Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 2004 Host specialization, intersterility, and taxonomy of populations of Ceratocystis fimbriata from sweet potato, sycamore, and cacao Christine Jeanette Baker Engelbrecht Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Agriculture Commons, and the Plant Pathology Commons Recommended Citation Engelbrecht, Christine Jeanette Baker, "Host specialization, intersterility, and taxonomy of populations of Ceratocystis fimbriata from sweet potato, sycamore, and cacao " (2004). Retrospective Theses and Dissertations. 935. https://lib.dr.iastate.edu/rtd/935 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 Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Host specialization, intersterility, and taxonomy of populations of Ceratocystis fimbriata from sweet potato, sycamore, and cacao by Christine Jeanette Baker Engelbrecht A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Major: Plant Pathology Program of Study Committee: Thomas C. Harrington, Major Professor Edward Braun Lynn Clark Russell Jurenka Gary Munkvold Iowa State University Ames, Iowa 2004 UMI Number: 3145635 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. 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For the Major Program iii TABLE OF CONTENTS ABSTRACT iv CHAPTER 1. GENERAL INTRODUCTION 1 CHAPTER 2. GENETIC VARIABILITY AND HOST SPECIALIZATION IN THE LATIN AMERICAN CLADE OF CERATOCYSTIS FIMBRIATA 29 CHAPTER 3. INTERSTERILITY, MORPHOLOGY, AND TAXONOMY OF CERATOCYSTIS FIMBRIATA ON SWEET POTATO, CACAO, AND SYCAMORE 71 CHAPTER 4. GENETIC ANALYSIS OF PROGENY FROM CROSSES WITHIN AND BETWEEN SPECIES IN THE CERATOCYSTIS FIMBRIA TA COMPLEX 108 CHAPTER 5. GENETIC VARIATION IN EASTERN NORTH AMERICAN AND PUTATIVELY INTRODUCED POPULATIONS OF CERATOCYSTIS PLATANI 134 CHAPTER 6. GENETIC VARIATION OF POPULATIONS OF THE CACAO WILT PATHOGEN CERATOCYSTIS CACAOFUNESTA 168 CHAPTER 7. CONCLUSIONS 201 iv ABSTRACT Ceratocystis fimbriata is a large complex of fungal species that cause wilt- type diseases on a variety of temperate and tropical plants. Within the Latin American clade of C. fimbriata, isolates from sweet potato (Ipomoea batatas), sycamore (Platanus spp.), and cacao (Theobroma cacao) form monophyletic lineages and are specialized to these three hosts. The sweet potato and sycamore lineages correspond to intersterility groups, and there are two intersterility groups within the cacao lineage, corresponding to a sublineage in western Ecuador and a second sublineage in Brazil, Colombia, and Costa Rica. The cacao lineage is described as a new species, C. cacaofunesta, and the sycamore lineage is raised from a form to a species, C. platani. Genetic analyses of progeny from crosses within and between C. fimbriata, C. cacaofunesta, and C. platani showed that these species likely have different numbers, sizes, or rearrangements of chromosomes. A population genetics study of C. platani using polymorphic microsatellite markers and nuclear and mitochondrial DNA fingerprints revealed that the population in the eastern USA is probably indigenous, and the pathogen was introduced to southern Europe and Modesto, California. A study of C. cacaofunesta using the same markers revealed that the populations of this pathogen in Ecuador and Rondônia (in the Upper Amazon of Brazil) are likely to be indigenous, while populations in Bahia Brazil, Colombia, and Costa Rica resulted from introductions of an Upper Amazonian population. 1 CHAPTER 1. GENERAL INTRODUCTION Ceratocystis fimbriata Ellis & Halsted is a fungal plant pathogen that causes serious diseases on a wide range of temperate and tropical plants. Economically important plants affected by this fungus include cacao, coffee, sycamore, poplar, hickory, almond, mango, eucalyptus, rubber tree, taro and sweet potato (on which it produces a black root rot) (CABI 2001). In woody hosts, the pathogen causes stem cankers, vascular stains, and wilt-type diseases. This pathogen is particularly important in Central and South America, the proposed center of origin of the Latin American clade of C. fimbriata (Harrington 2000). Within Ceratocystis fimbriata, groupings of isolates based on genetic sequences correlate to plant host groups, suggesting that this species is actually a collection of many lineages, which perhaps should be separated and defined as separate species. The definition of a species itself is debatable, though the species is the most basic unit of classification underlying all biology. Although many species concepts have been proposed, these models are often inappropriate for fungi, which have been underrepresented in studies of species and speciation despite their importance in other biological model systems. Effective species concepts and accurate systematic understandings of plant pathogenic fungi are essential for studying the epidemiology of plant diseases, understanding evolution and speciation of new pathogens, and developing effective methods for disease control. Additionally, human activities may complicate attempts to describe species of plant- pathogenic fungi by increasing their rates of evolution, creating new niches by 2 introducing the pathogens to new areas and hosts, and exerting additional selection pressures (Ennos and McConnell 1995). Because of its economic importance and diversity, the group of organisms currently classified as C. fimbriata is an ideal model system with which to test and verify a species concept suitable for plant pathogenic fungi. C. fimbriata varies greatly in several genetic, physiological, and ecological characteristics (CABI 2001). Although variation of several characters in the C. fimbriata complex has been noted, no study has systematically examined a wide array of characters in a large, diverse group of isolates of this fungus. As part of a larger phylogenetic study addressing the genus Ceratocystis, the purpose of this study was to investigate the ecological, morphological, and genetic variation within the Latin American clade of C. fimbriata, using C. fimbriata as a model to help test a species concept for the fungi. Additionally, this study examined the genetics of populations of the fungus on sycamore and cacao, to determine introduced and indigenous populations. Dissertation Organization This dissertation is organized into seven chapters. The first chapter contains an overview of the biology, ecology, and systematics of Ceratocystis fimbriata. The second chapter is a paper that has been published in Phytopathology concerning host specialization and phylogenetics within the Latin American clade of C. fimbriata. The third chapter is a paper submitted to Mycologia concerning intersterility 3 groupings within C. fimbriata and descriptions of new species. The fourth chapter is a paper that will be submitted to Current Genetics concerning genetic analysis of progeny from crosses. The fifth chapter is a paper that has been submitted to Molecular Ecology concerning the population genetics of the sycamore form of C. fimbriata. The sixth chapter is a paper that will be submitted to Phytopathology about the population genetics of the cacao form of C. fimbriata. I was the primary researcher and author of all of these papers. The final chapter gives overall conclusions from the study and directions for further research. Literature Review This chapter is an overview of species concepts and the biology, ecology, and systematics of Ceratocystis fimbriata. A. Species Concepts and Fungi The species is the basic unit used to categorize and describe populations of living organisms. The definition of a species, though, has been widely debated and revised, resulting in several opposing ideas of what constitutes a species. Maydin (1997) reviews twenty-two different species concepts; only a few representative models will be described here. Species concepts may be divided into two main groups, mechanistic concepts and historical concepts (Luckow 1995). Mechanistic species concepts 4 define species as participants in the process of speciation. For example, according to the biological species concept, one mechanistic concept, species are interbreeding