Host and symbiont genetic contributions to fitness in a <i>Trichogramma–Wolbachia</i> symbiosis

Abstract

The fitness effects associated with <i>Wolbachia</i> infection have wide-ranging ecological and evolutionary consequences for host species. How these effects are modulated by the relative influence of host and <i>Wolbachia</i> genomes has been described as a balancing act of genomic cooperation and conflict. For vertically transmitted symbionts, like cytoplasmic <i>Wolbachia</i>, concordant host-symbiont fitness interests would seem to select for genomic cooperation. However, <i>Wolbachia</i>'s ability to manipulate host reproductive systems and distort offspring sex ratios presents an evolutionary conflict of interest with infected hosts. In the parthenogenesis-inducing (PI) form of <i>Wolbachia</i> found in many haplodiploid insects, <i>Wolbachia</i> fitness is realized through females and is enhanced by their feminization of male embryos and subsequent parthenogenetic reproduction. In contrast, as long as <i>Wolbachia</i> is not fixed in a population and sexual reproduction persists, fitness for the host species is realized through both male and female offspring production. How these cooperating and competing interests interact and the relative influence of host and <i>Wolbachia</i> genomes were investigated in the egg parasitoid <i>Trichogramma kaykai</i>, where <i>Wolbachia</i> infection has remained at a low frequency in the field. A factorial design in which laboratory cultures of <i>Wolbachia</i>-infected <i>T. kaykai</i> were cured and re-infected with alternative <i>Wolbachia</i> strains was used to determine the relative influence of host and <i>Wolbachia</i> genomes on host fitness values. Our results suggest fitness variation is largely a function of host genetic background, except in the case of offspring sex ratio where a significant interaction between host and <i>Wolbachia</i> genomes was found. We also find a significant effect associated with the horizontal transfer of <i>Wolbachia</i> strains, which we discuss in terms of the potential for coadaptation in PI-<i>Wolbachia</i> symbioses.