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Date: 2025-01-01

Creator: Gracie Scheve

Access: Open access

Parasitism can influence host ecology and evolution in powerful ways, although the specific impacts on host fitness and life history may be context dependent and involve complex trade-offs. In this study, I investigated the effects of a novel microsporidian gut parasite on Daphnia ambigua, a freshwater zooplankton with a cyclical parthenogenetic life cycle. Combining extensive field sampling at Sewall Pond, Maine, with chronic exposure experiments in the lab, I assessed the parasite's impact on Daphnia fitness and propensity to shift from asexual to sexual reproduction. Field observations revealed a correlation between gut parasite prevalence and increased production of males and sexual females, independent of known sex inducers such as crowding, food limitation, and photoperiod. Lab experiments confirmed that chronic spore exposure significantly reduced Daphnia survival and reproductive output, particularly in clones previously naïve to this strain of the parasite. However, no induction of sex or male offspring was observed in response to parasite exposure under laboratory conditions. This suggests that more complex environmental interactions might be triggering sex in Daphnia. While sex provides the benefit of increased genetic diversity for future generations, I hypothesize that while Daphnia undergo sexual reproduction their ability to resist or tolerate parasite infection is diminished. Phylogenetic analyses indicate the parasite is closely related to the less virulent microsporidian Ordospora pajunii but genetically distinct, potentially constituting a new species or genus. These findings provide insight into the ecological and evolutionary tradeoffs involved in host-parasite interactions and introduce a new host-parasite system for this study.

Date: 2024-01-01

Creator: Mary Alta Rogalski, Elizabeth S Baker, Clara M Benadon

Access: Open access

Increasing application of road deicing agents (e.g., NaCl) has caused widespread salinization of freshwater environments. Chronic exposure to toxic NaCl levels can impact freshwater biota at genome to ecosystem scales, yet the degree of harm caused by road salt pollution is likely to vary among habitats and populations. The background ion chemistry of freshwater environments may strongly impact NaCl toxicity, with greater harm occurring in ion-poor, soft water conditions. In addition, populations exposed to salinization may evolve increased NaCl tolerance. Notably, if organisms are adapted to their natal lake water chemistry, toxicity responses may also vary among populations in a given test medium. We examined how this evolutionary and environmental context may interact in shaping NaCl toxicity with a pair of laboratory reciprocal transplant toxicity experiments, using natural populations of the water flea Daphnia ambigua from three lakes differing in ion availability. The lake water environment strongly influenced NaCl toxicity in both trials. NaCl greatly reduced reproduction and r in lake water from a low-ion/ calcium-poor environment compared with water from both a calcium-rich lake and an ion-rich coastal lake. Daphnia from this coastal lake were most robust to the effects of NaCl. A significant population x environment interaction shaped survival in both trials, suggesting that local adaptation to the test waters used contributed to toxicity responses. Our findings that the lake water environment, adaptation to that environment, and adaptation to a focal contaminant may shape toxicity demonstrate the importance of considering environmental and biological complexity in mitigating pollution impacts.