Online

Keller, Erin L.

Biology

2018

M.S.

Parasite life histories influence many aspects of infection dynamics, from the parasite infrapopulation diversity to the fitness of the parasite (the number of successfully transmitted parasites). Studies of medically important parasites, such as the parasite responsible for malaria (Plasmodium spp.), demonstrate the usefulness of investigating the life histories of parasites to better understand infection characteristics such as parasite load and probability of transmission. The gregarines are a diverse group of apicomplexan parasites that infect invertebrates, and are particularly common in insects and annelids. Given the great biodiversity and importance of their hosts, coupled with their close evolutionary relationship with important human pathogens such as Cryptosporidium spp., relatively little is known about gregarine life histories. The exemplar gregarine genus, Monocystis, is an excellent example of how a well-known gregarine parasite can have relatively little known about its life history. Specifically, the low reproductive output of Monocystis spp. and the absence of asexual replication makes the currently accepted life cycle untenable. More data are needed on the life history traits and strategies of Monocystis spp. that allow the parasite to be maintained at high prevalence and parasitemia. Here, a newly discovered species of Monocystis, infecting the invasive earthworm Amynthas agrestis, is described and investigated to determine key life history traits and strategies. First, I propose improvements to the current standard of gregarine species descriptions by standardizing nomenclature and biometrics and including molecular data. I described the newly discovered M. perplexa using the proposed improvements to gregarine species descriptions and found evidence of host species-specificity and widespread prevalence of the parasite in local earthworm populations. Such important data would not otherwise be collected with use of the current standard of gregarine species description and demonstrates the potential of implementing the suggested improvements. The life history of M. perplexa was investigated by measuring various life history traits throughout the course of infection. Key life history traits such as parasite phenology, resource allocation, and reproductive output were measured for infections at three local earthworm populations to determine if there is variation in parasite life histories within or among earthworm populations. Substantial variation in parasite life history traits and trade-offs between traits were identified. Notably, there was a trade-off between timing to parasite maturity, resource allocation, and reproductive output. Surprisingly, there was a near-complete lack of transmissible stages produced in one population, despite being maintained at high prevalence and parasitemia. To determine whether life history strategies, such as mode of transmission and asexual replication, can explain the currently untenable life cycle of M. perplexa, the presence of vertical transmission and asexual replication was revealed. Quantitative real-time PCR was used to detect and quantify minute amounts of parasite in the worm eggs and embryos and revealed a high rate of vertical transmission at all local sites. Evidence of parasite replication within the developing host embryo was found; however, no asexual replication was identified early in the host's season. Last, all worm tissues examined had high concentrations of parasite DNA, including the clitellum, the organ that produces the worm egg capsule.