Our studies combine various approaches including ecology, genetics, pollinator behavior and modeling.
|1.||Gene flow by distinct insect pollinators||Alfalfa||Columbine|
|2.||Impact of distinct pollinators on plant mating system and plant reproductive success||Alfalfa||Columbine|
|3.||Preferences of floral traits by distinct pollinators||Alfalfa||Columbine|
|4.||Dutch elm disease, genetic diversity and hybridization between native and invasive elm species||Elm|
|6.||Other Research Areas||Wheat and stripe rust||
1. Gene flow by distinct insect pollinators
Insects are important pollinators for many fruit, vegetable, forage, and oil-seed crops. Because many of these crops are visited and pollinated by wild pollinators, in addition to managed pollinators, it is important to consider the impact of different pollinators on gene flow. This has become especially important when considering the potential for transgene escape from genetically modified crops. As a first step, we used the Rocky Mountain columbine (Aquilegia coerulea) to establish that distinct pollinators differentially disperse pollen. We showed that distinct pollinators can differentially affect the outcrossing rate of plants (Brunet and Sweet 2006, Evolution) and, using genetic markers, that these different pollinators may differentially move pollen long distances (Brunet and Holmquist 2009, Molecular Ecology). In addition, we have shown that features of the landscape may differentially affect gene flow for distinct pollinators (Brunet and Stewart 2010, Psyche: A Journal of Entomology).
To expand these studies, we are using alfalfa (Medicago sativa) in an agricultural setting, where landscape features can be manipulated. Honey bees and leaf cutting bees are used as managed pollinators of alfalfa in seed production fields. Bumble bees also visit alfalfa fields (Brunet and Stewart, 2010). We are examining how different pollinators forage in alfalfa fields and how they move pollen around. We are looking at features of the alfalfa landscape such as patch size and isolation distances between patches that affect bee movement within and among patches. Our ultimate goal is to link pollinator behavior to gene flow in order to make a more predictive model of gene flow by insect pollinators. In collaboration with Dr. Murray Clayton in the departments of Statistics and Plant Pathology at the University of Wisconsin in Madison, we are developing a simulation model that links pollinator behavior to gene flow over the agricultural landscape. Such model will help predict gene flow and the risk of transgene escape for alfalfa seed production fields and for other insect-pollinated crops. We are looking at patterns of pollinator movements, at how pollinators move pollen as they forage between flowers and at the factors that affect the probability that the pollen deposited on the stigma will generate viable seeds. We are building models for pollinator movements, pollen dispersal and for gene flow (seed set). We are examining patterns of pollinatormovements within patches and decision making process of pollinators when selecting patches (i.e. moving between patches). We are also examining the foraging range and foraging effort of pollinators using Radio Frequency Identification Device (RFID) to determine how long they are gone from the hive and which pollen they bring back to the hive.
2. Impact of distinct pollinators on plant mating system and plant reproductive success
Earlier studies with A. coerulea indicated significant variation in selfing rate among plants within populations and identified some of the plant and floral traits that affected selfing rate (Brunet and Eckert 1998, Functional Ecology). Geitonogamous selfing contributed the majority of the selfing in this plant species while autogamous selfing was negliglibe (Brunet and Sweet 2006, IJPS). Both floral display size and pollinator type affected outcrossing, with an increase in outcrossing rate with increased hawkmoth abundance and larger floral displays, and both factors affected the levels of geitonogamous selfing (Brunet and Sweet 2006, Evolution). We concluded that selfing in this plant species was most likely a non-adaptive consequence of having more than one flower open at the same time on a plant (Brunet and Sweet 2006, Evolution; Brunet and Sweet 2006, IJPS). We later quantified the variation in pollination biology and floral traits among populations over part of the range of A. coerulea and looked for correlations between floral traits and pollinator species among populations (Brunet 2009, Annals of Botany). We then examined the impact of pollination and mating system on the geographical variation in genetic structure of A. coerulea populations (Brunet et al. 2012, IJPS ). The abundance of the two main pollinators, hawkmoths and bumble bees, and the selfing rate of the different populations, did not help explain differences in genetic diversity among populations from different regions. Within regions, fire and differences in flowering phenology with altitude, combined with aggregated populations, likely limited gene flow and increased the genetic differentiation of some geographically close populations. In a separate study, we examined the impact of temperature and water availability on floral traits that affected the selfing rate and determined that the increase in flower number as a plastic response to global warming will have the strongest influence on selfing rate by increasing the level of geitonogamous selfing (assuming pollinator availability) (Van Etten and Brunet 2013, IJPS). We also detected a genetic basis to flowering time in this plant species, together with phenotypic plasticity in flowering time in response to both water and temperature (Brunet and Larson-Rabin 2012). We did not detect genetic differentiation in phenotypic plasticity suggesting, as the most likely scenario for adaptation of this plant species to climate change, a rapid response via phenotypic plasticity followed by selection and micro-evolutionary changes in the mean phenotype.
In alfalfa we have found significant variation in selfing rate (0- 52%) among plants in an alfalfa seed-production fields (Riday et al. 2015, Crop Science). We are examining the separate impact of leafcutting bees, bumble bees and honey bees on selfing rate in alfalfa (Santa Martinez et al., in preparation). We are determining how floral traits and pollinator species interact to affect selfing rate estimates of individual alfalfa plants (Santa Martinez and Brunet, in preparation). We are also examining how these distinct pollinator species influence male and female reproductive success of alfalfa plants and whether pollen discounting occurs in alfalfa and how it may vary by pollinator species (Santa Martinez and Brunet, in preparation).
We are starting a project examining the impact of different management practices in alfalfa seed production fields to determine best management practices to reduce selfing. Selfing is associated with strong inbreeding depression which can reduce both seed production and hay yield, at least the first year.
3. Preferences of floral traits by distinct pollinators
In these studies, we are interested in identifying the plant traits, including floral volatiles, that are most attractive to different pollinators. In alfalfa, we are constrasting the visual floral traits that are most attracting to honeybees, leaf cutting bees and bumble bees and examining how this impacts the reproductive success of alfalfa (Bauer et al. submitted) and how such differences may influence the phenotypic selection exerted by these distinct pollinators on floral traits (Bauer and Brunet in preparation). We are also examining and contrasting the floral volatiles of different alfalfa cultivars and wild populations of alfalfa subspecies and examining the impact of such differences on pollinator preferences (Link and Brunet in preparation).
In the Rocky Mountain columbine, we examined how bumble bees and hawkmoths affected the maintenance of the flower color polymorphism commonly found in populations of the Rocky Mountain columbine (Brunet 2009). Hawkmoths preferred blue flowers under both day and dust light conditions and bumble bees quickly learn to associate flower color with pollen reward such that the pollinators did not help explain the maintenance of the flower color polymorphism in the Rocky Mountain columbine populations (Thairu and Brunet 2015, Annals of Botany). Using the Rocky Mountain columbine as a model system, we also examined the role of floral display size, flower size, and reward sizes on bumble bee choices in a dichogamous species where both male- and female-phase flowers are opened simultaneously on inflorescences (Brunet et al. 2015, IJPS). Bees could quantify the number of pollen-producing flowers on an inflorescence and preferred inflorescences with more pollen-producing flowers rather than inflorescences with more open flowers. Bee preference for floral traits was strongly associated with pollen reward and correlations between floral traits and pollen reward are likely to have a major impact on selection on floral traits by pollinators. In a separate project, we are characterizing the impact of temperature and water availability, two factors affected by global warming, on the visual floral traits influencing pollinator attraction (Brunet et al. in preparation). We have also examined, in collaboration with Ken Keefover-Ring, the volatile organic compounds that make up the floral scent of A. coerulea plants, and determined how volatile production was affected by temperature and water availability.
4. Dutch elm disease, genetic diversity and hybridization between native and invasive elm species
This work started when Dr. Juan Zalapa came to do postdoctoral work in the laboratory under an NSF minority fellowship. Together with Dr. Zalapa and Dr. Guries, the elm breeder at UW-Madison at the time (now retired), we have examined the pattern of hybridization between the native Red elm, Ulmus rubra, and an invasive exotic species, the Siberian elm, Ulmus pumila. We have developed species-specific microsatellite markers that permitted the genetic identification of putative hybrid individuals in contact zones between the two parental species (Zalapa et al. 2008, Molecular Ecology Resources); we have described the genetic diversity of U. pumila present at the UW Arboretum in Wisconsin and planted from seeds originally collected throughout China (Zalapa et al. 2008, Genome). We have confirmed genetically the presence of hybrids between U. pumila and the native U. rubra and identified a pattern of introgression biased towards U. pumila (Zalapa et al. 2009, American Journal of Botany). Finally, we have established the widespread presence of hybrids in naturalized U. pumila populations in Wisconsin and beyond (Zalapa et al. 2010, Evolutionary Applications). We have collaborated with Dr. Alberto Santini, a forest plant pathologist in Italy, to examine patterns of hybridization between U. pumila and a native European elm, Ulmus minor (Brunet et al. 2013, Biological Invasions). Our results are raising serious concerns about the long-term survival of native elm species in the U.S. and potentially Europe. We also collaborated with a german PhD student who examined whether hybrids existed when U. pumila was planted in regions that did not have native elm species (Hirsch et al. in review, Biological Invasions). Finally, we have examined the impact of Dutch elm disease (DED) on the genetic diversity and level of genetic differentiation among populations of the native Slippery elm, U. rubra, in Wisconsin (Brunet et al. 2016, Conservation Genetics) and determined that DED did not decrease the genetic diversity or increase the levels of genetic differentiation among populations of Slippery elm.
This research examines some of the factors that affect introgression or the spread of a transgene after it has been introduced into a wild species via gene flow and hybridization. We have examined the role of pollen competition in determining the success of transgenic squash as it introgresses into wild squash populations. Wild squash is Cucurbita pepo subsp. ovifera var. ozarkana and we used the transgenic cultivar, Destiny III, a yellow crookneck squash (Cucurbita pepo subsp. ovifera var. ovifera) (Brunet et al. in preparation). Destiny III is hemizygous with a single copy of the transgene that codes for three viral coat proteins and confers resistance to zucchini yellow mosaic virus (ZYMV), watermelon mosaic virus2 (WMV-2), and cucumber mosaic virus (CMV). We have produced first-generation hybrids (F1) and first- and second-generation backcrosses (BC1 and BC2) between the transgenic cultivar and wild squash to examine pollen competition.
We are studying the population dynamics of wild carrots and examining its impact, together with gene flow among wild populations, on the spread of genes from cultivated to wild carrot populations (Megan Van Etten and Brunet 2017 Acta Horticulturae). We have collaborated on a study of the genetic structure and domestication of carrots with Phil Simon’s lab., a carrot breeder (Iorizzo et al. 2013, AJB).
We examined the evolution of plant breeding system and pollination mode in the plant genus Thalictrum using phylogenies. in collaboration with Valerie Soza, Veronica Di Stilio and Aaron Liston (Soza et al. 2012, Molecular Phylogenetics and Evolution). Thalictrum is one of the few plant genus where both modes of pollination and breeding system vary among species of Thalictrum, permitting the examination of the associations between the evolution of pollination mechanism and plant breeding system using a phylogenetic approach. Dioecy and andro- and gynomonoecy occur together with hermaphroditism and both wind and insect pollination are found in the genus. We were interested in whether wind pollination preceded the evolution of unisexual flowers (dioecy or andro-/gynomonoecy) and found that wind pollination arose early in the genus and likely preceded the evolution of unisexual flowers in many cases.
I worked on stripe rust in wheat as a postdoctoral associate at Oregon State University and tested whether disease created frequency-dependent selection on its host which could help maintain polymorphism for resistance genes in the plant population ( (Brunet and Mundt 2000, Evolution; Brunet and Mundt 2000, Heredity; Brunet and Mundt 2000, Botany; Mundt et al. 2008, Evolutionary Ecology ). We found that disease did create frequency-dependent selection on its host but the frequency-dependence did not help maintain a polymorphism in resistance genes in the host population (Brunet and Mundt, Evolution 2000). We then examined whether interactions between disease and competition could prevent the maintenance of genetic polymorphism in a highly selfing plant like wheat where associations between traits are likely (Brunet and Mundt, Heredity 2000). We found that disease was unable to reverse the relative ranking of the two genotypes caused by competition, and create the negative frequency-dependent dependence on both genotypes in a mixture required for the maintenance of a genetic polymorphism. We then examined whether the maintenance of a genetic polymorphism could be affected by plant density, because competition would be reduced at low density. We found lower absolute fitness at higher density, likely due to competition, but no impact of plant density on the maintenance of genetic polymorphisms for resistance genes (Mundt et al., 2008 Evolutionary Ecology).
My graduate work examined floral sex allocation in hermaphrotidic plants both from a theoretical and experimental perspective. I experimentally examined the impact of different factors on floral sex allocation (Brunet 1996, Ecology) and developed a theoretical model explaining conditions where sex allocation is expected to vary among flowers on hermaphroditic plants published in collaboration with Dr. Deborah Charlesworth (Brunet and Charlesworth 1995, Evolution). I wrote a review on sex allocation (Brunet 1992, Trends in Ecology and Evolution (TREE)) and on the evolution of dioecy (Thomson and Brunet 1990, TREE).