1. Influence of temperature on the host-parasite interaction in the diatom Asterionella formosa and its parasite Zygorhizidium planktonicum A.Gsell, L. de Senerpont Domis, S. Wiezer, E. Van Donk, B. Ibelings a.gsell@nioo.knaw.nl Aquatic Food Web Studies, Center for Limnology, Netherlands Institute for Ecology, KNAW 1 BW Ibelings, A De Bruin, M Kagami, M Rijkeboer, M Brehm & E Van Donk. 2004. HOST PARASITE INTERACTIONS BETWEEN FRESHWATER PHYTOPLANKTON AND CHYTRID FUNGI (Chytridiomycota) J. Phycol. 40:3 pp 437–53. 2 A De Bruin, BW Ibelings, M Rijkeboer, M Brehm & E van Donk. 2004. GENETIC VARIATION IN ASTERIONELLA FORMOSA (BACILLARIOPHYCEAE): IS IT LINKED TO FREQUENT EPIDEMICS OF HOST-SPECIFIC PARASITIC FUNGI? J. Phycol. 40:5 pp 823-830. Error bars:  standard error Introduction The A. formosa population in Lake Maarsseveen (NL) typically blooms twice a year 1, and the spring bloom often is tracked by a chytrid (Z. planktonicum) epidemic. The host population shows a surprisingly high genetic diversity 2 considering the lack of gene flow or documented sexual reproduction. Parasite mediated, frequency dependent selection and / or seasonal temperature changes may maintain this high diversity of genotypes in the lake. To disentangle differential selection of host genotypes (G H ) by temperature (E T ) from parasite genotype(G P ) interactions, we conducted two experiments. Experimental set up Seven monoclonal A. formosa genotypes isolated from the same spring bloom in Lake Maarsseveen were acclimatised for two weeks to their experimental temperature at 1,6,11,16 or 21°C. Six replicates per genotype and temperature were set at 10 000 cells ml -1 and grown for 15 days at saturating light and nutrient conditions. Growth rates and surface to volume ratios were analysed for G H xE interactions. The same set up was repeated and infected with one monoclonal parasite strain. Parasite and host growth rate were analysed for G H xG P xE interactions. Conclusions G H xG P xE T experiment Temperature influences the overall level as well as the rank order of susceptibility of host strains to fungal infection, thus it may select for both strength and direction of the host parasite interaction. Conclusions G H xE T experiment Adaptation to different temperatures comes at differential costs for the tested host strains, thus changing temperatures can select for different genotypes and thereby maintain genetic variability in the host population. 2) Surface:volume ratio SA/V per genotype and temperature. SA/V is a proxy for nutrient uptake capacity, a low ratio means less uptake opportunities. The significant G H xE interaction suggests that adaptation to different temperatures is costly and not uniform for all genotypes. Two way ANOVA Genotype (G H ) p < 0.001  Temperature (E) p < 0.001  G H x E T p = 0.002  A. formosa colonies showing the extent of cell size differences Z. planktonicum asexual life cycle 1) Mean growth rate per day per genotype and temperature. Generally, each genotype expresses a singular phenotype, and growth rates increase with rising temperature. The significant G H xE interaction is re- flected in crossing reaction norm lines and suggests that both level and direction of the genotype effect is dependent on the level of the temperature. Two way ANOVA Genotype (G H ) p < 0.001  Temperature (E) p < 0.001  G H x E T p < 0.001  Results G H xE T experiment Parasite (G P ) Host (G H ) Environment (E T ) temperature controlled water baths Preliminary results G H xG P xE T experiment 3)Sporangia growth rate of sporangia numbers per ml per host genotype and temperature. Fungal growth rates are host strain dependent and increase with rising temperature, but drop rapidly at 21°C. This suggests that temperature can select for the strength of the host parasite outcome. Further, the crossing reaction norms suggest potential for a G H xG P xE T interaction. 1 2 3 4 5 6 7 sporangia ml -1 : rank order 1°C6°C11°C16°C21°C S24 3 1 17 2 S264 2 3 3 4 S371 4 61 5 S382 3551 S43 5 57 4 3 S49672 2 6 S53 7 6 4 6 7 4) Rank order changes in sporangia growth rate per host genotype and temperature. Rank order for the most infected strain (1) to the least infected host (7) changes for each temperature. This suggests that temperature may also select for the direction of the host parasite interaction. Mean growth rates of all strains across five temperatures SA/V of all strains across five temperatures Temperature in °C Growth rate d -1 growth rate of sporangia ml -1 d -1 on all host strains across five temperatures