| Cytoplasmic bacteria of the genus Wolbachia have a conflict with the host insect over the gender of the offspring. As most of the transmission of the bacteria occurs vertically, from one generation to the next, and male offspring do not transmit Wolbachia to the next generation, they favour a 100% female bias in the sex ratio. Parthenogenesis inducing bacteria seem to have won this conflict and cause diploidization of unfertilized eggs in infected females, resulting in all-female offspring. Although infection with Wolbachia may bring fitness costs to the host, it can still maintain itself in the host populations as long as infected hosts produce more daughters than uninfected ones. In most of the documented instances of PI-Wolbachia the bacteria have spread through the population and become fixed at 100%, although a model for vertical transmission does predict fixation. The very few exceptions are some Trichogramma populations with only a fraction of the females infected with PI-Wolbachia. Such populations provide the unique possibility to study the evolutionary dynamics of the Wolbachia-host interactions. We have recently discovered the first occurrence of a partly infected host population outside the genus Trichogramma, i.e. the Dutch populations of the eulophid Tetrastichus coeruleus. The fraction of infected females varies between populations, whilst North American populations originating from accidental introduction in the early 1900s are fixed for PI-Wolbachia. By comparing different populations and building models for the spread of a PI-Wolbachia after initial infection, we hope to elucidate the dynamics of co-evolution between PI-Wolbachia and their hosts. |
