Experimental and comparative analyses of the evolutionary ecology of parasitic nematodes

iii-iv CHAPTER 1 General Introduction 1.1 Phenotypic plasticity. 1-5 Adaptive and non-adaptive phenotypic plasticity. 1-5 Theoretical models of plasticity. 5 1.2 Plasticity in the genus Strongyloides. 5-6 1.3 Plasticity in Strongyloides ratti. 6-12 (i) Life cycle. 6-9 (ii) Conditional sexual reproduction. 9-10 (iii) Host specificity. 11 (iv) Maturation time. 11-12 1.4 Aims. 12 CHAPTER 2 Influence of rat strain on larval production by the parasitic nematode Strongyloides ratti 2.1 Summary.13 2.2 Introduction.13-14 2.3 Methods.14-15 2.4 Results and Discussion.15-18 CHAPTER 3 Host-specificity of Strongyloides raft! 3.1 Summary. 19 3.2 Introduction. 19-22 3.3 Methods. 23-27 Parasites and parasitology. 23-24 Establishment and expulsion. 24 Thymus-dependent immunity. 24-25 Selection. 25-26 Estimation of variance components. 26-27 Reciprocal cross infection. 27 (viii) Statistics . 27 3.4 Results. 28-35 Relative success of S. ratti in rats and mice. 28 Establishment, expulsion and fecundity.28-30 The role of thymus-dependent immunity.31 Outcome of selection and sources of variance. 31-35 3.5 Discussion. 36-39 CHAPTER 4 Host immunity acquired against another nematode does not induce sexuality in Strongyloides ratti 4.1 Summary.40 4.2 Introduction.40-42 4.3 Methods.42-46 Parasitology. 42-43 Immunisation. 43-44 Statistical analysis . 44-46 4.4 Results. 46 4.5 Discussion. 47 CHAPTER 5 Optimal timing of first reproduction in parasitic nematodes 5.1 Summary.48 5.2 Introduction. 48-50 5.3 Model. 50-54 5.4 Testing the model. 55-62 Estimating /3 . 55-59 maturation time and fecundity . 55 phylogeny. 55-56 regression. 56-59 Estimating mortality rates. 59-62 5.5 Observation and prediction. 63-67 5.6 Discussion.68-71 CHAPTER 6 Host immune status determines maturation time in a parasitic nematode 6 . 1 Summary.72 6.2 Introduction. 72-76 6 . 3 Methods. 76-81 Parasitology.76-77 Host immunity and worm output. 77-78 T-dependent immunity and parasite maturation. 78 Statistics.78-79 Literature survey. 79-81 6 . 4 Results.81-88 Immunity and parasite reproductive output. 81 T-dependent immunity and parasite maturation time.81-83 Published experiments . 83-88 6 . 5 Discussion.89-91 CHAPTER 7 General Discussion 7.1 Summary . 92 7.2 Main Findings.93-95 7.3 Future Directions.95-97 BIBLIOGRAPHY . 98-109 APPENDIX Publications during this period of study Gemmill, A. W. , Viney, M. E. and Read, A. F. (1997). Host immune status determines sexuality in a parasitic nematode. Evolution 51(2), 393-401. Gemmill, A. W. and Read, A. F. (1998). Counting the cost of disease resistance. Trends in Ecology & Evolution 13(1), 8-9. Gemniill, A. W. and West, S . A. (1998). Influence of rat strain on larval production by the parasitic nematode Strongyloides ratti. Journal of Parasitology 84(6), 1289-1291. Elvis on Worm Size This essay was one often runners-up in the 1998 WellcomeNew Scientist Millennial Science Essay Competition and recently appeared in the magazine BBSRC Business. ACKNOWLEDGEMENTS No man is an island (sadly) and so I must extend some woefully inadequate thanks to the following people. All of this work is dedicated to my dad, Jim Gemmill, who first told me about evolution. Thanks to him. The work was supported by a Natural Environment Research Council studentship, Award No. GT4/95/821T. Thanks to NERC. Mark Viney and Andrew Read are ("almost certainly") truly excellent supervisors as well as good friends and colleagues. Here is my tribute to both of them. Mark sees the biological "wood" despite the experimental "trees". He has more common sense than most other academic biologists of my acquaintance put together and knows how to use it to get science done. I would like to think that this virtue is learnable or at least that some of it might rub off. However, in Mark's case the ability appears to be congenitally hardwired and not under any voluntary control. Andrew Read enabled me to do the work I wanted to do sometimes even before I knew that I wanted to do it. He ushered me down many avenues of enquiry; some of which proved to be cul-desacs, and some not. These intellectual excursions (wild goose chases and otherwise) have formed the basis of my scientific training. Andrew encouraged me to go with my intuition when I thought I was right and gave me the confidence to have a good bellylaugh at my own mistakes when (not infrequently) I discovered I was wrong. In addition, he knows (and communicates) more about evolution than is probably decent (for him) or healthy (for everyone else). Both Mark and Andrew helped me out, hands on, when things were going "not quite according to plan". They always did so without qualification and I thank them both. So much for the supervisors. Lloyd Mitchell made all the equipment work. John Tweedie, Sheena Boath, Stuart Fleming and Helen Borthwick cared for my experimental animals and made sure I lived up to their exacting standards (and quite right too). Thanks to them. Louise Taylor, Aleta Graham, Nick Colegrave, Becky Timms, Lucy Crooks, Brian Chan, Margaret Mackinnon, Martin Waterfall and Kay Samuel helped to make academic life enjoyable. Martin and Kay were always happy to dispense advice on matters immunological. Lucy and Margaret are largely responsible for my small (but perfectly formed) knowledge of mathematics. Aleta did much of the culturing in Chapter 2 and administered the antihelminthics in Chapter 4. Thanks to all of them. Thanks also to the Glasgow Buddhist Centre for spiritual support. Angus Buckling faithfully perpetrated the same