Early male-killing (MK) bacteria are vertically transmitted reproductive parasites which kill male offspring that inherit them. Whereas their incidence is well documented, characteristics allowing originally non-MK bacteria to gradually evolve MK ability remain unclear. We show that horizontal transmission is a mechanism enabling vertically transmitted bacteria to evolve fully efficient MK under a wide range of host and parasite characteristics, especially when the efficacy of vertical transmission is high. We also show that an almost 100% vertically transmitted and 100% effective male-killer may evolve from a purely horizontally transmitted non-MK ancestor, and that a 100% efficient male-killer can form a stable coexistence only with a non-MK bacterial strain. Our findings are in line with the empirical evidence on current MK bacteria, explain their high efficacy in killing infected male embryos and their variability within and across insect taxa, and suggest that they may have evolved independently in phylogenetically distinct species.

Department of Biosystematics and Ecology Institute of Entomology Biology Centre CAS Branišovská 31 České Budějovice 37005 Czech Republic Institute of Mathematics and Biomathematics Faculty of Science University of South Bohemia Branišovská 1760 České Budějovice 37005 Czech Republic

Department of Mathematics and Statistics Faculty of Science Masaryk University Kotlářská 2 Brno 61137 Czech Republic

Department of Mathematics and Statistics Valparaiso University 1900 Chapel Drive Valparaiso IN 46383 USA

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