Evol Ecol Res 7: 1183-1190 (2005)     Full PDF if your library subscribes.

Competitive co-existence of vertically and horizontally transmitted parasites

Curtis M. Lively,* Keith Clay, Michael J. Wade and Clay Fuqua

Department of Biology, Indiana University, Bloomington, IN 47405-3700, USA

Author to whom all correspondence should be addressed.
e-mail: clively@indiana.edu

ABSTRACT

Questions: Can vertically transmitted parasites (VTPs) serve as indirect mutualists by excluding more virulent, horizontally transmitted parasites (HTPs)? How does the efficiency of vertical transmission affect: (1) the equilibrium number of hosts infected by each type of parasite; (2) the total number of hosts in the population; and (3) virulence of the horizontally transmitted parasite? How does reproductive output by the horizontally transmitted parasite affect the frequency of infection by both parasite types?

Methods and assumptions: We used a deterministic computer simulation to determine host-population dynamics following introduction of a vertically transmitted parasite. We assumed that the vertically transmitted parasite precludes infection by the horizontally transmitted parasite. We allowed for the possibility that virulence of the horizontally transmitted parasite depends on host density.

Conclusions: Vertically transmitted parasites can serve as indirect mutualists by excluding more virulent horizontally transmitted parasites. As such, selection would be expected to favour VTP strains that most efficiently exclude horizontally transmitted parasites. In addition, the total number of hosts at equilibrium increases (and HTP frequency decreases) with the efficiency of vertical transmission. Increasing the reproductive output of the horizontally transmitted parasite has little effect on the frequency of that parasite at equilibrium, but greatly increases the frequency of hosts infected with the vertically transmitted parasite. When virulence of the horizontally transmitted parasite is density dependent, the vertically transmitted parasite indirectly increases virulence by increasing host density at equilibrium.

Keywords: horizontal transmission, pathogen, symbiont, vertical transmission, virulence.

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        © 2005 Curtis M. Lively. All EER articles are copyrighted by their authors. All authors endorse, permit and license Evolutionary Ecology Ltd. to grant its subscribing institutions/libraries the copying privileges specified below without additional consideration or payment to them or to Evolutionary Ecology, Ltd. These endorsements, in writing, are on file in the office of Evolutionary Ecology, Ltd. Consult authors for permission to use any portion of their work in derivative works, compilations or to distribute their work in any commercial manner.

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