Storage length and temperature influence infectivity and spore yield of two common Daphnia parasites

Daphnia and their parasites have emerged as a model system for understanding the ecology and evolution of infectious diseases. Two of the most commonly studied Daphnia parasites are the bacterium Pasteuria ramosa and the fungus Metschnikowia bicuspidata. In addition to being the focus of numerous field studies, these two parasites have been used in many laboratory experiments. However, there is little information in the scientific literature about how the conditions under which these parasites are stored influence the infectivity and yield of transmission stages (“spores”). This is problematic because such information is critical for experiment design and data interpretation.We tested the influence of storage length (eight treatments ranging from 1 day to 1 year) and temperature (−20°C (freezer) vs. 4°C (refrigerator)) on spore infectivity and yield. We found that Pasteuria spores survived well at both −20°C and 4°C, and remained infective even after storage for one year. However, Pasteuria spore yields dropped over time, particularly at 4°C. In contrast, Metschnikowia spores were killed within days at −20°C. At 4°C, Metschnikowia infectivity declined steadily over a period of two months and, by four months, spores were no longer infective. Spore yield from Metschnikowia-infected hosts was not significantly impacted by storage length, but trended downwards.Scientists working with Pasteuria should be aware that spore yield declines during storage, particularly in the refrigerator. Scientists working with Metschnikowia should be aware that it is killed by freezer storage and that, even if it is stored in the refrigerator, infectivity declines within a few months. These results might have implications for parasite distributions in the field; for example, the high sensitivity of Metschnikowia to freezing might help explain why it tends to be more common in deep lakes than in ponds or rock pools.