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Title:Intraspecific variation in resource use, dormancy investment, and competitive ability in the facultative parthenogen Daphnia pulicaria
Author(s):Crawford, John Williams
Director of Research:Cáceres, Carla E.
Doctoral Committee Chair(s):Cáceres, Carla E.
Doctoral Committee Member(s):Suarez, Andrew V.; Fuller, Rebecca C.; Heath, Katy D.
Department / Program:School of Integrative Biology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Intraspecific variation
Power-efficiency trade-off
Juvenile growth rate
Clonal selection
Population dynamics
Sexual reproduction
Cyclic parthenogen
Facultative parthenogen
Fitness-associated sex
Consumer-resource theory
Daphnia pulicaria
Abstract:Ecological studies highlight that many species and most populations maintain high degrees of intraspecific (within species or within population) variation in a wide variety of life-history traits. This pattern prompts the question: how important is this variation? This is a question that is especially motivating for evolutionary ecologists as we are interested in how intraspecific variation influences evolutionary dynamics. While the maintenance of intraspecific variation is especially common in species that reproduce sexually, it is also common in clonal (and partially clonal) species. In my dissertation, I use the facultatively parthenogenetic zooplankton Daphnia pulicaria to address several questions related to the breadth and consequences of intraspecific variation. Freshwater zooplankton species are important subjects for ecologists. In particular, crustaceans in the genus Daphnia have been studied due to their importance as keystone species in many freshwater lakes. They provide ecosystem services including moving resources from the primary producers up to higher trophic levels and have served as models for grazers in other systems. Decades of research on Daphnia demonstrate substantial interspecific (between species) and intraspecific (within species) variation in how individuals respond to available food resources. These studies targeted both spatial variation in the quality and quantity of algal resources between neighboring lakes and temporal variation (including changes due to decades of eutrophication). This prior research has shown that there is interspecific variation in the ability of species to capitalize on different resource quantities and qualities. One limitation of prior research in this system is that it has concentrated almost exclusively on interspecific variation in resource acquisition and allocation and has largely ignored intraspecific variation within and among populations. Variation in resource quality in natural systems along with trade-offs between and within populations in response to resources makes the Daphnia-algae system ideal for examining intraspecific variation in key life-history traits. Prior work on Daphnia suggests both an interspecific and intraspecific trade-off in response to resource quality between “powerful” and “efficient” species and individuals. In several studies, Alan Tessier and colleagues found that some Daphnia species were better able to capitalize on rich resources and that these “powerful” individuals maximized their growth on both high-quantity and rich-quality resources but were “sensitive” to the decline in resource quality or quantity. Other “efficient” species were better able to maintain their growth on low quantity or poor-quality resources, and were not sensitive to the decline in resource quantity or quality. Further work by Spencer Hall and colleagues extended this examination to intraspecific variation in individual ability to use resources of different qualities. They found similar variation in terms of powerful and efficient strategies occurring among individuals of Daphnia ambigua. My research goal was to examine how intraspecific variation in response to resource quality may (1) be influenced by the population or season from which an individual was collected, (2) influence the likelihood of an individual to invest in sexually-produced dormant offspring, and (3) influence competitive ability. In the first chapter, I explore intraspecific variation in response to resource quality in 143 clones from six populations of Daphnia pulicaria in Michigan. In freshwater lakes, the quality of algal resources varies seasonally as the rich-quality resources of the spring are replaced by poor-quality resources in the summer. Concurrently, there is a decline in population density of large bodied grazers (such as D. pulicaria) through the combined effects of competition for resources, predation, and parasitism. In some “non-persisting” populations the decline in density is so dramatic that populations are reduced to undetectably low levels by the summer. As individuals from these non-persisting populations do not experience poor-quality resources in the summer, I predicted that these individuals would grow relatively poorly on poor-quality algal diets in the laboratory (i.e. would be sensitive to changes in resource quality). I also predicted populations that persisted through the summer would have fewer individuals who were sensitive to the decline in resource quality as these individuals are exposed to both rich- and poor-quality resources in the field. Although I found significant variation in response to resource quality, my results did not support the prediction that sensitivity to resource quality is greater in the non-persisting populations. I further examined the genetic consequences of resource sensitivity by looking at turnover in clone identity. I predicted a turnover in genotypes between spring and summer in the persisting populations as more efficient genotypes persevered and the sensitive genotypes were selected against. Although there was evidence for rapid evolution between spring and summer, my results did not support the prediction that the distributions of growth rates were driven by changing qualities of resources in the field. Daphnia exhibit considerable intraspecific variation in the likelihood of investing in sexually-produced dormant offspring, but why some genotypes are more likely to invest in sex/dormancy is less understood. For many Daphnia species (including D. pulicaria), sexual reproduction is the only means of producing dormant offspring; therefore, investment in sex/dormancy can be viewed as a cost-benefit analysis in which individuals forgo current production of numerous clonal daughters for the future hatching of fewer but sexually-produced daughters. In chapter two, I examined whether this variation is due to the ability of a genotype to grow on different qualities of resources, an indication of current fitness. Using 121 of the clones from chapter one, I assessed the likelihood of a genotype to invest in sexual reproduction and dormancy by quantifying investment in clonal daughters, clonal sons, and haploid eggs awaiting fertilization. Although the observed variation suggests other factors contribute to the likelihood of allocating to sex/dormancy, I found that individuals with lower mean growth had higher investment in sexually-produced dormant offspring. There are three possible explanations for this finding. First, dormancy constitutes a temporal escape from poor environmental conditions such as the onset or expected onset of higher competition, predation, and parasitism. Second, individuals with low current fitness may be more likely to invest in sexual reproduction for the potential fitness benefits to their offspring. Or third, the investment in sexually-produced dormant offspring could be for the joint benefits of both sexual reproduction and dormancy. In chapter three, I explored whether the laboratory-assessed sensitivity to resource quality (from chapter one) drives competitive dynamics and predicts growth on field-collected resources. I used three genotypes that were sensitive to changes in resource quality and three that were efficient and maintained equivalent growth on both rich- and poor-quality resources. As resource quality in the field progresses through a seasonal succession from rich-quality algae in the spring to poor-quality, toxic, and/or digestion resistant algae in the summer, my first prediction was that sensitive individuals would be able to grow better (have higher expected fitness) on spring- and poorly on summer-collected resources. My results did not support the prediction that laboratory-assayed sensitivity predicted an individual’s growth on field-collected resources. Secondly, I predicted that the outcome of a laboratory competition assay would mirror these results; high sensitivity genotypes were predicted to perform better in rich-quality competition diets. My results show that individuals of both sensitivities reached equivalent densities in both diet treatments and there was no competitive advantage in the 21-day experiment. Despite the difference in performance by these genotypes on rich-quality resources documented in chapter one, sensitivity to resource quality does not appear to drive competition dynamics. Instead, high overall densities and resource limitation may have constrained population growth and the effects of intra-strain and inter-strain competition may have had an equivalent effect in this low volume, short-term, and high-density experiment. Although sensitivity to resource quality is not governing performance on field-collected resources or in short-term competition, I suggest that other factors such as a genotype’s ability to maintain growth on poor-quality resources may be a more important metric for future studies seeking to predict individuals’ growth in the field or longer-term competitive ability. In conclusion, I demonstrated several interesting results in the study of intraspecific variation. First, there is significant variation in response to resource quality both between and within populations of what has been previously described as a generalist grazer. Second, this intraspecific variation in growth contributes to an individual’s likelihood of investing in sexually-produced dormant offspring. Third, while my analysis failed to link sensitivity to resource quality with competitive ability, there is intraspecific variation in other growth traits that should be explored. My results indicate two important factors that evolutionary ecologists should continue to consider. First, using the mean trait of a species misses a lot of interesting and potentially important variation. Secondly, variation in suites of ecologically important traits can influence other suites of traits. To better understand biological phenomena, we must consider the importance of intraspecific variation and the joint-effects of variation on other suites of traits.
Issue Date:2017-04-19
Rights Information:Copyright 2017 John Crawford
Date Available in IDEALS:2017-08-10
Date Deposited:2017-05

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