential that nonresistant migrants germinate from the seed bank.     

Higher parasite resistance in Daphnia populations with recent epidemics

Natural populations often show genetic variation in parasite resistance, forming the basis for evolutionary response to selection imposed by parasitism. We investigated whether previous epidemics selected for higher resistance to novel parasite isolates in a Daphnia galeata–microparasite system by comparing susceptibility of host clones from populations with varying epidemic history. We manipulated resource availability to evaluate whether diet influences Daphnia susceptibility as epidemics are common in nutrient‐rich lakes. Exposing clones from 10 lakes under two food treatments to an allopatric protozoan parasite, we found that Daphnia originating from lakes (mainly nutrient rich) with previous epidemics better resist infection. Despite this result, there was a tendency of higher susceptibility in the low food treatment, suggesting that higher resistance of clones from populations with epidemic background is not directly caused by lake nutrient level. Rather, our results imply that host populations respond to parasite‐mediated selection by evolving higher parasite resistance.     

Herkogamy and mate diversity in the wild daffodil Narcissus longispathus: beyond the selfing–outcrossing paradigm in the evolution of mixed mating

Spatial separation of male and female reproductive structures (herkogamy) is a widespread floral trait that has traditionally been viewed as an adaptation that reduces the likelihood of self‐pollination. Here we propose that increased herkogamy may also influence another important aspect of plant mating: the diversity of pollen donors siring seeds within fruits. We test this hypothesis in Narcissus longispathus, a wild daffodil species with extensive variation in anther–stigma separation. To study the morphological basis of variation in herkogamy, floral measurements were undertaken in 16 populations of N. longispathus. We then quantified multilocus outcrossing rates and the correlation of outcrossed paternity in three of these populations sampled over several years. Mating system estimates were calculated for each population and year, and also separately for groups of plants that differed markedly in herkogamy within each population and year. In N. longispathus herkogamy was much more variable than other floral traits, and was more closely related to style length than to anther position. Averaged across populations and years, plants with high herkogamy had similar outcrossing rates (0.683) to plants with intermediate (0.648) or low herkogamy (0.590). However, a significant linear trend was found for correlation of outcrossed paternity, which increased monotonically from high herkogamy (0.221), through intermediate herkogamy (0.303) to low herkogamy (0.463) plants. The diversity of pollen donors siring seeds of high herkogamy Narcissus flowers was thus consistently greater than the diversity of pollen donors siring seeds of low herkogamy flowers. Results of this study contribute to the emerging consensus that floral traits can simultaneously influence several aspects of plant mating system in complex ways, thus extending the traditional focus centred exclusively on patterns and relative importance of self‐ and cross‐fertilisation.     

Small‐scale spatial variation in infection risk shapes the evolution of a Borrelia resistance gene in wild rodents

Spatial variation in pathogen‐mediated selection is predicted to influence the evolutionary trajectory of host populations and lead to spatial variation in their immunogenetic composition. However, to date few studies have been able to directly link small‐scale spatial variation in infection risk to host immune gene evolution in natural, nonhuman populations. Here, we use a natural rodent–Borrelia system to test for associations between landscape‐level spatial variation in Borrelia infection risk along replicated elevational gradients in the Swiss Alps and Toll‐like receptor 2 (TLR2) evolution, a candidate gene for Borrelia resistance, across bank vole (Myodes glareolus) populations. We found that Borrelia infection risk (i.e., the product of Borrelia prevalence in questing ticks and the average tick load of voles at a sampling site) was spatially variable and significantly negatively associated with elevation. Across sampling sites, Borrelia prevalence in bank voles was significantly positively associated with Borrelia infection risk along the elevational clines. We observed a significant association between naturally occurring TLR2 polymorphisms in hosts and their Borrelia infection status. The TLR2 variant associated with a reduced likelihood of Borrelia infection was most common in rodent populations at lower elevations that face a high Borrelia infection risk, and its frequency changed in accordance with the change in Borrelia infection risk along the elevational clines. These results suggest that small‐scale spatial variation in parasite‐mediated selection affects the immunogenetic composition of natural host populations, providing a striking example that the microbial environment shapes the evolution of the host's immune system in the wild.     

THE ORIGIN OF SPECIFICITY BY MEANS OF NATURAL SELECTION: EVOLVED AND NONHOST RESISTANCE IN HOST–PATHOGEN INTERACTIONS

Most species seem to be completely resistant to most pathogens and parasites. This resistance has been called “nonhost resistance” because it is exhibited by species that are considered not to be part of the normal host range of the pathogen. A conceptual model is presented suggesting that failure of infection on nonhosts may be an incidental by‐product of pathogen evolution leading to specialization on their source hosts. This model is contrasted with resistance that results from hosts evolving to resist challenge by their pathogens, either as a result of coevolution with a persistent pathogen or as the result of one‐sided evolution by the host against pathogens that are not self‐sustaining on those hosts. Distinguishing evolved from nonevolved resistance leads to contrasting predictions regarding the relationship between resistance and genetic distance. An analysis of cross‐inoculation experiments suggests that the resistance is often the product of pathogen specialization. Understanding the contrasting evolutionary origins of resistance is critical for studies on the genetics and evolution of host–pathogen interactions in human, agricultural, and natural populations. Research on human infectious disease using animal models may often study resistances that have quite contrasting evolutionary origins, and therefore very different underlying genetic mechanisms.     

Natural and human‐mediated selection in a landrace of Thai rice (Oryza sativa)

Landrace rice in Thailand consists of managed populations grown under traditional and long‐standing agricultural practices. These populations evolve both in response to environmental conditions within the local agro‐ecosystem and in response to human activities. Single landraces are grown across varying environments and recently have experienced temporal changes in local environments due to climate change. Here we assess the interplay between natural selection in a changing climate and human‐mediated selection on the population genetic structure of Muey Nawng, a local landrace of Thai rice. Genetic diversity and population structure of landrace rice were assessed by a STRUCTURE analysis of 20 microsatellite loci. The first exon–intron junction of the waxy gene was sequenced to determine genotypes for glutinous or non‐glutinous grain starch. Muey Nawng rice is genetically variable and is structured based on starch grain types and the level of resistance to gall midge pest. A strong positive correlation was found between genetic diversity and the percentage of gall midge infestation. Variation in the waxy locus is correlated with starch quality; selection for non‐glutinous rice appears to involve additional genes. The dynamics of genetic diversity within Muey Nawng rice depends on three factors: (a) a genetic bottleneck caused by strong selection associated with gall midge infestation, (b) selection by local farmers for starch quality and (c) variation introduced by farmer practices for cultivation and seed exchange. These results, when taken in total, document the ability of landrace rice to quickly evolve in response to both natural and human‐mediated selection.     

INDIRECT SELECTION OF STIGMA POSITION IN IPOMOPSIS AGGREGATA VIA A GENETICALLY CORRELATED TRAIT

Experimental manipulation of a trait can be used to distinguish direct selection from selection of correlated traits and to identify mechanisms of selection. Here we use experiments to investigate phenotypic selection of stigma position in angiosperm flowers. In natural populations of the subalpine herb Ipomopsis aggregata, plants with more strongly exserted stigmas receive more pollen per flower, indicating selection favoring stigma exsertion during the pollination stage of the life cycle. We pose four hypotheses for this association, two involving direct selection on stigma position and two involving indirect selection of a correlated floral trait. The first three hypotheses were tested using hand pollinations that mimicked natural hummingbird visitation, and by presenting captive hummingbirds with a series of flowers that differed in stigma and anther positions, sex ratio, and presence of anthers. In these experiments, pollen deposition either was independent of stigma exsertion or was highest on inserted stigmas, suggesting direct selection against exserted stigmas. In natural populations, however, stigma exsertion is highly correlated with time spent by the protandrous flowers in the pistillate phase. When we manipulated the latter trait in the field, pollen deposition increased with duration of exposure to hummingbirds, indicating indirect selection for stigma exsertion. Stigma exsertion and time spent in the pistillate phase are genetically and phenotypically correlated, as shown by a quantitative genetic experiment conducted in the field with paternal half sibships. Our results suggest that the evolution of stigma position can be driven by selection of a genetically correlated trait.     

Rapid genome‐wide evolution in Brassica rapa populations following drought revealed by sequencing of ancestral and descendant gene pools

There is increasing evidence that evolution can occur rapidly in response to selection. Recent advances in sequencing suggest the possibility of documenting genetic changes as they occur in populations, thus uncovering the genetic basis of evolution, particularly if samples are available from both before and after selection. Here, we had a unique opportunity to directly assess genetic changes in natural populations following an evolutionary response to a fluctuation in climate. We analysed genome‐wide differences between ancestors and descendants of natural populations of Brassica rapa plants from two locations that rapidly evolved changes in multiple phenotypic traits, including flowering time, following a multiyear late‐season drought in California. These ancestor‐descendant comparisons revealed evolutionary shifts in allele frequencies in many genes. Some genes showing evolutionary shifts have functions related to drought stress and flowering time, consistent with an adaptive response to selection. Loci differentiated between ancestors and descendants (F_ST outliers) were generally different from those showing signatures of selection based on site frequency spectrum analysis (Tajima's D), indicating that the loci that evolved in response to the recent drought and those under historical selection were generally distinct. Very few genes showed similar evolutionary responses between two geographically distinct populations, suggesting independent genetic trajectories of evolution yielding parallel phenotypic changes. The results show that selection can result in rapid genome‐wide evolutionary shifts in allele frequencies in natural populations, and highlight the usefulness of combining resurrection experiments in natural populations with genomics for studying the genetic basis of adaptive evolution.     

Host resistance and pathogen infectivity in host populations with varying connectivity

Theory predicts that hosts and pathogens will evolve higher resistance and aggressiveness in systems where populations are spatially connected than in situations in which populations are isolated and dispersal is more local. In a large cross‐inoculation experiment we surveyed patterns of host resistance and pathogen infectivity in anther‐smut diseased Viscaria alpina populations from three contrasting areas where populations range from continuous, through patchy but spatially connected to highly isolated demes. In agreement with theory, isolated populations of V. alpina were more susceptible on average than either patchily distributed or continuous populations. While increased dispersal in connected systems increases disease spread, it may also increase host gene flow and the potential for greater host resistance to evolve. In the Viscaria–Microbotryum system, pathogen infectivity mirrored patterns of host resistance with strains from the isolated populations being the least infective and strains from the more resistant continuous populations being the most infective on average, suggesting that high resistance selects for high infectivity. To our knowledge this study is the first to characterize the impacts of varying spatial connectivity on patterns of host resistance and pathogen infectivity in a natural system.     

Rapid evolution of parasitoids when faced with the symbiont-mediated resistance of their hosts

Insects harbour a wild diversity of symbionts that can spread and persist within populations by providing benefits to their host. The pea aphid Acyrthosiphon pisum maintains a facultative symbiosis with the bacterium Hamiltonella defensa, which provides enhanced resistance against the aphid parasitoid Aphidius ervi. Although the mechanisms associated with this symbiotic‐mediated protection have been investigated thoroughly, little is known about its evolutionary effects on parasitoid populations. We used an experimental evolution procedure in which parasitoids were exposed either to highly resistant aphids harbouring the symbiont or to low innate resistant hosts free of H. defensa. Parasitoids exposed to H. defensa gained virulence over time, reaching the same parasitism rate as those exposed to low aphid innate resistance only. A fitness reduction was associated with this adaptation as the size of parasitoids exposed to H. defensa decreased through generations. This study highlighted the considerable role of symbionts in host–parasite co‐evolutionary dynamics.     

Simultaneous inbreeding modifies inbreeding depression in a plant–herbivore interaction

Because inbreeding is common in natural populations of plants and their herbivores, herbivore‐induced selection on plants, and vice versa, may be significantly modified by inbreeding and inbreeding depression. In a feeding assay with inbred and outbred lines of both the perennial herb, Vincetoxicum hirundinaria, and its specialist herbivore, Abrostola asclepiadis, we discovered that plant inbreeding increased inbreeding depression in herbivore performance in some populations. The effect of inbreeding on plant resistance varied among plant and herbivore populations. The among‐population variation is likely to be driven by variation in plant secondary compounds across populations. In addition, inbreeding depression in plant resistance was substantial when herbivores were outbred, but diminished when herbivores were inbred. These findings demonstrate that in plant–herbivore interactions expression of inbreeding depression can depend on the level of inbreeding of the interacting species. Furthermore, our results suggest that when herbivores are inbred, herbivore‐induced selection against self‐fertilisation in plants may diminish.     

Metabolic constraints on the evolution of antibiotic resistance

Despite our continuous improvement in understanding antibiotic resistance, the interplay between natural selection of resistance mutations and the environment remains unclear. To investigate the role of bacterial metabolism in constraining the evolution of antibiotic resistance, we evolved Escherichia coli growing on glycolytic or gluconeogenic carbon sources to the selective pressure of three different antibiotics. Profiling more than 500 intracellular and extracellular putative metabolites in 190 evolved populations revealed that carbon and energy metabolism strongly constrained the evolutionary trajectories, both in terms of speed and mode of resistance acquisition. To interpret and explore the space of metabolome changes, we developed a novel constraint‐based modeling approach using the concept of shadow prices. This analysis, together with genome resequencing of resistant populations, identified condition‐dependent compensatory mechanisms of antibiotic resistance, such as the shift from respiratory to fermentative metabolism of glucose upon overexpression of efflux pumps. Moreover, metabolome‐based predictions revealed emerging weaknesses in resistant strains, such as the hypersensitivity to fosfomycin of ampicillin‐resistant strains. Overall, resolving metabolic adaptation throughout antibiotic‐driven evolutionary trajectories opens new perspectives in the fight against emerging antibiotic resistance.     

Evolutionary consequences of ecological factors: pollinator reliability predicts mating‐system traits of a perennial plant

The reproductive‐assurance hypothesis predicts that mating‐system traits will evolve towards increased autonomous self‐pollination in plant populations experiencing unreliable pollinator service. We tested this long‐standing hypothesis by assessing geographic covariation among pollinator reliability, outcrossing rates, heterozygosity and relevant floral traits across populations of Dalechampia scandens in Costa Rica. Mean outcrossing rates ranged from 0.16 to 0.49 across four populations, and covaried with the average rates of pollen arrival on stigmas, a measure of pollinator reliability. Across populations, genetically based differences in herkogamy (anther–stigma distance) were associated with variation in stigmatic pollen loads, outcrossing rates and heterozygosity. These observations are consistent with the hypothesis that, when pollinators are unreliable, floral traits promoting autonomous selfing evolve as a mechanism of reproductive assurance. Extensive covariation between floral traits and mating system among closely related populations further suggests that floral traits influencing mating systems track variation in adaptive optima generated by variation in pollinator reliability.     

Elevational disease distribution in a natural plant–pathogen system: insights from changes across host populations and climate

Understanding the factors determining the distribution of parasites and pathogens in natural systems is essential for making predictions about the spread of emerging infectious disease. Here, we report the distribution of the fungal anther‐smut disease, caused by Microbotryum spp., on populations of the European wildflower Silene vulgaris over a range of elevations. A survey of several geographically distinct mountains in the southern French alps found that anther‐smut disease was restricted to high elevations, rarely observed below 1300 m despite availability of hosts below this elevation. Anther smut causes host‐sterility, and is recognized as a model system for natural host–pathogen interactions, sharing common features with vector‐borne and sexually‐transmitted disease in animals. In such systems, many biotic and abiotic factors likely to change over ecological gradients can influence disease epidemiology, including host spatial structure, pathogen infectivity, host resistance, and vector behavior. Here, we tested whether host population size, density, or connectivity also declined across elevation, and whether these epidemiologically relevant factors explained the observed disease distribution. We found that while none of these factor means changed across elevation, disease was significantly more likely to occur at both higher elevations and in larger populations, the majority of which were found above 1300 m. The break in disease incidence was also associated with an apparent scarcity of these larger host populations between 1000 and 1300 m in elevation. Examining variation in climatic factors among host populations, we also showed that the probability of disease was higher in areas with historically colder, wetter, and more stable conditions. The restricted distribution of anther‐smut disease in high‐elevation S. vulgaris provides an opportunity for empirical study on range limits and disease distribution in natural alpine communities that are considered particularly sensitive to the effects of climate change.     

The evolutionary significance of parasitism: do parasite‐driven genetic dynamics occur ex silico?

It has long been recognized that reciprocal antagonism might lock host and parasite populations into a process of constant change, adapting and reacting in open‐ended coevolution. A significant body of theory supports this intuition: dynamic genetic polymorphisms are a common outcome of computer simulations of host–parasite coevolution. These in silico experiments have also shown that dynamical interactions could be responsible for high levels of genetic diversity in host populations, and even be the principle determinant of rates of genetic recombination and sexuality. The evolutionary significance of parasitism depends on the strength and prevalence of parasite‐mediated selection in nature. Here I appraise whether parasitism is a pervasive agent of evolutionary change by detailing empirical evidence for selection. Although there is considerable evidence of genetic variation for resistance, and hence the potential for selection, direct observation of parasite‐driven genetic change is lacking.