Øystein H. Opedal

Linking small-scale topography with microclimate, plant species diversity and intra-specific trait variation in an alpine landscape

Background: Small-scale topographic complexity is a characteristic feature of alpine landscapes, with important effects on alpine plant distribution. Aims: We investigated the links between small-scale topographic complexity and resultant microclimatic heterogeneity, vascular-plant species richness and beta diversity, and realised niche width and trait variation of some target species. Methods: We recorded temperature and soil moisture within 10 sites (40 m × 40 m) of differing topographic complexity in alpine terrain at Finse, Norway (N 60° 36ʹ, E 7° 33ʹ). Plant species occurrence and traits of target species were recorded in 16 sample plots at each site. Results: Sites differed significantly in microclimatic heterogeneity, and topographically rough sites were always more heterogeneous than flatter ones. Greater species richness and turnover was associated with greater microclimatic heterogeneity, and rough sites contained 15–55% more species than flatter ones. Plant species had on average wider realised niches when growing at rough sites. Individuals of Bistorta vivipara, but not those of Luzula spicata, tended to exhibit greater phenotypic variation at rough sites. Conclusions: Rough alpine terrains create small-scale variation in microclimate, promoting species richness and beta diversity. In the event of climate change, small-scale microclimatic heterogeneity might allow plant species to escape from regional climate change by short-distance migration to local micro-refugia. This study suggests that the opportunity for such responses would be greater in topographically complex terrains.

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.

Does stronger pollen competition improve offspring fitness when pollen load does not vary

PREMISE OF THE STUDY Competition among pollen grains from a single donor is expected to increase the quality of the offspring produced because of the recessive deleterious alleles expressed during pollen-tube growth. However, evidence for such an effect is inconclusive; a large number of studies suffer from confounding variation in pollen competition with variation in pollen load. METHODS In this study, we tested the effect of pollen competition on offspring performance independently of pollen-load variation. We compared seed mass and early seedling performance in Dalechampia scandens (Euphorbiaceae) between crosses in which variation in pollen competition was achieved, without variation in pollen load, by manipulating the dispersion of pollen grains on the stigmas. KEY RESULTS Despite a large sample size (211 crosses on 20 maternal plants), we failed to find an effect of pollen competition on seed characteristics or early seedling performance. Paternal effects were always limited, and pollen competition never reduced the within-father (residual) variance. CONCLUSION These results suggest that limited within-donor variation in genetic quality of pollen grains reduces the potential benefits of pollen competition in the study population. The lack of paternal effects on early sporophyte performance further suggests that benefits of pollen competition among pollen from multiple donors should be limited as well, and it raises questions about the significance of pollen competition as a mechanism of sexual selection.

Euglossine bees mediate only limited long-distance gene flow in a tropical vine.

Euglossine bees (Apidae: Euglossini) have long been hypothesized to act as long-distance pollinators of many low-density tropical plants. We tested this hypothesis by the analysis of gene flow and genetic structure within and among populations of the euglossine bee-pollinated vine Dalechampia scandens. Using microsatellite markers, we assessed historical gene flow by the quantification of regional-scale genetic structure and isolation by distance among 18 populations, and contemporary gene flow by the estimation of recent migration rates among populations. To assess bee-mediated pollen dispersal on a smaller scale, we conducted paternity analyses within a focal population, and quantified within-population spatial genetic structure in four populations. Gene flow was limited to certain nearby populations within continuous forest blocks, whereas drift appeared to dominate on larger scales. Limited long-distance gene flow was supported by within-population patterns; gene flow was biased towards nearby plants, and significant small-scale spatial genetic structure was detected within populations. These findings suggest that, although female euglossine bees might be effective at moving pollen within populations, and perhaps within forest blocks, their contribution to gene flow on the regional scale seems too limited to counteract genetic drift in patchily distributed tropical plants. Among-population gene flow might have been reduced following habitat fragmentation.

The evolvability of herkogamy: Quantifying the evolutionary potential of a composite trait

Accurate estimates of trait evolvabilities are central to predicting the short‐term evolutionary potential of populations, and hence their ability to adapt to changing environments. We quantify and evaluate the evolvability of herkogamy, the spatial separation of male and female structures in flowers, a key floral trait associated with variation in mating systems. We compiled genetic‐variance estimates for herkogamy and related floral traits, computed evolvabilities, and compared these among trait groups and among species differing in their mating systems. When measured in percentage of its own size, the median evolvability of herkogamy was an order of magnitude greater than the evolvability of other floral size measurements, and was generally not strongly constrained by genetic covariance between its components (pistil and stamen lengths). Median evolvabilities were similar across mating systems, with only a tendency toward reduction in highly selfing taxa. We conclude that herkogamy has the potential to evolve rapidly in response to changing environments. This suggests that the extensive variation in herkogamy commonly observed among closely related populations and species may result from rapid adaptive tracking of fitness optima determined by variation in pollinator communities or other selective factors.

Multiple Effects of Drought on Pollination and Mating-System Traits in Dalechampia scandens

Premise of research. Selfing rates in mixed-mating plant species are often found to fluctuate greatly across time and space. Environmentally induced changes in floral traits may mediate changes in selfing rates through several mechanisms, including direct effects via changes in traits influencing autofertility rates and indirect effects via changes in traits affecting the rate of pollinator visitation and/or the efficiency of cross-pollination. In this study, we tested how experimentally induced drought affected traits related to these three components of plant mating systems. Methodology. We subjected two populations from each of two species in the Dalechampia scandens species complex to a series of experimental drought events in the greenhouse. We measured drought effects on advertisement (signaling) traits, reward traits, herkogamy, dichogamy, autofertility, and pollination accuracy and compared these across populations and species. Pivotal results. Blossom size (advertisement and reward traits) and dichogamy were consistently reduced under dry conditions. In contrast, the effects of drought on herkogamy and autofertility were population specific. Similarly, despite consistent effects of drought on traits functionally related to pollen transfer, changes in pollination accuracy differed among populations. When plants were returned to a benign moisture environment, phenotypic changes were largely reversed. Conclusions. These results show that environmental variation may simultaneously affect multiple traits related to plant mating systems and, thus, mediate spatial and temporal variation in selfing rates. However, except for size reductions in advertisement and reward traits, these effects tend to be population specific and, therefore, difficult to predict.

Intersexual conflict over seed size is stronger in more outcrossed populations of a mixed-mating plant

Significance Intersexual conflict over maternal resource allocation to offspring can lead to the evolution of imprinted genes with parent-of-origin–specific expression. However, the precise mechanism involved in the evolution of such imprinted genes is less well understood, and few clear predictions have been presented. We resolve this issue, and, using different populations of a mixed-mating plant, we demonstrate that more outcrossed paternal populations produce larger seeds when crossed with less outcrossed maternal populations, and vice versa. This provides clear support for a “tug-of-war” mechanism operating between maternally and paternally imprinted genes. Such a mechanism can have important consequences for local adaptation in offspring size in the presence of gene flow between populations with different mating systems. In polyandrous species, fathers benefit from attracting greater maternal investment toward their offspring at the expense of the offspring of other males, while mothers should usually allocate resources equally among offspring. This conflict can lead to an evolutionary arms race between the sexes, manifested through antagonistic genes whose expression in offspring depends upon the parent of origin. The arms race may involve an increase in the strength of maternally versus paternally derived alleles engaged in a “tug of war” over maternal provisioning or repeated “recognition-avoidance” coevolution where growth-enhancing paternally derived alleles evolve to escape recognition by maternal genes targeted to suppress their effect. Here, we develop predictions to distinguish between these two mechanisms when considering crosses among populations that have reached different equilibria in this intersexual arms race. We test these predictions using crosses within and among populations of Dalechampia scandens (Euphorbiaceae) that presumably have experienced different intensities of intersexual conflict, as inferred from their historical differences in mating system. In crosses where the paternal population was more outcrossed than the maternal population, hybrid seeds were larger than those normally produced in the maternal population, whereas when the maternal population was more outcrossed, hybrid seeds were smaller than normal. These results confirm the importance of mating systems in determining the intensity of intersexual conflict over maternal investment and provide strong support for a tug-of-war mechanism operating in this conflict. They also yield clear predictions for the fitness consequences of gene flow among populations with different mating histories.

Microenvironment and functional‐trait context dependence predict alpine plant community dynamics

Predicting the structure and dynamics of communities is difficult. Approaches linking functional traits to niche boundaries, species co-occurrence and demography are promising, but have so far had limited success. We hypothesized that predictability in community ecology could be improved by incorporating more accurate measures of fine-scale environmental heterogeneity and the context-dependent function of traits. We tested these hypotheses using long term whole-community demography data from an alpine plant community in Colorado. Species distributions along microenvironmental gradients covaried with traits important for below-ground processes. Positive associations between species distributions across life stages could not be explained by abiotic microenvironment alone, consistent with facilitative processes. Rates of growth, survival, fecundity and recruitment were predicted by the direct and interactive effects of trait, microenvironment, macroenvironment and neighbourhood axes. Synthesis. Context-dependent interactions between multiple traits and microenvironmental axes are needed to predict fine-scale community structure and dynamics. (Less)

Digest: Toward predicting evolutionary response to environmental change: The power of integrated experimental and genetic studies*: DIGESTS

How do populations become locally adapted? Answering this requires accurate estimates of two core components of evolution: natural selection acting on phenotypic traits in natural populations and the genetic architecture of these traits. Despite an ever growing number of studies, our current understanding is incomplete for both components. In this issue, Ågren et al. (2016) examine the genetic basis of differences between two locally adapted populations of thale cress (Arabidopsis thaliana). Their study is exemplary in jointly investigating both components of evolution in the wild and combining several of the most powerful experimental approaches available to students of evolution. Such integrated studies yield unique insights into the evolutionary process and contribute to our ability to predict adaptation to changing environments. Natural selection is notoriously difficult to measure. Restricted phenotypic variation in well-adapted natural populations makes unfit individuals rare and the expected selection against them hard to detect. Ågren et al. elegantly overcame this problem by generating experimental populations via crossing parents from highly differentiated populations from Italy and Sweden and transplanting these back into the parental environments. A second challenge in selection studies is to separate true trait–fitness relationships from spurious correlations arising from joint environmental effects on fitness and traits. This problem was overcome