title: The evolution of transmission mode.
abstract: This article reviews research on the evolutionary mechanisms leading to different transmission modes. Such modes are often under genetic control of the host or the pathogen, and often in conflict with each other via trade-offs. Transmission modes may vary among pathogen strains and among host populations. Evolutionary changes in transmission mode have been inferred through experimental and phylogenetic studies, including changes in transmission associated with host shifts and with evolution of the unusually complex life cycles of many parasites. Understanding the forces that determine the evolution of particular transmission modes presents a fascinating medley of problems for which there is a lack of good data and often a lack of conceptual understanding or appropriate methodologies. Our best information comes from studies that have been focused on the vertical versus horizontal transmission dichotomy. With other kinds of transitions, theoretical approaches combining epidemiology and population genetics are providing guidelines for determining when and how rapidly new transmission modes may evolve, but these are still in need of empirical investigation and application to particular cases. Obtaining such knowledge is a matter of urgency in relation to extant disease threats.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.
authors.affiliation: Department of Biology, University of Virginia, Charlottesville, VA 22904, USA ja8n@virginia.edu.
authors.forename: Janis
authors.identifier: http://orcid.org/0000-0002-5189-1611
authors.initials: J
authors.lastname: Antonovics
authors.affiliation: Integrative Entomology group, Vector-borne Viral Diseases programme, The Pirbright Institute, Pirbright GU24 0NF, UK.
authors.forename: Anthony J
authors.identifier: http://orcid.org/0000-0002-2000-2914
authors.initials: AJ
authors.lastname: Wilson
authors.affiliation: Department of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B7.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Forbes
authors.affiliation: Department of Biodiversity and Molecular Ecology, Research and Innovation Centre, Fondazione Edmund Mach, Via E. Mach 1, 38010 S. Michele all'Adige, Trentino, Italy.
authors.forename: Heidi C
authors.initials: HC
authors.lastname: Hauffe
authors.affiliation: Department of Biological and Environmental Science, University of Jyvaskyla, PO Box 35, 40014 Jyvaskyla, Finland.|Department of Ecology, University of Oulu, PO Box 3000, 90014 Oulu, Finland.
authors.forename: Eva R
authors.initials: ER
authors.lastname: Kallio
authors.affiliation: Department of Genetics, University of Cambridge, Cambridge CB2 3EH, UK.
authors.forename: Helen C
authors.identifier: http://orcid.org/0000-0001-8758-355X
authors.initials: HC
authors.lastname: Leggett
authors.affiliation: Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Cornwall TR10 9FE, UK.
authors.forename: Ben
authors.identifier: http://orcid.org/0000-0001-6936-1697
authors.initials: B
authors.lastname: Longdon
authors.affiliation: Department of Life Sciences, Natural History Museum, Cromwell Road, London SW5 7BD, UK.
authors.forename: Beth
authors.initials: B
authors.lastname: Okamura
authors.affiliation: School of Biology, University of Leeds, Leeds LS2 9JT, UK.
authors.forename: Steven M
authors.initials: SM
authors.lastname: Sait
authors.affiliation: Department of Pathology and Pathogen Biology, Centre for Emerging, Endemic and Exotic Diseases, Royal Veterinary College, University of London, London AL9 7TA, UK.
authors.forename: Joanne P
authors.identifier: http://orcid.org/0000-0001-8616-4919
authors.initials: JP
authors.lastname: Webster
mesh_terms: D000820:Animal Diseases / Q000635:transmission*; D000818:Animals; D005075:Biological Evolution; D006790:Host-Parasite Interactions; D054884:Host-Pathogen Interactions
keywords: complex life cycles; host shifts; infectious disease; spill-over