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Yucca Research Photos | Relevant Publications | Udovic Home Page |
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(1) The Ecological &Evolutionary Dynamics of Plant-Pollinator mutualismsAn important area of research in evolutionary ecology is to decipher the detailed manner in which interacting populations influence each other’s fitness (and consequently their evolutionary trajectories). The kinds of interactions most commonly studied are competition, predation and parasitism. Much less is known about the role of mutualistic interactions (in which individuals in both populations potentially benefit), and even less is known about the complexities of multiple-species interactions in which some of the pair-wise interactions are mutualistic. Hesperoyucca whipplei provides an unusual opportunity to examine some of the questions mentioned above. First, the number of species associated with flowering and fruiting is small and most of these species are highly specialized. The sole pollinator is the specialist yucca moth, Tegeticula maculata, which also acts as a seed predator. Anthoneus agavensis, a sap beetle whose larvae destroy developing flower buds, is another specialist. In addition, there are several other species (beetles and moths that are also fairly specialized) that act as seed predators. Second, Hesperoyucca whipplei flowers only once in its lifetime, and does not reproduce vegetatively. Consequently, we can get a clear estimate of reproductive success by looking at seed production (and pollen dissemination) during the single flowering episode. Third, we can determine with relative ease the fate of every flower produced on every flowering plant in a population, and we can relate floral fates to specific interactions with floral associates. For example, flowers that abort as buds are almost always infested by a sap beetle larva. Likewise, pollinated flowers have always been visited by yucca moths. A long-term goal of our lab is to develop a framework for understanding how these species influence each other’s population dynamics and evolution. The apparent simplicity of this system masks a complex set of interactions with inherent non-linearities. For example, the flow diagram below indicates the potential complexities involved in understanding the selection pressures influencing the evolution of the plant’s strategies for allocating resources to flowering and fruiting. In our earlier studies, my students and I addressed the subset of these interactions that define the role that pollinators play in determining fruit set. Our results indicated that within broad limits, pollinators do not limit fruit production, and that yuccas are able to regulate fruit production by aborting excess fruit. These findings provided insights and raised new questions about the evolution and stability of the mutualistic interaction between the plants and moths, which have been the focus of a number of studies by other investigators over the last decade. We have recently begun a new phase of this research (in collaboration with Judith Bronstein at the University of Arizona and Kari Segraves, University of Idaho), turning attention to the influence of florivory on fruit production and pollen dissemination, and how that is in turn influenced by the interaction between H. whipplei and its pollinators. Although these studies are in their initial stages, they indicate the direction that we plan to pursue over the next few years.
Damage to yucca inflorescences resulting from florivory by Anthoneus is quite variable both within and between populations, occasionally exceeding 50% of the flowers on an inflorescence. Infested flowers are too severely damaged to serve as pollen donors or to produce mature fruits. Since plants regulate fruit production by aborting fruit, I hypothesized that yuccas infested with Anthoneus compensate for floral damage by aborting fewer fruits. Bethany Rader and I tested this hypothesis by determining the fate of each flower on 100 inflorescences at a study site near San Diego, CA. We constructed floral life tables for each inflorescence, using key factor analysis to determine the relative contribution of different sources of flower and fruit mortality. We also compared our results with analyses of a similar data set that I collected in 1981. Our preliminary analyses indicate that Anthoneus appears to have only a small affect on the production of mature fruits (Udovic 2001). Yucca whipplei’s “low-fruiting hermaphrodite” flowering strategy of producing many flowers but relatively few fruits, and typically aborting many “excess fruits,” allows it to compensate for beetle damage. Even in years in which fruit production appears limited by pollinators (e.g., 2001), fruit production is not correlated with damage caused by bud predators. One possible explanation is that beetle damage is concentrated in flowers that are relatively high up on inflorescences, and that would open after most pollination has occurred. These results support the hypothesis that, at least within broad limits, Anthoneus infestations have little affect on the current ecological dynamics of H. whipplei or Tegeticula maculata populations. Evolutionary implications are more difficult to discern. So far, our studies address only the female component of plant fitness. It seems likely, however, that plants with heavier beetle infestations will have less of their pollen collected by yucca moths, resulting in fewer fruits “sired”. Consequently, Anthoneus is still likely to be an important element in the selection regimes of both Hesperoyucca and Tegeticula. We are just beginning a large-scale analysis of patterns of pollen donation (using AFLP’s for parentage analysis in a technique similar to DNA fingerprinting). Our hope is to be able to identify the male parents for a significant fraction of the seeds produced at our study sites. Since we will know both parents, we will have a better estimate of each plant’s overall fitness, and we will be able to relate that to a variety of factors, including inflorescence size and location, timing of flowering, and levels of pollinator visitation and beetle infestation. Those results can then be combined with models of the spatial and temporal patterns of this multiple-species interaction to provide a more complete analysis of the complexities of this system. (2) The response of an obligate plant-pollinator mutualism to fire disturbanceJudith Bronstein (University of Arizona) and I are currently studying the response of Hesperoyucca whipplei and its pollinator Tegeticula maculata to fire. These studies take advantage of recent fires in southern California in the fall of 2003 that burned one of our long-term field sites. Comparing flowering and pollination data pre-and post-burn and with data from an unburned site will provide insights into the sensitivity, resilience and re-establishment of a highly specialized obligate mutualism after a major disturbance. Surveys during the 2004 flowering season indicated that most yuccas survived at our burned site (the Elliot Chapparal Reserve near San Diego), despite the destruction of almost all other above-ground vegetation. Furthermore, more yuccas flowered than in the previous year. Fruit production was greatly diminished, however, because the pollinator population was severely reduced. In contrast, pollinator density at our unburned site (on the Santa Margarita Ecological Reserve near Fallbrook) was extremely high in 2004, resulting in normal to high fruit production. We plan to continue to monitor these sites to determine the influence of the fire on the mutualism propogates through time This collaborative research project is funded by an NSF SGER (DEB- Ecology - 0412729). (3) Assessing and conserving genetic diversity in the Hesperoyucca whipplei species complexOur research using molecular approaches to determining parentage also allows us to assess genetic diversity in H. whipplei populations. With funding from the University of California's Genetic Resources Conservation Program, Kari Segraves (University of Idaho) and I have been working on a study designed to measure and help preserve within population and between-population genetic diversity in Hesperoyucca. In conjunction with this project we are archiving materials that can be used by future researchers. The archival collection consists of extracted DNA from preserved (dried or frozen) leaf tissue from adult flowering plants as well as from seeds. Materials are currently available from two locations: the Elliot Chaparral Reserve, near San Diego (administrated by UCSD) and the Santa Margarita Ecological Reserve, near Temecula, CA (administrated by SDSU). A long-term goal of this project is to add collections from populations throughout the species’ range in an effort to broaden our understanding of the diversity and evolution of Hesperoyucca whipplei and its specialized insect associates and to assist in the development of conservation policies and plans that insure the long-term persistence of this unique, highly specialized group of species. For information about archived materials, contact me (udovic@uoregon.edu). The following tables (click here) provide a list of available materials. These tables will continue to be updated as additional materials become available. Detailed information is available about flowering and fruit production for many of these plants. For available photos of flowering individuals, see photo gallery.
AFLP bands from H. whipplei (photo courtesy of Kari Segraves) |
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