Variation in Plant Defense Traits and Population Genetics Within A
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Iowa State University Capstones, Theses and Graduate Theses and Dissertations Dissertations 2012 Variation in plant defense traits and population genetics within a Sonoran Desert cotton endemic, Gossypium davidsonii and boll weevil, Anthonomus grandis Adam Kuester Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/etd Part of the Entomology Commons, Evolution Commons, and the Plant Sciences Commons Recommended Citation Kuester, Adam, "Variation in plant defense traits and population genetics within a Sonoran Desert cotton endemic, Gossypium davidsonii and boll weevil, Anthonomus grandis" (2012). Graduate Theses and Dissertations. 12371. https://lib.dr.iastate.edu/etd/12371 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Variation in plant defense traits and population genetics within a Sonoran Desert cotton endemic, Gossypium davidsonii and boll weevil, Anthonomus grandis by Adam P. Kuester A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Major: Genetics Program of Study Committee: John Nason, Major Professor Thomas Sappington Jonathan Wendel Fredric Janzen Matthew O’Neal Iowa State University Ames, Iowa 2012 ii TABLE OF CONTENTS CHAPTER1. General Introduction 1 CHAPTER 2. Abiotic and biotic stress affects on defense trait 12 phenotypes in a wild cotton species, Gossypium davidsonii CHAPTER 3. Response of herbivory to defense traits, geographic and 54 genetic structure within a Sonoran Desert wild cotton species, Gossypium davidsonii CHAPTER 4. Comparison of quantitative defense traits and genetics 90 in determining evolution of defense syndromes in a wild Sonoran Desert cotton endemic, Gossypium davidsonii CHAPTER 5. Covariation in the geographic genetic structures of a 137 wild cotton, Gossypium davidsonii, and associated boll weevil, Anthonomus grandis CHAPTER 6. Population structure and genetic diversity of the boll 182 weevil, Anthonomus grandis, in North America CHAPTER 7. General Conclusions 234 APPENDIX. DNA Sequencing 238 1 CHAPTER 1. General Introduction Introduction Understanding the evolutionary mechanisms behind plant defense can be very important in determining how plant populations have evolved to cope with associated insect herbivores. Not surprisingly, vascular plants have developed complex chemical and physical strategies to defend themselves against them. While such strategies can be effective at reducing herbivory, investment in defenses often comes as a tradeoff with other processes (as reviewed in Herms and Mattson, 1992; Simms, 1992; Hakulinen et al., 1995; Haukioja et al., 1998; Haugen et al., 2008; Moreno et al., 2009). Defense trait variation may, in fact, create quite different responses by associated insect herbivores. For instance, specialist herbivores that rely on Gossypium plants for development are attracted by particular colors of cotton leaves and high levels of plant terpenoids (Lincoln and Boyer 1975, Hedin and McCarty 1995), whereas generalist folivores tend to tolerate lower levels of plant toxins and varying degrees of leaf pubescence depending on feeding habit (Lincoln and Boyer 1975; Stipanovic et al. 2008). Additional levels of plant defense through response of tertiary trophic predators can drastically alter herbivore communities. The initial reason for host preference across an insect herbivore’s species range is frequently related to host availability and distribution within a particular location, but also can stem from variation in host plant phenotypes. An insect may have higher reproductive output or faster growth rates on plants that exhibit particular characters, leading to greater selection on plant populations against less fit defense syndromes. 2 Recent studies suggest that patterns of gene flow within interacting species can be an important determinant of their coevolutionary dynamics (Thompson and Burdon, 1992; Nuismer et al., 1999; Forde and Thompson, 2004; Hoeksema and Forde, 2008; Vogwill et al., 2008; Gandon and Nuismer, 2009). Indeed, that outcomes of inter-species interactions are influenced by the level and spatial patterning of gene flow between populations of both species is a central component of the geographic mosaic theory of coevolution (Thompson, 2005; Gomulkiewicz et al. 2007). In plant-insect systems, the nature of the inter-specific interaction can influence the amount and geographical symmetry of gene flow between populations of both organisms. In particular, interactions in which an insect directly affects plant dispersal through pollination and seed distribution are more likely to contribute to the geographical symmetry of gene flow between the two species. Regardless of this symmetry, very low rates of gene flow within taxa tend to limit local adaptive genetic variation and the rates of coevolutionary change, while very high levels of gene flow can cause local maladaptation by limiting the effectiveness of reciprocal selection. Theoretical studies indicate, however, that with greater symmetry of gene flow between symbionts, localities with stronger reciprocal selection tend to dominate global coevolutionary patterns (Nuismer et al. 1999; Gomulkiewicz et al. 2000). In addition to host association and gene flow within species of insect herbivores, inter-species dynamics is also of interest by illuminating geographical effects and host race associations through genetic structuring of insect herbivores that have broader host diets. Insect species that are hosted by one to several species of plants may be genetically structured simply as a result of isolation by distance or by prefence of a particular plant host. 3 Research Rationale: Recognizing the mechanisms and patterns of evolution of natural plant defenses not only allows us to understand how to reasonably control host associated pest populations, but also to direct better-suited breeding and selection programs for traits associated with tolerance or resistance to otherwise uncontrollable pests (Gould 1988; Cortesero et al. 1999; Rausher 2001; Thrall et al. 2011). Though this thesis work focuses on Gossypium davidsonii, similar defense traits and responses to herbivory can be seen in many of the New World cotton species (Rudgers et al. 2004). Understanding how plants respond to herbivory in natural systems should provide us with insights into variation in traits that may provide resistance within agronomic settings. Additionally, population genetic studies of associated pest insects have identified approaches that best control associated insect herbivore populations (Porreta et al., 2007). Study System Gossypium davidsonii is a wild diploid cotton species that is endemic to the Sonoran Desert of northwestern Mexico, found in the subsection Integrifolia within the genus Gossypium (Family Malvaceae). It is commonly characterized as a branched shrub between 1-3 meters in height, with lobed to entire cordate leaves. The species is found from sea level to 400 m elevation with primary range within the lower Cape region of the Baja Peninsula. Habitat ranges from open to thorn-scrub vegetation in disturbed or rocky areas. Like most cotton species, G. davidsonii exhibits several characteristic defense traits. Though variation in nectary presence has been previously noted (Phillips and Clements, 1967), we detect no biological reason for selection on nectaries, as no myrmechophyllic 4 interaction with associated ants has been observed within populations of the species (pers. obs.). Another common defense trait among cotton species is presence of pigmented glands on aerial plant parts in which gossypol and related terpene aldehyde anti-herbivore defense compounds are stored. Pigment gland densities on fruit capsules have been noted to vary among populations from 250-300 glands per capsule and foliar quantities of glands make up between 1.8 – 2.0 % of leaf dry weight within the species (Phillips and Clements, 1967). Gossypium davidsonii expresses higher concentrations of gossypol in seed and leaf tissue than any other species (Khan, 1999; Stipanovic et al., 2005). Gossypol concentration varies up to 6-fold among populations from Baja California’s Cape Region in both seed and leaf tissues, despite sample sites being located <150 km apart (Stipanovic et al., 2005). Little attention has been devoted, however, to understanding why G. davidsonii (or any other cotton) produces large amounts of this compound, or why levels of gossypol expression vary so extensively among populations. Wild New World cotton species have been well studied in terms of their variation for desired traits, such as fiber production (Mei et al. 2004; Park et al., 2005), chemical defenses (Stipanovic, 1986; Stipanovic et al., 2003), and oil content (Gotmare et al., 2004). Other aspects of their natural history, however, are not well understood. On the one hand, many of the New World species have large, showy flowers, suggesting that they are attractive for pollinators, have out-crossing mating systems, and experience appreciable gene flow. On the other hand, studies of wild cottons have found low levels of genetic variability at putatively neutral genetic marker loci, including G. davidsonii (AFLPs: