A DISEASE ECOLOGIST S GUIDE TO EVOLUTION: EVIDENCE FROM HOST- PARASITE RELATIONSHIPS SARAH A. ORLOFSKE TEACHING EVOLUTION WORKSHOP UNIVERSITY OF COLORADO BOULDER sarah.orlofske@colorado.edu Ph.D. Candidate ~ Disease Ecology M.S. Student ~ Physiological Ecology B.S. ~ Biology & Wildlife Ecology Amphibian diseases and parasites Role of parasites in food webs Tropical amphibian conservation Amphibian physiology Evolution of host-parasite interactions WHY USE DISEASES TO TEACH EVOLUTION? Evolution of pathogens (agents that cause disease) has important applications for future human and wildlife health Control and treatment methods Characteristics of pathogens make them useful models for many evolutionary concepts Short generation time, high mutation rate Students have innate curiosity about diseases and many misconceptions Many existing resources available (videos, interactive models, case-studies and more) 1
OBJECTIVES Introduce evolutionary concepts in the context of disease ecology Malaria and Sickle Cell Anemia Mutation, Alleles, Natural Selection Evolution of Complex Life Cycles Ancestral states, Adaptation, Diversity Host Specificity Speciation, Co-evolution, Phylogenetics Provide suggestions/ideas for discussions and hands-on activities for use in the classroom MALARIA AND SICKLE CELL ANEMIA Genetic basis of evolution Disease as a mechanism of natural selection WHAT IS SICKLE CELL ANEMIA? Single point mutation Change in bases in DNA replication Results in a change in the amino-acid sequence Demonstration Effects of DNA Mutation on Protein Sequences 2
WHAT IS SICKLE CELL ANEMIA? Specifically affects the structure of hemoglobin Thr-Pro-Val-Gal-Lys Thr-Pro-Glu-Gal-Lys SICKLE CELL DISEASE Destruction of blood cells leads to anemia Sickle cells damage the heart, brain and other organs Symptoms include: frequent infections, paralysis, heart failure, severe pain, fevers, mental disorders and other aliments CONSEQUENCES Less than 20% of individuals with this disease reproduce So why do we still see it in the population? When can sickle cell disease be a good thing? 3
MALARIA Appoximately 250 million cases among 3.3 billion at risk for malaria PLASMODIUM SPP. Malaria is the disease caused by the parasitic protozoa Plasmodium
DISEASE Rupture of red blood cells occurs every 48-72 hours Fevers and Chills Decreases blood ph Death results from blockages of vital organs SICKLE CELL AND MALARIA Individuals heterozygous for Sickle Cell trait are more resistant to malaria Heterozygous superiority Make it less hospitable for the trophs Areas of high malaria risk heterozygous individuals have 20% lower death rate GENETICS OF SICKLE CELL DISEASE Homozygous vs. Heterozygous Hb A Hb a Hb A Normal Hemoglobin Hb a Sickel Cell Hemoglobin Hb A Hb a Hb A Hb A Hb A Hb a Hb A Hb a Hb a Hb a 5
EVOLUTION Evolution - changes in geneticallyinfluenced characteristics over time Microevolution - changes in relative allele frequencies in a population Processes of Evolution Mutation, Recombination, Gene flow, Genetic drift, non-random mating and Natural selection NATURAL SELECTION Requirements: Genetic variation Selective force Traits are heritable Malaria? Demonstration Fitness model for gene frequencies in population ALTERNATIVE TEACHING IDEAS http://www.pbs.org/wgbh/evolution/library/01/2/ l_012_02.htmlgenetic variation
EVOLUTION OF COMPLEX LIFE CYCLES Trophic Transmission Behavioral and morphological modification by parasites DEFINITION OF PARASITE Parasites require multiple hosts or organisms to complete their life cycle Examples Protozoa: Malaria, Toxoplasma, Trypanosoma Worms: Cestodes, Nematodes and Trematodes BUILD A COMPLEX LIFE CYCLE A. Adult parasites in bird host F. Infected tadpole is eaten by bird B. Parasite leaves snail and swims to tadpole D. Second larval stage is in the developing tadpole G. Parasite larva infects snail host C. Snail host harbors larval parasites. E. Parasite eggs are contained in host feces. 7
BUILD A COMPLEX LIFE CYCLE A. Adult parasites in bird host E. Parasite eggs are contained in host feces. G. Parasite larva infects snail host F. Infected tadpole is eaten by bird D. Second larval stage is in the developing tadpole C. Snail host harbors larval parasites B. Parasite leaves snail and swims to tadpole EVOLUTIONARY ORIGINS OF COMPLEX LIFE CYCLES Upward Incorporation Downward Incorporation Parasite Host 2 Host 1 Host 2 Host 1 Parasite Parasite Schimid-Hempel 2011 Evolutionary Parasitology EVOLUTIONARY CONCEPTS Natural Selection Predation is a strong selective force influencing the parasites fitness = number of off spring Genetic variation in the ability of parasites to survive and develop in the predator = new host
MODIFICATION BY PARASITES Taking advantage of the food web Is it Adaptive or not? ADAPTATION Adaptation is used to refer to both an organism s trait and the process leading to the evolution of that trait. A genetically determined feature that has become or is becoming prevalent in a population because it confers a selective advantage to its bearer through an improvement in some function. Poulin 1995 CRITERIA 1. What is the mechanism? 1. Is the modification intentionally caused by the parasite or is it a side effect of infection? 2. What is the benefit to the parasite? 1. Does it enhance the parasite s fitness (survival, reproduction or transmission)
EVIDENCE Evidence for pathology-independent changes in host biochemistry (Toxoplasma and others) Small modifications in behavior/morphology lead to large changes in transmission However, more research is needed in many systems. SUGGESTED TEACHING ACTIVITIES Assign individual or small groups of students a particular host-parasite system exhibiting behavior/morphological modification Ask them to Describe the parasite life cycle and the role of the modified host Characterize the type of modification Synthesize the evidence from scientific sources (Does the system meet the criteria?) Design an experiment to test the modifications (Derive hypotheses) Evaluate whether or not the system is an example of Adaptation HOST SPECIFICITY Co-evolution Macro and Micro Evolution Clayton-Bush Lab Website 10
SPECIALISTS VS. GENERALISTS Numbers of host species a particular parasite species uses COMPARING PHYLOGENIES Relevant for other associations Plants and Pollinators Host Parasite CO-SPECIATION Speciation: the evolutionary process by which new biological species arise Speciation in parallel Phylogenies es Mirror Host Parasite specific, co speciates with host Parasite
HOST SWITCHING Parasites evolve to use a new host species Potentially leads to disease emergence Host Some parasites not host specific evidence of past host switching Parasite WHAT LEADS TO THESE PATTERNS? Natural Selection Opportunities for colonization and availability of suitable host species Preening removal of lice by the birds was an important selective force in what species were able to establish on a particular host. Bush, S.E. and D.H. Clayton. 2006 CASE STUDY TEACHING A Tale of Three Lice By Joan Sharp and Erin Barley Human Evolution Co-evolution The head louse, Pediculus humanus capitus The pubic louse, Phthirus pubis The body louse, Pediculus humanus corporis
MORE DISEASE EXAMPLES OF EVOLUTION Evolution of Drug Resistance http://evolution.berkeley.edu/evolibrary/news/091201_malaria Evolution of Pesticide Resistance - Vectors of Disease http://www.pbs.org/wgbh/evolution/library/10/1/l_101_02.html Evolution of Virulence (How damaging should a pathogen be to its host?) http://www.pbs.org/wgbh/evolution/library/01/6/l_016_06.html ADDITIONAL TEACHING RESOURCES Mathematica Demonstrations Website http://demonstrations.wolfram.com/ Other topics in addition to evolution National Center for Case Study Teaching http://sciencecases.lib.buffalo.edu/cs/index.asp Disease Evolution Case Studies Parasite of the Day Blog http://dailyparasite.blogspot.com/ Short articles on parasites including evolutionary aspects ACKNOWLEDGEMENTS Dr. John Basey and all my students in General Biology Robert Jadin and Joe Mihaljevic Organizers and Volunteers All of you! Please contact me with comments/questions Requests for additional info or materials 13