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Open Access Publications from the University of California

Scholarly Works (46 results)

  • Article
  • Peer Reviewed
UC Santa Barbara Previously Published Works (2019)

Premise of the study

Wild plant species that require the services of pollen-feeding insects for reliable pollination may evolve features that attract and reward their mutualistic partners. Heterantherous species have been proposed to exhibit a "division of labor" whereby "feeding anthers" (which produce pollen that may be consumed by an insect) are distinguished from "reproductive anthers" (which produce pollen more likely to contribute to reproduction). In some heterantherous species, including Clarkia unguiculata (Onagraceae), these two anther types differ with respect to stamen length, anther size, pollen production, and pollen color.

Methods

The primary goal of this study was to test one component of the "division of labor" hypothesis by comparing the performance of the pollen produced by each type of anther in C. unguiculata. To achieve this goal, under greenhouse conditions, we hand pollinated and assessed pollen performance (using epifluorescence microscopy) within ~228 flowers.

Key results

The pollen produced by the two anther types differed significantly with respect to both stigma and style penetration. The inner series of anthers produce pollen with higher performance than the outer series of longer, dark red anthers.

Conclusions

These findings contradict previous descriptions of the genus, reporting that the inner diminutive series of anthers in Clarkia produce "abortive and nonfunctional" pollen. We outline the future research required to demonstrate the ecological function of heteranthery in this iconic wildflower group.
    Cover page: Heteranthery in Clarkia: pollen performance of dimorphic anthers contradicts expectations
    • Article
    • Peer Reviewed
    UC Santa Barbara Previously Published Works (2016)
    Shifts in plant sex ratios help plant species to expand upslope in a warmer climate
      Cover page: How climate change affects plants' sex lives
      • Article
      • Peer Reviewed
      UC Santa Barbara Previously Published Works (2021)

      Premise

      Forecasting how species will respond phenologically to future changes in climate is a major challenge. Many studies have focused on estimating species- and community-wide phenological sensitivities to climate to make such predictions, but sensitivities may vary within species, which could result in divergent phenological responses to climate change.

      Methods

      We used 743 herbarium specimens of the mountain jewelflower (Streptanthus tortuosus, Brassicaceae) collected over 112 years to investigate whether individuals sampled from relatively warm vs. cool regions differ in their sensitivity to climate and whether this difference has resulted in divergent phenological shifts in response to climate warming.

      Results

      During the past century, individuals sampled from warm regions exhibited a 20-day advancement in flowering date; individuals in cool regions showed no evidence of a shift. We evaluated two potential drivers of these divergent responses: differences between regions in (1) the degree of phenological sensitivity to climate and (2) the magnitude of climate change experienced by plants, or (3) both. Plants sampled from warm regions were more sensitive to temperature-related variables and were subjected to a greater degree of climate warming than those from cool regions; thus our results suggest that the greater temporal shift in flowering date in warm regions is driven by both of these factors.

      Conclusions

      Our results are among the first to demonstrate that species exhibited intraspecific variation in sensitivity to climate and that this variation can contribute to divergent responses to climate change. Future studies attempting to forecast temporal shifts in phenology should consider intraspecific variation.
        Cover page: Region‐specific phenological sensitivities and rates of climate warming generate divergent temporal shifts in flowering date across a species' range
        • Article
        • Peer Reviewed
        UC Santa Barbara Previously Published Works (2016)

        Premise of the study

        Climate change models for California predict a warmer, drier future, potentially resulting in shorter growing seasons. If phenotypic differences between closely related species currently distributed across a moisture and temperature gradient represent adaptations to their abiotic environment, then as conditions become warmer and drier, populations presently adapted to cooler and wetter conditions may evolve to become more similar to those adapted to warmer and drier conditions. Two sister species, Clarkia unguiculata and C. exilis, are distributed across a moisture and temperature gradient in the southern Sierra Nevada, providing an opportunity to predict how this process may occur.

        Methods

        In a greenhouse experiment using wild-collected seeds from 11 populations in the southern Sierra Nevada, we examined relationships among elevation, climatic conditions, and population means for each trait, then evaluated bivariate relationships among maternal family means, using raw values and controlling for population and seed mass effects on phenotype.

        Key results

        Clarkia exilis occupied warmer, drier conditions, typically at lower elevations, than C. unguiculata did and flowered earlier and faster, producing smaller flowers with lower herkogamy. In C. unguiculata, petal area, herkogamy, and the rate of flower production were positively correlated with days to first flower.

        Conclusions

        If selection favors earlier flowering, smaller petals, or faster flower production in C. unguiculata, then the genetic correlations among these traits should reinforce their joint evolution. Moreover, the correlations between these traits and herkogamy may promote the evolution of self-fertilization as an indirect response to selection, a previously unrecognized potential outcome of climate change.
          Cover page: Geographic variation in climate as a proxy for climate change: Forecasting evolutionary trajectories from species differentiation and genetic correlations
          • Article
          • Peer Reviewed
          UC Santa Barbara Previously Published Works (2016)
          For most species, a precise understanding of how climatic parameters determine the timing of seasonal life cycle stages is constrained by limited long-term data. Further, most long-term studies of plant phenology that have examined relationships between phenological timing and climate have been local in scale or have focused on single climatic parameters. Herbarium specimens, however, can expand the temporal and spatial coverage of phenological datasets. Using Trillium ovatum specimens collected over > 100 yr across its native range, we analyzed how seasonal climatic conditions (mean minimum temperature (Tmin ), mean maximum temperature and total precipitation (PPT)) affect flowering phenology. We then examined long-term changes in climatic conditions and in the timing of flowering across T. ovatum's range. Warmer Tmin advanced flowering, whereas higher PPT delayed flowering. However, Tmin and PPT were shown to interact: the advancing effect of warmer Tmin was strongest where PPT was highest, and the delaying effect of higher PPT was strongest where Tmin was coldest. The direction of temporal change in climatic parameters and in the timing of flowering was dependent on geographic location. Tmin , for example, decreased across the observation period in coastal regions, but increased in inland areas. Our results highlight the complex effects of climate and geographic location on phenology.
            Cover page: Historical changes in flowering phenology are governed by temperature × precipitation interactions in a widespread perennial herb in western North America
            • Thesis
            • Peer Reviewed
            UC Santa Barbara Electronic Theses and Dissertations (2020)

            Geographic variation in fitness-related traits among populations and species may be driven by long-term climatic conditions, which may contribute to local adaptation, as well as by inter-annual variation in climate, which can cause both rapid, short-term evolution and plastic responses. Few studies, however, have assessed both factors, limiting our ability to predict how species will respond to future climate change. In my dissertation, I evaluated both long- and short-term climatic drivers of geographic variation in two traits that influence individual fitness in many species: flowering date and seed mass. To detect drivers of geographic variation in flowering time, I analyzed ~750 herbarium specimens of Streptanthus tortuosus, a widespread California jewelflower (Brassicaceae). To detect climatic influences on seed mass – a much more evolutionary conservative trait than flowering time – I examined 88 populations representing six Streptanthus species. In both studies, I used linear models to detect the independent and interacting effects of long-term and inter-annual climate conditions on the focal trait. With respect to flowering time, S. tortuosus is phenologically sensitive to both long-term and inter-annual variation in temperature and precipitation. Additionally, S. tortuosus exhibits high regional variation in the response of flowering time to local climatic conditions. Plants sampled from warm regions are phenologically more sensitive to temperature-related variables and experienced a greater increase in temperature over the last century than plants sampled from cool regions. Together, these differences resulted in significantly greater phenological advancement in warm regions than in cool regions during the past century. To apply these results to the future, I used region-specific phenological models in combination with species distribution models to forecast the effects of upcoming climate change on both the phenology and the geographic distribution of S. tortuosus. These models predict range loss and flowering time divergence between warm and cool regions during the next century. My investigation of seed mass variation similarly detected sensitivity to long-and short-term variation in temperature-mediated growing season length and precipitation. Specifically, both long-term precipitation and inter-annual precipitation anomalies affected population mean seed mass, but their effects differed in direction and magnitude. Relatively large seeds were produced at chronically wet sites but also during drier-than-average years, suggesting that these associations may be generated by different mechanisms (i.e., adaptive evolution vs. phenotypic plasticity). Collectively, these studies inform our understanding of how traits respond to long-vs. short-term variation in climate both within and among species and will improve our ability to predict species’ responses to future climate change.

              Cover page: Ecological and evolutionary causes of geographic variation in reproductive phenology and seed mass in the California jewelflowers (Streptanthus, Brassicaceae)
              • Thesis
              • Peer Reviewed
              UC Santa Barbara Electronic Theses and Dissertations (2023)

              I. Seed size is a critical factor in determining various aspects of a plant’s life cycle, such as germination rate, seedling size, and survival. The sources of variation in the trait are essential to understand its adaptive potential to changing environments. This study evaluates the genetic and environmental influences on seed size variation within and among four wild populations of N. menziesii, a widespread California annual herb. By leveraging a greenhouse breeding experiment with a nested half-sibling mating design, this study examines the magnitude of additive genetic variance and heritability of seed size within populations, the concordance between different heritability estimation methods, and the contribution of the maternal and paternal parents to seed size. Mean seed size differed significantly among populations, regardless of growing location, while the magnitude of plasticity in response to environmental conditions was similar among populations. The study revealed a disparity between the two methods used to estimate heritability – parent-offspring regression and the nested half-sibling analysis. We found evidence for maternal variance in seed size in all four populations, but significant additive genetic variance was only present in two populations.

              II. Seed size is also a critical trait that affects life-history attributes, reproductive output, and overall fitness of adult plants. Despite the competitive advantages of larger seeds, seed size variation persists within and among plant populations due to additive genetic effects, non-additive genetic effects, environmental influences, and differential seed provisioning by the maternal sporophyte. This study investigates the factors contributing to seed size variation and its fitness consequences in the California-native annual species, Nemophila menziesii, across multiple conspecific populations that were reintroduced to their home environments following a one-generation greenhouse breeding experiment. Seeds produced in greenhouse conditions were generally larger than those produced in the field, and environmental factors were significant determinants of offspring seed size across populations. Parental genotype and phenotype effects on seed size were population-specific, with maternal influences tending to exceed paternal effects. The impact of maternal seed size on reproductive traits and fitness was also population-specific, with larger maternal seed size associated with increased fruit production in three of the four populations, and increased fecundity and reproductive yield in two of the four populations. Tradeoffs between seed mass and the number of seeds produced per fruit by a reproductive individual were detected in two of the four populations.

              • 1 supplemental ZIP
              Cover page: Causes and consequences of seed size variation in Nemophila menziesii, a California native annual plant
              • Article
              • Peer Reviewed
              UC Santa Barbara Previously Published Works (2022)
              Offspring size is a key functional trait that can affect subsequent life history stages; in many species, it exhibits both local adaptation and phenotypic plasticity. Variation among populations in offspring size may be explained by various factors, including local climatic conditions. However, geographic variation in climate may be partitioned into long-term and interannual sources of variation, which may differ in their effects on population mean offspring size. To assess environmental correlates of offspring size, we evaluated geographic variation in seed mass among 88 populations representing 6 species of Streptanthus (Brassicaceae) distributed across a broad climatic gradient in California. We examined the effects of temperature-mediated growing season length and precipitation on population mean seed mass to determine whether it is best explained by (1) long-term mean climatic conditions; (2) interannual climate anomalies (i.e., deviations in climate from long-term means) during the year of seed development, or (3) interactions between climate variables. Both long-term mean climate and climate anomalies in the year of collection were associated with population mean seed mass, but their effects differed in direction and magnitude. Relatively large seeds were produced at chronically wet sites but also during drier-than-average years. This contrast indicates that these associations may be generated by different mechanisms (i.e., adaptive evolution vs. phenotypic plasticity) and may be evidence of countergradient plasticity in seed mass. In addition, populations occurring in locations characterized by relatively long growing seasons produced comparatively large seeds, particularly among chronically dry sites. This study highlights the need to consider that the responses of seed mass to long-term versus recent climatic conditions may differ and that climate variables may interact to predict seed mass. Such considerations are especially important when using these patterns to forecast the long- and short-term responses of seed mass to climate change. The results presented here also contribute to our broader understanding of how climate drives long-term (e.g., local adaptation) and short-term (e.g., phenotypic plasticity) variation in functional traits, such as offspring size across landscapes.
                Cover page: Geographic variation in offspring size: Long‐ and short‐term climate affect mean seed mass of Streptanthus populations
                • Article
                • Peer Reviewed
                UC Santa Barbara Previously Published Works (2016)
                Ecologists and evolutionary biologists have been interested in the functional biology of pollen since the discovery in the 1800s that pollen grains encompass tiny plants (male gametophytes) that develop and produce sperm cells. After the discovery of double fertilization in flowering plants, botanists in the early 1900s were quick to explore the effects of temperature and maternal nutrients on pollen performance, while evolutionary biologists began studying the nature of haploid selection and pollen competition. A series of technical and theoretic developments have subsequently, but usually separately, expanded our knowledge of the nature of pollen performance and how it evolves. Today, there is a tremendous diversity of interests that touch on pollen performance, ranging from the ecological setting on the stigma, structural and physiological aspects of pollen germination and tube growth, the form of pollen competition and its role in sexual selection in plants, virus transmission, mating system evolution, and inbreeding depression. Given the explosion of technical knowledge of pollen cell biology, computer modeling, and new methods to deal with diversity in a phylogenetic context, we are now more than ever poised for a new era of research that includes complex functional traits that limit or enhance the evolution of these deceptively simple organisms.
                  Cover page: Pollen—Tiny and ephemeral but not forgotten: New ideas on their ecology and evolution
                  • Article
                  • Peer Reviewed
                  UC Santa Barbara Previously Published Works (2019)

                  Premise of the study

                  Predicting the flowering times of angiosperm taxa is a goal of mounting importance in the face of future climate change, with applications not only in plant biology and ecology, but also horticulture, agriculture, and invasive species management. To date, no tool is available to facilitate predictions of flowering phenology using multivariate phenoclimatic models. Such a tool is needed by researchers and other stakeholders who need to predict phenological activity, but are unfamiliar with phenoclimate modeling techniques. PhenoForecaster allows users of any background to conduct species-specific phenological predictions using an intuitive graphical interface and provides an estimate of each prediction's accuracy.

                  Methods and results

                  Elastic net regression techniques were used to develop species-specific models capable of predicting the flowering dates of 2320 angiosperm species.

                  Conclusions

                  PhenoForecaster is the first stand-alone package to make phenological modeling directly accessible to users without the need for in-depth phenological observations.
                    Cover page: PhenoForecaster: A software package for the prediction of flowering phenology
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