title: Host control and nutrient trading in a photosynthetic symbiosis.
abstract: Photosymbiosis is one of the most important evolutionary trajectories, resulting in the chloroplast and the subsequent development of all complex photosynthetic organisms. The ciliate Paramecium bursaria and the alga Chlorella have a well established and well studied light dependent endosymbiotic relationship. Despite its prominence, there remain many unanswered questions regarding the exact mechanisms of the photosymbiosis. Of particular interest is how a host maintains and manages its symbiont load in response to the allocation of nutrients between itself and its symbionts. Here we construct a detailed mathematical model, parameterised from the literature, that explicitly incorporates nutrient trading within a deterministic model of both partners. The model demonstrates how the symbiotic relationship can manifest as parasitism of the host by the symbionts, mutualism, wherein both partners benefit, or exploitation of the symbionts by the hosts. We show that the precise nature of the photosymbiosis is determined by both environmental conditions (how much light is available for photosynthesis) and the level of control a host has over its symbiont load. Our model provides a framework within which it is possible to pose detailed questions regarding the evolutionary behaviour of this important example of an established light dependent endosymbiosis; we focus on one question in particular, namely the evolution of host control, and show using an adaptive dynamics approach that a moderate level of host control may evolve provided the associated costs are not prohibitive.
authors.affiliation: Department of Biology, University of York, Wentworth Way, York YO10 5DD, UK; Department of Mathematics, University of York, York YO10 5DD, UK. Electronic address: andrew.dean@york.ac.uk.
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Dean
authors.affiliation: Department of Biology, University of York, Wentworth Way, York YO10 5DD, UK.
authors.forename: Ewan J A
authors.initials: EJ
authors.lastname: Minter
authors.affiliation: Department of Biology, University of York, Wentworth Way, York YO10 5DD, UK.
authors.forename: Megan E S
authors.initials: ME
authors.lastname: Sørensen
authors.affiliation: Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Penryn Campus, Treliever Road, Penryn TR10 9FE, UK.
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Lowe
authors.affiliation: Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield S10 2TN, UK.
authors.forename: Duncan D
authors.initials: DD
authors.lastname: Cameron
authors.affiliation: Department of Biology, University of York, Wentworth Way, York YO10 5DD, UK.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Brockhurst
authors.affiliation: Department of Biology, University of York, Wentworth Way, York YO10 5DD, UK; Department of Mathematics, University of York, York YO10 5DD, UK.
authors.forename: A
authors.initials: A
authors.lastname: Jamie Wood
mesh_terms: D000222:Adaptation, Physiological / Q000528:radiation effects; D002708:Chlorella / Q000254:growth & development / Q000502:physiology*; D008027:Light; D010247:Paramecium / Q000254:growth & development / Q000502:physiology*; D010788:Photosynthesis / Q000502:physiology* / Q000528:radiation effects; D013559:Symbiosis / Q000502:physiology* / Q000528:radiation effects
keywords: Adaptive dynamics; Algae; Endosymbiosis; Protozoa