Infection dynamics and host mortality.
The amount of spores in the host increased exponentially, regardless of infection route, and reached a plateau. In vertically infected hosts, spores appeared earlier and reached a higher plateau. This is expected, as the parasite can start developing earlier (already when the embryo is inside the mother) than in horizontal infection, which can occur only after the Daphnia has hatched. This delay, however, was shorter than expected, suggesting that vertical infections start slower (as found in other microsporidia). Horizontal infections were more aggressive and produced a higher and earlier mortality, probably due to higher initial doses and/or a faster initial growth than in vertical infections. This, however, did not translate into an advantage for the parasite, as the final spore-load was much lower! It is thus possible that by wasting less resources at the beginning of the infection, the parasite can redirect more resources later for its own growth.
Two of the three different types of spores differed clearly in their dynamics, suggesting that their functions in the parasite's life-history may also differ. One spore-type was produced mainly at the earlier phase, and perhaps is responsable of the internal spread and the vertical transmission. The other spore-type was accumulated during time, suggesting that it is responsible for external, horizontal transmission.
Vizoso, D. B. & Ebert, D. 2004. Within-host dynamics of a microsporidium with horizontal and vertical transmission: Octosporea bayeri in Daphnia magna. Parasitology, 128: 31:38. [pdf]
Phenotypic plasticity in host and parasite.
We conducted a life-table experiment to compare D. magna infected with O. bayeri either horizontally or vertically, using three different parasite isolates. Both the infection route and the parasite isolate had significant effects on host life-history. Hosts matured at different ages depending on the parasite isolate, and at a size that varied with infection route. The frequency of host sterility and the host's life-time reproductive success were affected by both the infection route and the parasite isolate. The infection route also affected parasite life-history. The production of parasite spores was much higher in vertically than in horizontally infected hosts. We found a trade-off between the production of spores (the parasite’s horizontal fitness component) and the production of infected host offspring (the parasite’s vertical fitness component). This study shows that hosts and parasites can react plastically to different routes of infection, suggesting that ecological factors that may influence the relative importance of horizontal and vertical transmission can shape the evolution of host and parasite life histories, and, consequently, the evolution of virulence.
Vizoso, D. B. & Ebert, D. 2005. Phenotypic plasticity of host-parasite interactions in response to the route of infection. Journal of Evolutionary Biology. 18: 911-921. [pdf]
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